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re(3erl)			     Erlang Module Definition				 re(3erl)

NAME
       re - Perl like regular expressions for Erlang

DESCRIPTION
       This module contains regular expression matching functions for strings and binaries.

       The  regular  expression syntax and semantics resemble that of Perl. This library replaces
       the deprecated pure-Erlang regexp library; it has a richer syntax,  more  options  and  is
       faster.

       The  library's matching algorithms are currently based on the PCRE library, but not all of
       the PCRE library is interfaced and some parts of the library go beyond what  PCRE  offers.
       The  sections  of  the  PCRE  documentation which are relevant to this module are included
       here.

   Note:
       The Erlang literal syntax for strings uses the "\"  (backslash)	character  as  an  escape
       code.  You  need  to  escape  backslashes in literal strings, both in your code and in the
       shell, with an additional backslash, i.e.: "\\".

DATA TYPES
	   iodata() = iolist() | binary()
	   iolist() = [char() | binary() | iolist()]
	     - a binary is allowed as the tail of the list

	   unicode_binary() = binary() with characters encoded in UTF-8 coding standard
	   unicode_char() = integer() representing a valid unicode codepoint

	   chardata() = charlist() | unicode_binary()

	   charlist() = [unicode_char() | unicode_binary() | charlist()]
	     - a unicode_binary is allowed as the tail of the list

	   mp() = Opaque datatype containing a compiled regular expression.
	     - The mp() is guaranteed to be a tuple() having the atom
	    're_pattern' as its first element, to allow for matching in
	       guards. The arity of the tuple() or the content of the other fields
	    may change in future releases.

EXPORTS
       compile(Regexp) -> {ok, MP} | {error, ErrSpec}

	      Types  Regexp = iodata()

	      The same as compile(Regexp,[])

       compile(Regexp,Options) -> {ok, MP} | {error, ErrSpec}

	      Types  Regexp = iodata() | charlist()
		     Options = [ Option ]
		     Option = unicode | anchored | caseless | dollar_endonly | dotall |  extended
		     |	firstline | multiline | no_auto_capture | dupnames | ungreedy | {newline,
		     NLSpec}| bsr_anycrlf | bsr_unicode
		     NLSpec = cr | crlf | lf | anycrlf | any
		     MP = mp()
		     ErrSpec = {ErrString, Position}
		     ErrString = string()
		     Position = int()

	      This function compiles a regular expression with the syntax described below into an
	      internal format to be used later as a parameter to the run/2,3 functions.

	      Compiling  the  regular expression before matching is useful if the same expression
	      is to be used in matching against multiple subjects during the program's	lifetime.
	      Compiling  once  and executing many times is far more efficient than compiling each
	      time one wants to match.

	      When the unicode option is given, the regular expression should be given as a valid
	      unicode charlist() , otherwise as any valid iodata() .

	      The options have the following meanings:

		unicode :
		  The regular expression is given as a unicode charlist() and the resulting regu-
		  lar expression code is to be run against a valid unicode charlist() subject.

		anchored :
		  The pattern is forced to be "anchored", that is, it  is  constrained	to  match
		  only	at  the  first	matching  point in the string that is being searched (the
		  "subject string"). This effect can also be achieved by  appropriate  constructs
		  in the pattern itself.

		caseless :
		  Letters  in  the pattern match both upper and lower case letters. It is equiva-
		  lent to Perl's /i option, and it can be changed within  a  pattern  by  a  (?i)
		  option  setting.  Uppercase  and  lowercase  letters	are  defined  as  in  the
		  ISO-8859-1 character set.

		dollar_endonly :
		  A dollar metacharacter in the pattern matches only at the end  of  the  subject
		  string. Without this option, a dollar also matches immediately before a newline
		  at the end of the  string  (but  not	before	any  other  newlines).	The  dol-
		  lar_endonly  option  is  ignored  if multiline is given. There is no equivalent
		  option in Perl, and no way to set it within a pattern.

		dotall :
		  A dot in the pattern matches all characters, including those that indicate new-
		  line.  Without  it, a dot does not match when the current position is at a new-
		  line. This option is equivalent to Perl's /s option,	and  it  can  be  changed
		  within a pattern by a (?s) option setting. A negative class such as [^a] always
		  matches newline characters, independent of this option's setting.

		extended :
		  Whitespace data characters in the pattern are ignored except	when  escaped  or
		  inside  a  character class. Whitespace does not include the VT character (ASCII
		  11). In addition, characters between an unescaped # outside a  character  class
		  and the next newline, inclusive, are also ignored. This is equivalent to Perl's
		  /x option, and it can be changed within a pattern by	a  (?x)  option  setting.
		  This	option makes it possible to include comments inside complicated patterns.
		  Note, however, that this applies only to data characters. Whitespace characters
		  may  never  appear within special character sequences in a pattern, for example
		  within the sequence (?( which introduces a conditional subpattern.

		firstline :
		  An unanchored pattern is required to match before or at the  first  newline  in
		  the subject string, though the matched text may continue over the newline.

		multiline :
		  By  default,	PCRE  treats the subject string as consisting of a single line of
		  characters (even if  it  actually  contains  newlines).  The	"start	of  line"
		  metacharacter  (^)  matches  only at the start of the string, while the "end of
		  line" metacharacter ($) matches only at the end of the string, or before a ter-
		  minating newline (unless dollar_endonly is given). This is the same as Perl.

		  When	multiline  it  is given, the "start of line" and "end of line" constructs
		  match immediately following or immediately before internal newlines in the sub-
		  ject string, respectively, as well as at the very start and end. This is equiv-
		  alent to Perl's /m option, and it can be changed within a  pattern  by  a  (?m)
		  option setting. If there are no newlines in a subject string, or no occurrences
		  of ^ or $ in a pattern, setting multiline has no effect.

		no_auto_capture :
		  Disables the use of numbered capturing parentheses in the pattern. Any  opening
		  parenthesis  that is not followed by ? behaves as if it were followed by ?: but
		  named parentheses can still be used for capturing (and they acquire numbers  in
		  the usual way). There is no equivalent of this option in Perl.

		dupnames :
		  Names  used  to  identify capturing subpatterns need not be unique. This can be
		  helpful for certain types of pattern when it is known that only one instance of
		  the  named subpattern can ever be matched. There are more details of named sub-
		  patterns below

		ungreedy :
		  This option inverts the "greediness" of the quantifiers so that  they  are  not
		  greedy  by  default, but become greedy if followed by "?". It is not compatible
		  with Perl. It can also be set by a (?U) option setting within the pattern.

		{newline, NLSpec} :
		  Override the default definition of a newline in the subject string, which is LF
		  (ASCII 10) in Erlang.

		  cr :
		    Newline is indicated by a single character CR (ASCII 13)

		  lf :
		    Newline is indicated by a single character LF (ASCII 10), the default

		  crlf :
		    Newline  is  indicated  by the two-character CRLF (ASCII 13 followed by ASCII
		    10) sequence.

		  anycrlf :
		    Any of the three preceding sequences should be recognized.

		  any :
		    Any of the newline sequences above, plus the Unicode sequences  VT	(vertical
		    tab,  U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line sepa-
		    rator, U+2028), and PS (paragraph separator, U+2029).

		bsr_anycrlf :
		  Specifies specifically that \R is to match only the cr, lf or  crlf  sequences,
		  not the Unicode specific newline characters.

		bsr_unicode :
		  Specifies  specifically  that \R is to match all the Unicode newline characters
		  (including crlf etc, the default).

       run(Subject,RE) -> {match, Captured} | nomatch

	      Types  Subject = iodata() | charlist()
		     RE = mp() | iodata() | charlist()
		     Captured = [ CaptureData ]
		     CaptureData = {int(),int()}

	      The same as run(Subject,RE,[]) .

       run(Subject,RE,Options) -> {match, Captured} | match | nomatch

	      Types  Subject = iodata() | charlist()
		     RE = mp() | iodata() | charlist()
		     Options = [ Option ]
		     Option = anchored | global | notbol | noteol | notempty | {offset, int()}	|
		     {newline, NLSpec} | bsr_anycrlf | bsr_unicode | {capture, ValueSpec} | {cap-
		     ture, ValueSpec, Type} | CompileOpt
		     Type = index | list | binary
		     ValueSpec = all | all_but_first | first | none | ValueList
		     ValueList = [ ValueID ]
		     ValueID = int() | string() | atom()
		     CompileOpt = see compile/2 above
		     NLSpec = cr | crlf | lf | anycrlf | any
		     Captured = [ CaptureData ] | [ [ CaptureData ] ... ]
		     CaptureData = {int(),int()} | ListConversionData | binary()
		     ListConversionData = string() | {error, string(), binary()}  |  {incomplete,
		     string(), binary()}

	      Executes a regexp matching, returning match/{match, Captured} or nomatch . The reg-
	      ular expression can be given either as iodata() in which case it	is  automatically
	      compiled	(as  by  re:compile/2  ) and executed, or as a pre compiled mp() in which
	      case it is executed against the subject directly.

	      When compilation is involved, the exception badarg is thrown if a compilation error
	      occurs. Call re:compile/2 to get information about the location of the error in the
	      regular expression.

	      If the regular expression is previously compiled, the option list can only  contain
	      the  options  anchored  ,  global  , notbol , noteol , notempty , {offset, int()} ,
	      {newline, NLSpec} and {capture, ValueSpec}/{capture, ValueSpec, Type}  .	Otherwise
	      all  options  valid  for	the  re:compile/2  function  are allowed as well. Options
	      allowed both for compilation and execution of a match, namely  anchored  and  {new-
	      line,  NLSpec} , will affect both the compilation and execution if present together
	      with a non pre-compiled regular expression.

	      If the regular expression was previously compiled with the  option  unicode  ,  the
	      Subject  should  be provided as a valid Unicode charlist() , otherwise any iodata()
	      will do. If compilation is involved and the option unicode is given, both the  Sub-
	      ject and the regular expression should be given as valid Unicode charlists() .

	      The {capture, ValueSpec}/{capture, ValueSpec, Type} defines what to return from the
	      function upon successful matching. The capture tuple may contain both a value spec-
	      ification  telling  which of the captured substrings are to be returned, and a type
	      specification, telling how captured substrings are to be returned (as index tuples,
	      lists or binaries). The capture option makes the function quite flexible and power-
	      ful. The different options are described in detail below.

	      If the capture options describe that no substring capturing at all is to be done	(
	      {capture,  none}	), the function will return the single atom match upon successful
	      matching, otherwise the tuple {match, ValueList} is returned.  Disabling	capturing
	      can be done either by specifying none or an empty list as ValueSpec .

	      The options relevant for execution are:

		anchored :
		  Limits  re:run/3  to	matching at the first matching position. If a pattern was
		  compiled with anchored , or turned out to be anchored by  virtue  of	its  con-
		  tents,  it  cannot be made unanchored at matching time, hence there is no unan-
		  chored option.

		global :
		  Implements global (repetitive) search (the g	flag  in  Perl).  Each	match  is
		  returned  as	a  separate  list()  containing the specific match as well as any
		  matching subexpressions (or as specified by the capture option ). The  Captured
		  part of the return value will hence be a list() of list() s when this option is
		  given.

		  The interaction of the global option with a regular expression which matches an
		  empty  string  surprises  some users. When the global option is given, re:run/3
		  handles empty matches in the same way as Perl: a zero-length match at any point
		  will	be  retried with the options [anchored, notempty] as well. If that search
		  gives a result of length > 0, the result is included. For example:

		    re:run("cat","(|at)",[global]).

		  The following matching will be performed:

		  At offset 0 :
		    The regexp (|at) will first match at the initial position of the string cat ,
		    giving  the  result  set [{0,0},{0,0}] (the second {0,0} is due to the subex-
		    pression marked by the parentheses). As the length of  the	match  is  0,  we
		    don't advance to the next position yet.

		  At offset 0 with [anchored, notempty] :
		    The search is retried with the options [anchored, notempty] at the same posi-
		    tion, which does not give any interesting result of  longer  length,  so  the
		    search position is now advanced to the next character ( a ).

		  At offset 1 :
		    This  time, the search results in [{1,0},{1,0}] , so this search will also be
		    repeated with the extra options.

		  At offset 1 with [anchored, notempty] :
		    Now the ab alternative is found and the result  will  be  [{1,2},{1,2}].  The
		    result  is added to the list of results and the position in the search string
		    is advanced two steps.

		  At offset 3 :
		    The search now once again matches the empty string, giving [{3,0},{3,0}] .

		  At offset 1 with [anchored, notempty] :
		    This will give no result of length > 0 and we are at the  last  position,  so
		    the global search is complete.

		  The result of the call is:

		     {match,[[{0,0},{0,0}],[{1,0},{1,0}],[{1,2},{1,2}],[{3,0},{3,0}]]}

		notempty :
		  An  empty string is not considered to be a valid match if this option is given.
		  If there are alternatives in the pattern, they are tried. If all  the  alterna-
		  tives  match the empty string, the entire match fails. For example, if the pat-
		  tern

		    a?b?

		  is applied to a string not beginning with "a" or "b", it would  normally  match
		  the  empty  string  at the start of the subject. With the notempty option, this
		  match is not valid, so re:run/3 searches further into  the  string  for  occur-
		  rences of "a" or "b".

		  Perl	has no direct equivalent of notempty , but it does make a special case of
		  a pattern match of the empty string within its split() function, and when using
		  the  /g  modifier.  It  is possible to emulate Perl's behavior after matching a
		  null string by first trying the match again at the same  offset  with  notempty
		  and  anchored  , and then, if that fails, by advancing the starting offset (see
		  below) and trying an ordinary match again.

		notbol :
		  This option specifies that the first character of the subject string is not the
		  beginning  of  a  line, so the circumflex metacharacter should not match before
		  it. Setting this without multiline (at compile time) causes circumflex never to
		  match.  This	option only affects the behavior of the circumflex metacharacter.
		  It does not affect \\A.

		noteol :
		  This option specifies that the end of the subject string is not the  end  of	a
		  line,  so the dollar metacharacter should not match it nor (except in multiline
		  mode) a newline immediately before it. Setting this without multiline (at  com-
		  pile	time) causes dollar never to match. This option affects only the behavior
		  of the dollar metacharacter. It does not affect \\Z or \\z.

		{offset, int()} :
		  Start matching at the offset (position) given in the subject string. The offset
		  is zero-based, so that the default is {offset,0} (all of the subject string).

		{newline, NLSpec} :
		  Override the default definition of a newline in the subject string, which is LF
		  (ASCII 10) in Erlang.

		  cr :
		    Newline is indicated by a single character CR (ASCII 13)

		  lf :
		    Newline is indicated by a single character LF (ASCII 10), the default

		  crlf :
		    Newline is indicated by the two-character CRLF (ASCII 13  followed	by  ASCII
		    10) sequence.

		  anycrlf :
		    Any of the three preceding sequences should be recognized.

		  any :
		    Any  of  the newline sequences above, plus the Unicode sequences VT (vertical
		    tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line  sepa-
		    rator, U+2028), and PS (paragraph separator, U+2029).

		bsr_anycrlf :
		  Specifies  specifically  that \R is to match only the cr, lf or crlf sequences,
		  not the Unicode specific newline characters. (overrides compilation option)

		bsr_unicode :
		  Specifies specifically that \R is to match all the Unicode  newline  characters
		  (including crlf etc, the default).(overrides compilation option)

		{capture, ValueSpec} / {capture, ValueSpec, Type} :
		  Specifies  which  captured  substrings  are  returned  and  in  what format. By
		  default, re:run/3 captures all of the matching part of the substring as well as
		  all  capturing  subpatterns (all of the pattern is automatically captured). The
		  default return type is (zero-based)  indexes	of  the  captured  parts  of  the
		  string, given as {Offset,Length} pairs (the index Type of capturing).

		  As an example of the default behavior, the following call:

		    re:run("ABCabcdABC","abcd",[]).

		  returns,  as	first  and  only captured string the matching part of the subject
		  ("abcd" in the middle) as a index pair {3,4} , where	character  positions  are
		  zero	based,	just as in offsets. The return value of the call above would then
		  be:

		    {match,[{3,4}]}

		  Another (and quite common) case is where the regular expression matches all  of
		  the subject, as in:

		    re:run("ABCabcdABC",".*abcd.*",[]).

		  where the return value correspondingly will point out all of the string, begin-
		  ning at index 0 and being 10 characters long:

		    {match,[{0,10}]}

		  If the regular expression contains capturing subpatterns, like in the following
		  case:

		    re:run("ABCabcdABC",".*(abcd).*",[]).

		  all of the matched subject is captured, as well as the captured substrings:

		    {match,[{0,10},{3,4}]}

		  the  complete matching pattern always giving the first return value in the list
		  and the rest of the subpatterns being added in the order they occurred  in  the
		  regular expression.

		  The capture tuple is built up as follows:

		  ValueSpec :
		    Specifies  which captured (sub)patterns are to be returned. The ValueSpec can
		    either be an atom describing a predefined set of return  values,  or  a  list
		    containing either the indexes or the names of specific subpatterns to return.

		    The predefined sets of subpatterns are:

		    all :
		      All  captured  subpatterns  including the complete matching string. This is
		      the default.

		    first :
		      Only the first captured subpattern, which is always the  complete  matching
		      part of the subject. All explicitly captured subpatterns are discarded.

		    all_but_first :
		      All but the first matching subpattern, i.e. all explicitly captured subpat-
		      terns, but not the complete matching part of the subject	string.  This  is
		      useful  if  the  regular	expression as a whole matches a large part of the
		      subject, but the part you're interested in is  in  an  explicitly  captured
		      subpattern.  If  the  return type is list or binary , not returning subpat-
		      terns you're not interested in is a good way to optimize.

		    none :
		      Do not return matching subpatterns at all, yielding the single  atom  match
		      as  the  return value of the function when matching successfully instead of
		      the {match, list()} return. Specifying an empty list gives the same  behav-
		      ior.

		    The  value	list  is  a  list of indexes for the subpatterns to return, where
		    index 0 is for all of the pattern, and 1 is for the first explicit	capturing
		    subpattern in the regular expression, and so forth. When using named captured
		    subpatterns (see below) in the regular expression, one can use  atom()  s  or
		    string()  s  to specify the subpatterns to be returned. For example, consider
		    the regular expression:

		      ".*(abcd).*"

		    matched against the string ""ABCabcdABC", capturing only the "abcd" part (the
		    first explicit subpattern):

		      re:run("ABCabcdABC",".*(abcd).*",[{capture,[1]}]).

		    The call will yield the following result:

		      {match,[{3,4}]}

		    as	the  first explicitly captured subpattern is "(abcd)", matching "abcd" in
		    the subject, at (zero-based) position 3, of length 4.

		    Now consider the same regular expression, but with the subpattern  explicitly
		    named 'FOO':

		      ".*(?<FOO>abcd).*"

		    With  this	expression,  we could still give the index of the subpattern with
		    the following call:

		      re:run("ABCabcdABC",".*(?<FOO>abcd).*",[{capture,[1]}]).

		    giving the same result as before. But, since the subpattern is named, we  can
		    also specify its name in the value list:

		      re:run("ABCabcdABC",".*(?<FOO>abcd).*",[{capture,['FOO']}]).

		    which would yield the same result as the earlier examples, namely:

		      {match,[{3,4}]}

		    The  values  list  might  specify indexes or names not present in the regular
		    expression, in which case the return values vary depending on  the	type.  If
		    the  type is index , the tuple {-1,0} is returned for values having no corre-
		    sponding subpattern in the regexp, but for the other types ( binary and  list
		    ), the values are the empty binary or list respectively.

		  Type :
		    Optionally	specifies how captured substrings are to be returned. If omitted,
		    the default of index is used. The Type can be one of the following:

		    index :
		      Return captured substrings as pairs of byte indexes into the subject string
		      and  length of the matching string in the subject (as if the subject string
		      was flattened  with  iolist_to_binary/1  or  unicode:characters_to_binary/2
		      prior  to  matching). Note that the unicode option results in byte-oriented
		      indexes in a (possibly virtual) UTF-8 encoded binary. A  byte  index  tuple
		      {0,2}  might  therefore  represent one or two characters when unicode is in
		      effect. This might seem counter-intuitive, but has  been	deemed	the  most
		      effective  and  useful  way  to way to do it. To return lists instead might
		      result in simpler code if that is desired. This return type is the default.

		    list :
		      Return matching substrings as lists of characters (Erlang string()  s).  It
		      the unicode option is used in combination with the \C sequence in the regu-
		      lar expression, a captured subpattern can contain bytes that are not  valid
		      UTF-8 (\C matches bytes regardless of character encoding). In that case the
		      list capturing may result in the same types of tuples that  unicode:charac-
		      ters_to_list/2  can  return, namely three-tuples with the tag incomplete or
		      error , the successfully converted characters and the invalid UTF-8 tail of
		      the  conversion  as  a  binary.  The best strategy is to avoid using the \C
		      sequence when capturing lists.

		    binary :
		      Return matching substrings as binaries. If  the  unicode	option	is  used,
		      these  binaries are in UTF-8. If the \C sequence is used together with uni-
		      code the binaries may be invalid UTF-8.

		  In general, subpatterns that were  not  assigned  a  value  in  the  match  are
		  returned  as	the  tuple {-1,0} when type is index . Unassigned subpatterns are
		  returned as the empty binary or list, respectively,  for  other  return  types.
		  Consider the regular expression:

		    ".*((?<FOO>abdd)|a(..d)).*"

		  There are three explicitly capturing subpatterns, where the opening parenthesis
		  position determines the order in the	result,  hence	((?<FOO>abdd)|a(..d))  is
		  subpattern  index 1, (?<FOO>abdd) is subpattern index 2 and (..d) is subpattern
		  index 3. When matched against the following string:

		    "ABCabcdABC"

		  the subpattern at index 2 won't match, as "abdd" is not present in the  string,
		  but  the  complete pattern matches (due to the alternative a(..d) . The subpat-
		  tern at index 2 is therefore unassigned and the default return value will be:

		    {match,[{0,10},{3,4},{-1,0},{4,3}]}

		  Setting the capture Type to binary would give the following:

		    {match,[<<"ABCabcdABC">>,<<"abcd">>,<<>>,<<"bcd">>]}

		  where the empty binary ( <<>> ) represents the unassigned  subpattern.  In  the
		  binary  case,  some  information about the matching is therefore lost, the <<>>
		  might just as well be an empty string captured.

		  If differentiation between empty matches and non existing subpatterns is neces-
		  sary,  use  the  type  index	and do the conversion to the final type in Erlang
		  code.

		  When the option global is given, the capture specification affects  each  match
		  separately, so that:

		    re:run("cacb","c(a|b)",[global,{capture,[1],list}]).

		  gives the result:

		    {match,[["a"],["b"]]}

	      The options solely affecting the compilation step are described in the re:compile/2
	      function.

       replace(Subject,RE,Replacement) -> iodata() | charlist()

	      Types  Subject = iodata() | charlist()
		     RE = mp() | iodata()
		     Replacement = iodata() | charlist()

	      The same as replace(Subject,RE,Replacement,[]) .

       replace(Subject,RE,Replacement,Options) -> iodata() | charlist() | binary() | list()

	      Types  Subject = iodata() | charlist()
		     RE = mp() | iodata() | charlist()
		     Replacement = iodata() | charlist()
		     Options = [ Option ]
		     Option = anchored | global | notbol | noteol | notempty | {offset, int()}	|
		     {newline,	NLSpec} | bsr_anycrlf | bsr_unicode | {return, ReturnType} | Com-
		     pileOpt
		     ReturnType = iodata | list | binary
		     CompileOpt = see compile/2 above
		     NLSpec = cr | crlf | lf | anycrlf | any

	      Replaces the matched part of the Subject string with the contents of Replacement .

	      The permissible options are the same as for re:run/3  ,  except  that  the  capture
	      option  is  not  allowed.  Instead  a  {return, ReturnType} is present. The default
	      return type is iodata , constructed in a way to minimize copying. The iodata result
	      can  be  used directly in many i/o-operations. If a flat list() is desired, specify
	      {return, list} and if a binary is preferred, specify {return, binary} .

	      As in the re:run/3 function, an mp() compiled with the unicode option requires  the
	      Subject to be a Unicode charlist() . If compilation is done implicitly and the uni-
	      code compilation option is given to this function, both the regular expression  and
	      the Subject should be given as valid Unicode charlist() s.

	      The  replacement	string	can  contain  the special character & , which inserts the
	      whole matching expression in the result, and the special sequence \ N (where  N  is
	      an  integer  >  0), resulting in the subexpression number N will be inserted in the
	      result. If no subexpression with that number is generated by  the  regular  expres-
	      sion, nothing is inserted.

	      To  insert  an & or \ in the result, precede it with a \ . Note that Erlang already
	      gives a special meaning to \ in literal strings, so a single \ has to be written as
	      "\\" and therefore a double \ as "\\\\" . Example:

		  re:replace("abcd","c","[&]",[{return,list}]).

	      gives

		  "ab[c]d"

	      while

		  re:replace("abcd","c","[\\&]",[{return,list}]).

	      gives

		  "ab[&]d"

	      As  with re:run/3 , compilation errors raise the badarg exception, re:compile/2 can
	      be used to get more information about the error.

       split(Subject,RE) -> SplitList

	      Types  Subject = iodata() | charlist()
		     RE = mp() | iodata()
		     SplitList = [ iodata() | charlist() ]

	      The same as split(Subject,RE,[]) .

       split(Subject,RE,Options) -> SplitList

	      Types  Subject = iodata() | charlist()
		     RE = mp() | iodata() | charlist()
		     Options = [ Option ]
		     Option = anchored | global | notbol | noteol | notempty | {offset, int()}	|
		     {newline,	NLSpec}  |  bsr_anycrlf  |  bsr_unicode  | {return, ReturnType} |
		     {parts, NumParts} | group | trim | CompileOpt
		     NumParts = int() | infinity
		     ReturnType = iodata | list | binary
		     CompileOpt = see compile/2 above
		     NLSpec = cr | crlf | lf | anycrlf | any
		     SplitList = [ RetData ] | [ GroupedRetData ]
		     GroupedRetData = [ RetData ]
		     RetData = iodata() | charlist() | binary() | list()

	      This function splits the input into parts by finding tokens according to the  regu-
	      lar expression supplied.

	      The  splitting  is done basically by running a global regexp match and dividing the
	      initial string wherever a match occurs. The matching part of the string is  removed
	      from the output.

	      As  in the re:run/3 function, an mp() compiled with the unicode option requires the
	      Subject to be a Unicode charlist() . If compilation is done implicitly and the uni-
	      code  compilation option is given to this function, both the regular expression and
	      the Subject should be given as valid Unicode charlist() s.

	      The result is given as a list of "strings", the preferred  datatype  given  in  the
	      return option (default iodata).

	      If  subexpressions are given in the regular expression, the matching subexpressions
	      are returned in the resulting list as well. An example:

		  re:split("Erlang","[ln]",[{return,list}]).

	      will yield the result:

		  ["Er","a","g"]

	      while

		  re:split("Erlang","([ln])",[{return,list}]).

	      will yield

		  ["Er","l","a","n","g"]

	      The text matching the subexpression (marked by the parentheses in  the  regexp)  is
	      inserted	in the result list where it was found. In effect this means that concate-
	      nating the result of a split where the whole regexp is a single  subexpression  (as
	      in the example above) will always result in the original string.

	      As  there  is no matching subexpression for the last part in the example (the "g"),
	      there is nothing inserted after that. To make the group of strings  and  the  parts
	      matching	the  subexpressions  more  obvious, one might use the group option, which
	      groups together the part of the subject string with the parts matching  the  subex-
	      pressions when the string was split:

		  re:split("Erlang","([ln])",[{return,list},group]).

	      gives:

		  [["Er","l"],["a","n"],["g"]]

	      Here  the  regular  expression  matched first the "l", causing "Er" to be the first
	      part in the result. When the regular expression matched, the  (only)  subexpression
	      was  bound  to the "l", so the "l" is inserted in the group together with "Er". The
	      next match is of the "n", making "a" the next part to be returned. Since the subex-
	      pression	is bound to the substring "n" in this case, the "n" is inserted into this
	      group. The last group consists of the rest of the string, as no  more  matches  are
	      found.

	      By default, all parts of the string, including the empty strings, are returned from
	      the function. For example:

		  re:split("Erlang","[lg]",[{return,list}]).

	      will return:

		  ["Er","an",[]]

	      since the matching of the "g" in the end of the string leaves an empty  rest  which
	      is  also	returned.  This behaviour differs from the default behaviour of the split
	      function in Perl, where empty strings at the end are by default removed. To get the
	      "trimming" default behavior of Perl, specify trim as an option:

		  re:split("Erlang","[lg]",[{return,list},trim]).

	      The result will be:

		  ["Er","an"]

	      The  "trim"  option  in  effect says; "give me as many parts as possible except the
	      empty ones", which might be useful in some circumstances. You can also specify  how
	      many parts you want, by specifying {parts, N } :

		  re:split("Erlang","[lg]",[{return,list},{parts,2}]).

	      This will give:

		  ["Er","ang"]

	      Note that the last part is "ang", not "an", as we only specified splitting into two
	      parts, and the splitting stops when enough parts are given, which is why the result
	      differs from that of trim .

	      More than three parts are not possible with this indata, so

		  re:split("Erlang","[lg]",[{return,list},{parts,4}]).

	      will  give  the  same  result as the default, which is to be viewed as "an infinite
	      number of parts".

	      Specifying 0 as the number of parts gives the same effect as the option trim  .  If
	      subexpressions  are  captured,  empty  subexpression  matches  at  the end are also
	      stripped from the result if trim or {parts,0} is specified.

	      If you are familiar with Perl, the trim behaviour corresponds exactly to	the  Perl
	      default,	the  {parts,N}	where  N is a positive integer corresponds exactly to the
	      Perl behaviour with a positive numerical third parameter and the default	behaviour
	      of re:split/3 corresponds to that when the Perl routine is given a negative integer
	      as the third parameter.

	      Summary of options not previously described for the re:run/3 function:

		{return,ReturnType} :
		  Specifies how the parts of the original string  are  presented  in  the  result
		  list. The possible types are:

		  iodata :
		    The variant of iodata() that gives the least copying of data with the current
		    implementation (often a binary, but don't depend on it).

		  binary :
		    All parts returned as binaries.

		  list :
		    All parts returned as lists of characters ("strings").

		group :
		  Groups together the part of the string with the parts of  the  string  matching
		  the subexpressions of the regexp.

		  The  return  value from the function will in this case be a list() of list() s.
		  Each sublist begins with the string picked out of the subject string,  followed
		  by  the parts matching each of the subexpressions in order of occurrence in the
		  regular expression.

		{parts,N} :
		  Specifies the number of parts the subject string is to be split into.

		  The number of parts should be a positive integer for a specific maximum on  the
		  number  of  parts  and  infinity  for the maximum number of parts possible (the
		  default). Specifying {parts,0} gives as many	parts  as  possible  disregarding
		  empty parts at the end, the same as specifying trim

		trim :
		  Specifies that empty parts at the end of the result list are to be disregarded.
		  The same as specifying {parts,0} . This corresponds to the default behaviour of
		  the split built in function in Perl.

PERL LIKE REGULAR EXPRESSIONS SYNTAX
       The following sections contain reference material for the regular expressions used by this
       module. The regular expression reference is based on the PCRE documentation, with  changes
       in cases where the re module behaves differently to the PCRE library.

PCRE REGULAR EXPRESSION DETAILS
       The  syntax  and  semantics  of	the  regular  expressions  that are supported by PCRE are
       described in detail below. Perl's regular expressions are described in its own  documenta-
       tion,  and  regular expressions in general are covered in a number of books, some of which
       have copious examples. Jeffrey Friedl's	"Mastering  Regular  Expressions",  published  by
       O'Reilly,  covers  regular expressions in great detail. This description of PCRE's regular
       expressions is intended as reference material.

       The reference material is divided into the following sections:

	 * Newline conventions

	 * Characters and metacharacters

	 * Backslash

	 * Circumflex and dollar

	 * Full stop (period, dot)

	 * Matching a single byte

	 * Square brackets and character classes

	 * POSIX character classes

	 * Vertical bar

	 * Internal option setting

	 * Subpatterns

	 * Duplicate subpattern numbers

	 * Named subpatterns

	 * Repetition

	 * Atomic grouping and possessive quantifiers

	 * Back references

	 * Assertions

	 * Conditional subpatterns

	 * Comments

	 * Recursive patterns

	 * Subpatterns as subroutines

	 * Backtracking control

NEWLINE CONVENTIONS
       PCRE supports five different conventions for indicating line breaks in strings:	a  single
       CR  (carriage  return)  character,  a  single  LF  (linefeed) character, the two-character
       sequence CRLF , any of the three preceding, or any Unicode newline sequence.

       It is also possible to specify a newline convention by starting a pattern string with  one
       of the following five sequences:

	 (*CR) :
	   carriage return

	 (*LF) :
	   linefeed

	 (*CRLF) :
	   carriage return, followed by linefeed

	 (*ANYCRLF) :
	   any of the three above

	 (*ANY) :
	   all Unicode newline sequences

       These  override	the default and the options given to re:compile/2 . For example, the pat-
       tern:

       (*CR)a.b

       changes the convention to CR. That pattern matches "a\nb" because LF is no longer  a  new-
       line. Note that these special settings, which are not Perl-compatible, are recognized only
       at the very start of a pattern, and that they must be in upper case. If more than  one  of
       them is present, the last one is used.

       The  newline  convention  does not affect what the \R escape sequence matches. By default,
       this is any Unicode newline  sequence,  for  Perl  compatibility.  However,  this  can  be
       changed;  see  the  description of \R in the section entitled "Newline sequences" below. A
       change of \R setting can be combined with a change of newline convention.

CHARACTERS AND METACHARACTERS
       A regular expression is a pattern that is matched against a subject string  from  left  to
       right.  Most  characters  stand	for  themselves in a pattern, and match the corresponding
       characters in the subject. As a trivial example, the pattern

       The quick brown fox

       matches a portion of a subject string that is identical to itself. When caseless  matching
       is specified (the caseless option), letters are matched independently of case.

       The  power of regular expressions comes from the ability to include alternatives and repe-
       titions in the pattern. These are encoded in the pattern by the use  of	metacharacters	,
       which do not stand for themselves but instead are interpreted in some special way.

       There  are two different sets of metacharacters: those that are recognized anywhere in the
       pattern except within square brackets, and those that are recognized within square  brack-
       ets. Outside square brackets, the metacharacters are as follows:

	 \ :
	   general escape character with several uses

	 ^ :
	   assert start of string (or line, in multiline mode)

	 $ :
	   assert end of string (or line, in multiline mode)

	 . :
	   match any character except newline (by default)

	 [ :
	   start character class definition

	 | :
	   start of alternative branch

	 ( :
	   start subpattern

	 ) :
	   end subpattern

	 ? :
	   extends the meaning of (, also 0 or 1 quantifier, also quantifier minimizer

	 * :
	   0 or more quantifier

	 + :
	   1 or more quantifier, also "possessive quantifier"

	 { :
	   start min/max quantifier

       Part of a pattern that is in square brackets is called a "character class". In a character
       class the only metacharacters are:

	 \ :
	   general escape character

	 ^ :
	   negate the class, but only if the first character

	 - :
	   indicates character range

	 [ :
	   POSIX character class (only if followed by POSIX syntax)

	 ] :
	   terminates the character class

       The following sections describe the use of each of the metacharacters.

BACKSLASH
       The backslash character has several uses. Firstly, if it is followed by a non-alphanumeric
       character,  it  takes  away any special meaning that character may have. This use of back-
       slash as an escape character applies both inside and outside character classes.

       For example, if you want to match a * character, you write \* in the pattern. This  escap-
       ing  action  applies whether or not the following character would otherwise be interpreted
       as a metacharacter, so it is always safe to precede a non-alphanumeric with  backslash  to
       specify	that  it  stands for itself. In particular, if you want to match a backslash, you
       write \\.

       If a pattern is compiled with the extended option, whitespace in the pattern  (other  than
       in  a  character  class) and characters between a # outside a character class and the next
       newline are ignored. An escaping backslash can be used to include a whitespace or #  char-
       acter as part of the pattern.

       If  you want to remove the special meaning from a sequence of characters, you can do so by
       putting them between \Q and \E. This is different from Perl in that $ and @ are handled as
       literals  in \Q...\E sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
       tion. Note the following examples:

	 Pattern	   PCRE matches   Perl matches

	 \Qabc$xyz\E	   abc$xyz	  abc followed by the contents of $xyz
	 \Qabc\$xyz\E	   abc\$xyz	  abc\$xyz
	 \Qabc\E\$\Qxyz\E  abc$xyz	  abc$xyz

       The \Q...\E sequence is recognized both inside and outside character classes.

       Non-printing characters

       A second use of backslash provides a way of encoding non-printing characters  in  patterns
       in a visible manner. There is no restriction on the appearance of non-printing characters,
       apart from the binary zero that terminates a pattern, but when a pattern is being prepared
       by  text  editing,  it is usually easier to use one of the following escape sequences than
       the binary character it represents:

	 \a :
	   alarm, that is, the BEL character (hex 07)

	 \cx :
	   "control-x", where x is any character

	 \e :
	   escape (hex 1B)

	 \f :
	   formfeed (hex 0C)

	 \n :
	   linefeed (hex 0A)

	 \r :
	   carriage return (hex 0D)

	 \t :
	   tab (hex 09)

	 \ddd :
	   character with octal code ddd, or backreference

	 \xhh :
	   character with hex code hh

	 \x{hhh..} :
	   character with hex code hhh..

       The precise effect of \cx is as follows: if x is a lower case letter, it is  converted  to
       upper case. Then bit 6 of the character (hex 40) is inverted. Thus \cz becomes hex 1A, but
       \c{ becomes hex 3B, while \c; becomes hex 7B.

       After \x, from zero to two hexadecimal digits are read (letters can be in upper	or  lower
       case). Any number of hexadecimal digits may appear between \x{ and }, but the value of the
       character code must be less than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode.
       That  is,  the maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger than the
       largest Unicode code point, which is 10FFFF.

       If characters other than hexadecimal digits appear between \x{ and }, or if  there  is  no
       terminating  },	this  form  of	escape is not recognized. Instead, the initial \x will be
       interpreted as a basic hexadecimal escape, with no following digits,  giving  a	character
       whose value is zero.

       Characters  whose  value is less than 256 can be defined by either of the two syntaxes for
       \x. There is no difference in the way they are handled. For example, \xdc is  exactly  the
       same as \x{dc}.

       After \0 up to two further octal digits are read. If there are fewer than two digits, just
       those that are present are used. Thus the sequence \0\x\07 specifies two binary zeros fol-
       lowed by a BEL character (code value 7). Make sure you supply two digits after the initial
       zero if the pattern character that follows is itself an octal digit.

       The handling of a backslash followed by a digit other than 0  is  complicated.  Outside	a
       character class, PCRE reads it and any following digits as a decimal number. If the number
       is less than 10, or if there have been at least that many previous capturing  left  paren-
       theses in the expression, the entire sequence is taken as a back reference . A description
       of how this works is given later, following the discussion of parenthesized subpatterns.

       Inside a character class, or if the decimal number is greater than 9 and  there	have  not
       been that many capturing subpatterns, PCRE re-reads up to three octal digits following the
       backslash, and uses them to generate a data character. Any  subsequent  digits  stand  for
       themselves.  The  value	of a character specified in octal must be less than \400. In non-
       UTF-8 mode, the value of a character specified in octal must be less than \400.	In  UTF-8
       mode, values up to \777 are permitted. For example:

	 \040 :
	   is another way of writing a space

	 \40 :
	   is the same, provided there are fewer than 40 previous capturing subpatterns

	 \7 :
	   is always a back reference

	 \11 :
	   might be a back reference, or another way of writing a tab

	 \011 :
	   is always a tab

	 \0113 :
	   is a tab followed by the character "3"

	 \113 :
	   might be a back reference, otherwise the character with octal code 113

	 \377 :
	   might be a back reference, otherwise the byte consisting entirely of 1 bits

	 \81 :
	   is  either  a  back reference, or a binary zero followed by the two characters "8" and
	   "1"

       Note that octal values of 100 or greater must not be introduced by a leading zero, because
       no more than three octal digits are ever read.

       All the sequences that define a single character value can be used both inside and outside
       character classes. In addition, inside a character class, the sequence \b  is  interpreted
       as  the	backspace  character (hex 08), and the sequences \R and \X are interpreted as the
       characters "R" and "X", respectively. Outside a character class, these sequences have dif-
       ferent meanings (see below).

       Absolute and relative back references

       The  sequence  \g  followed  by	an  unsigned or a negative number, optionally enclosed in
       braces, is an absolute or relative back reference. A named back reference can be coded  as
       \g{name}.  Back	references are discussed later, following the discussion of parenthesized
       subpatterns.

       Generic character types

       Another use of backslash is for specifying generic  character  types.  The  following  are
       always recognized:

	 \d :
	   any decimal digit

	 \D :
	   any character that is not a decimal digit

	 \h :
	   any horizontal whitespace character

	 \H :
	   any character that is not a horizontal whitespace character

	 \s :
	   any whitespace character

	 \S :
	   any character that is not a whitespace character

	 \v :
	   any vertical whitespace character

	 \V :
	   any character that is not a vertical whitespace character

	 \w :
	   any "word" character

	 \W :
	   any "non-word" character

       Each  pair of escape sequences partitions the complete set of characters into two disjoint
       sets. Any given character matches one, and only one, of each pair.

       These character type sequences can appear both inside and outside character classes.  They
       each  match one character of the appropriate type. If the current matching point is at the
       end of the subject string, all of them fail, since there is no character to match.

       For compatibility with Perl, \s does not match the VT character (code 11). This	makes  it
       different from the POSIX "space" class. The \s characters are HT(9), LF(10), FF(12), CR(13), and space(32). If "use locale;" is included in a Perl script, \s may match  the  VT
       character. In PCRE, it never does.

       In  UTF-8  mode,  characters  with  values greater than 128 never match \d, \s, or \w, and
       always match \D, \S, and \W. This is true even when Unicode character property support  is
       available.  These  sequences  retain their original meanings from before UTF-8 support was
       available, mainly for efficiency reasons.

       The sequences \h, \H, \v, and \V  are  Perl  5.10  features.  In  contrast  to  the  other
       sequences,  these  do  match  certain high-valued codepoints in UTF-8 mode. The horizontal
       space characters are:

	 U+0009 :
	   Horizontal tab

	 U+0020 :
	   Space

	 U+00A0 :
	   Non-break space

	 U+1680 :
	   Ogham space mark

	 U+180E :
	   Mongolian vowel separator

	 U+2000 :
	   En quad

	 U+2001 :
	   Em quad

	 U+2002 :
	   En space

	 U+2003 :
	   Em space

	 U+2004 :
	   Three-per-em space

	 U+2005 :
	   Four-per-em space

	 U+2006 :
	   Six-per-em space

	 U+2007 :
	   Figure space

	 U+2008 :
	   Punctuation space

	 U+2009 :
	   Thin space

	 U+200A :
	   Hair space

	 U+202F :
	   Narrow no-break space

	 U+205F :
	   Medium mathematical space

	 U+3000 :
	   Ideographic space

       The vertical space characters are:

	 U+000A :
	   Linefeed

	 U+000B :
	   Vertical tab

	 U+000C :
	   Formfeed

	 U+000D :
	   Carriage return

	 U+0085 :
	   Next line

	 U+2028 :
	   Line separator

	 U+2029 :
	   Paragraph separator

       A "word" character is an underscore or any character less than 256 that	is  a  letter  or
       digit.  The  definition of letters and digits is controlled by PCRE's low-valued character
       tables, which are always ISO-8859-1.

       Newline sequences

       Outside a character class, by default, the escape sequence \R matches any Unicode  newline
       sequence.  This	is a Perl 5.10 feature. In non-UTF-8 mode \R is equivalent to the follow-
       ing:

       (?>\r\n|\n|\x0b|\f|\r|\x85)

       This is an example of an "atomic group", details of which are given below.

       This particular group matches either the two-character sequence CR followed by LF, or  one
       of  the	single characters LF (linefeed, U+000A), VT (vertical tab, U+000B), FF (formfeed,
       U+000C), CR (carriage return, U+000D), or  NEL  (next  line,  U+0085).  The  two-character
       sequence is treated as a single unit that cannot be split.

       In  UTF-8 mode, two additional characters whose codepoints are greater than 255 are added:
       LS (line separator, U+2028) and PS (paragraph separator, U+2029). Unicode character  prop-
       erty support is not needed for these characters to be recognized.

       It  is  possible to restrict \R to match only CR, LF, or CRLF (instead of the complete set
       of Unicode line endings) by setting the option bsr_anycrlf either at compile time or  when
       the  pattern  is matched. (BSR is an abbreviation for "backslash R".) This can be made the
       default when PCRE is built; if this is the case, the other behaviour can be requested  via
       the  bsr_unicode  option. It is also possible to specify these settings by starting a pat-
       tern string with one of the following sequences:

       (*BSR_ANYCRLF) CR, LF, or CRLF only (*BSR_UNICODE) any Unicode newline sequence

       These override the default and the options given to re:compile/2 , but they can	be  over-
       ridden  by  options  given  to  re:run/3 . Note that these special settings, which are not
       Perl-compatible, are recognized only at the very start of a pattern, and that they must be
       in upper case. If more than one of them is present, the last one is used. They can be com-
       bined with a change of newline convention, for example, a pattern can start with:

       (*ANY)(*BSR_ANYCRLF)

       Inside a character class, \R matches the letter "R".

       Unicode character properties

       When PCRE is built with	Unicode  character  property  support,	three  additional  escape
       sequences  that match characters with specific properties are available. When not in UTF-8
       mode, these sequences are of course limited to testing  characters  whose  codepoints  are
       less than 256, but they do work in this mode. The extra escape sequences are:

       \p{  xx } a character with the xx property \P{ xx } a character without the xx property \X
       an extended Unicode sequence

       The property names represented by xx above are limited to the Unicode  script  names,  the
       general	category  properties, and "Any", which matches any character (including newline).
       Other properties such as "InMusicalSymbols" are not currently supported by PCRE. Note that
       \P{Any} does not match any characters, so always causes a match failure.

       Sets  of  Unicode characters are defined as belonging to certain scripts. A character from
       one of these sets can be matched using a script name. For example:

       \p{Greek} \P{Han}

       Those that are not part of an identified script are lumped together as "Common". The  cur-
       rent list of scripts is:

	 * Arabic

	 * Armenian

	 * Balinese

	 * Bengali

	 * Bopomofo

	 * Braille

	 * Buginese

	 * Buhid

	 * Canadian_Aboriginal

	 * Cherokee

	 * Common

	 * Coptic

	 * Cuneiform

	 * Cypriot

	 * Cyrillic

	 * Deseret

	 * Devanagari

	 * Ethiopic

	 * Georgian

	 * Glagolitic

	 * Gothic

	 * Greek

	 * Gujarati

	 * Gurmukhi

	 * Han

	 * Hangul

	 * Hanunoo

	 * Hebrew

	 * Hiragana

	 * Inherited

	 * Kannada

	 * Katakana

	 * Kharoshthi

	 * Khmer

	 * Lao

	 * Latin

	 * Limbu

	 * Linear_B

	 * Malayalam

	 * Mongolian

	 * Myanmar

	 * New_Tai_Lue

	 * Nko

	 * Ogham

	 * Old_Italic

	 * Old_Persian

	 * Oriya

	 * Osmanya

	 * Phags_Pa

	 * Phoenician

	 * Runic

	 * Shavian

	 * Sinhala

	 * Syloti_Nagri

	 * Syriac

	 * Tagalog

	 * Tagbanwa

	 * Tai_Le

	 * Tamil

	 * Telugu

	 * Thaana

	 * Thai

	 * Tibetan

	 * Tifinagh

	 * Ugaritic

	 * Yi

       Each character has exactly one general category property, specified by a two-letter abbre-
       viation. For compatibility with Perl, negation can be specified by including a  circumflex
       between	the  opening  brace  and  the  property name. For example, \p{^Lu} is the same as
       \P{Lu}.

       If only one letter is specified with \p or \P, it includes all the general category  prop-
       erties  that  start  with that letter. In this case, in the absence of negation, the curly
       brackets in the escape sequence are optional; these two examples have the same effect:

	 * \p{L}

	 * \pL

       The following general category property codes are supported:

	 C :
	   Other

	 Cc :
	   Control

	 Cf :
	   Format

	 Cn :
	   Unassigned

	 Co :
	   Private use

	 Cs :
	   Surrogate

	 L :
	   Letter

	 Ll :
	   Lower case letter

	 Lm :
	   Modifier letter

	 Lo :
	   Other letter

	 Lt :
	   Title case letter

	 Lu :
	   Upper case letter

	 M :
	   Mark

	 Mc :
	   Spacing mark

	 Me :
	   Enclosing mark

	 Mn :
	   Non-spacing mark

	 N :
	   Number

	 Nd :
	   Decimal number

	 Nl :
	   Letter number

	 No :
	   Other number

	 P :
	   Punctuation

	 Pc :
	   Connector punctuation

	 Pd :
	   Dash punctuation

	 Pe :
	   Close punctuation

	 Pf :
	   Final punctuation

	 Pi :
	   Initial punctuation

	 Po :
	   Other punctuation

	 Ps :
	   Open punctuation

	 S :
	   Symbol

	 Sc :
	   Currency symbol

	 Sk :
	   Modifier symbol

	 Sm :
	   Mathematical symbol

	 So :
	   Other symbol

	 Z :
	   Separator

	 Zl :
	   Line separator

	 Zp :
	   Paragraph separator

	 Zs :
	   Space separator

       The special property L& is also supported: it matches a character that has the Lu, Ll,  or
       Lt property, in other words, a letter that is not classified as a modifier or "other".

       The Cs (Surrogate) property applies only to characters in the range U+D800 to U+DFFF. Such
       characters are not valid in UTF-8 strings (see RFC 3629) and so cannot be tested by  PCRE,
       unless UTF-8 validity checking has been turned off (see the discussion of no_utf8_check in
       the pcreapi page).

       The long synonyms for these properties that Perl supports (such	as  \p{Letter})  are  not
       supported by PCRE, nor is it permitted to prefix any of these properties with "Is".

       No  character that is in the Unicode table has the Cn (unassigned) property. Instead, this
       property is assumed for any code point that is not in the Unicode table.

       Specifying caseless matching does not affect these escape sequences. For  example,  \p{Lu}
       always matches only upper case letters.

       The  \X	escape	matches  any  number  of Unicode characters that form an extended Unicode
       sequence. \X is equivalent to

       (?>\PM\pM*)

       That is, it matches a character without the "mark" property,  followed  by  zero  or  more
       characters  with  the  "mark"  property,  and  treats the sequence as an atomic group (see
       below). Characters with the "mark" property are typically accents that affect the  preced-
       ing character. None of them have codepoints less than 256, so in non-UTF-8 mode \X matches
       any one character.

       Matching characters by Unicode property is not fast, because PCRE has to search	a  struc-
       ture  that contains data for over fifteen thousand characters. That is why the traditional
       escape sequences such as \d and \w do not use Unicode properties in PCRE.

       Resetting the match start

       The escape sequence \K, which is a Perl 5.10 feature, causes any previously matched  char-
       acters not to be included in the final matched sequence. For example, the pattern:

       foo\Kbar

       matches	"foobar",  but	reports  that  it has matched "bar". This feature is similar to a
       lookbehind assertion (described below). However, in this case, the  part  of  the  subject
       before  the  real  match does not have to be of fixed length, as lookbehind assertions do.
       The use of \K does not interfere with the setting of  captured  substrings.  For  example,
       when the pattern

       (foo)\Kbar

       matches "foobar", the first substring is still set to "foo".

       Simple assertions

       The final use of backslash is for certain simple assertions. An assertion specifies a con-
       dition that has to be met at a particular point in a match, without consuming any  charac-
       ters  from  the	subject string. The use of subpatterns for more complicated assertions is
       described below. The backslashed assertions are:

	 \b :
	   matches at a word boundary

	 \B :
	   matches when not at a word boundary

	 \A :
	   matches at the start of the subject

	 \Z :
	   matches at the end of the subject also matches before a newline at the end of the sub-
	   ject

	 \z :
	   matches only at the end of the subject

	 \G :
	   matches at the first matching position in the subject

       These  assertions  may  not  appear in character classes (but note that \b has a different
       meaning, namely the backspace character, inside a character class).

       A word boundary is a position in the subject string where the current  character  and  the
       previous  character  do not both match \w or \W (i.e. one matches \w and the other matches
       \W), or the start or end of the string if the first or last character matches \w,  respec-
       tively.

       The \A, \Z, and \z assertions differ from the traditional circumflex and dollar (described
       in the next section) in that they only ever match at the very start and end of the subject
       string,	whatever  options  are	set.  Thus, they are independent of multiline mode. These
       three assertions are not affected by the notbol or noteol options, which affect	only  the
       behaviour  of  the circumflex and dollar metacharacters. However, if the startoffset argu-
       ment of re:run/3 is non-zero, indicating that matching is to start at a point  other  than
       the beginning of the subject, \A can never match. The difference between \Z and \z is that
       \Z matches before a newline at the end of the string as well as at the very  end,  whereas
       \z matches only at the end.

       The  \G assertion is true only when the current matching position is at the start point of
       the match, as specified by the startoffset argument of re:run/3 . It differs from \A  when
       the  value of startoffset is non-zero. By calling re:run/3 multiple times with appropriate
       arguments, you can mimic Perl's /g option, and it is in this kind of implementation  where
       \G can be useful.

       Note,  however,	that  PCRE's  interpretation of \G, as the start of the current match, is
       subtly different from Perl's, which defines it as the end of the previous match. In  Perl,
       these  can  be  different  when the previously matched string was empty. Because PCRE does
       just one match at a time, it cannot reproduce this behaviour.

       If all the alternatives of a pattern begin with \G, the	expression  is	anchored  to  the
       starting  match	position,  and the "anchored" flag is set in the compiled regular expres-
       sion.

CIRCUMFLEX AND DOLLAR
       Outside a character class, in the default matching mode, the circumflex	character  is  an
       assertion  that	is true only if the current matching point is at the start of the subject
       string. If the startoffset argument of re:run/3 is non-zero, circumflex can never match if
       the  multiline  option is unset. Inside a character class, circumflex has an entirely dif-
       ferent meaning (see below).

       Circumflex need not be the first character of the pattern if a number of alternatives  are
       involved,  but it should be the first thing in each alternative in which it appears if the
       pattern is ever to match that branch. If all possible alternatives start  with  a  circum-
       flex, that is, if the pattern is constrained to match only at the start of the subject, it
       is said to be an "anchored" pattern. (There are also other constructs  that  can  cause	a
       pattern to be anchored.)

       A  dollar  character is an assertion that is true only if the current matching point is at
       the end of the subject string, or immediately before a newline at the end  of  the  string
       (by default). Dollar need not be the last character of the pattern if a number of alterna-
       tives are involved, but it should be the last item in any branch in which it appears. Dol-
       lar has no special meaning in a character class.

       The  meaning  of  dollar  can  be  changed  so that it matches only at the very end of the
       string, by setting the dollar_endonly option at compile time. This does not affect the  \Z
       assertion.

       The  meanings  of the circumflex and dollar characters are changed if the multiline option
       is set. When this is the case, a circumflex matches immediately after internal newlines as
       well  as  at  the start of the subject string. It does not match after a newline that ends
       the string. A dollar matches before any newlines in the string, as well	as  at	the  very
       end,  when multiline is set. When newline is specified as the two-character sequence CRLF,
       isolated CR and LF characters do not indicate newlines.

       For example, the pattern /^abc$/ matches the subject string "def\nabc"  (where  \n  repre-
       sents  a  newline)  in  multiline mode, but not otherwise. Consequently, patterns that are
       anchored in single line mode because all branches start with ^ are not anchored in  multi-
       line  mode,  and  a  match  for	circumflex  is	possible when the startoffset argument of
       re:run/3 is non-zero. The dollar_endonly option is ignored if multiline is set.

       Note that the sequences \A, \Z, and \z can be used to match the start and end of the  sub-
       ject  in both modes, and if all branches of a pattern start with \A it is always anchored,
       whether or not multiline is set.

FULL STOP (PERIOD, DOT)
       Outside a character class, a dot in the pattern matches any one character in  the  subject
       string  except  (by  default) a character that signifies the end of a line. In UTF-8 mode,
       the matched character may be more than one byte long.

       When a line ending is defined as a single character, dot  never	matches  that  character;
       when  the  two-character sequence CRLF is used, dot does not match CR if it is immediately
       followed by LF, but otherwise it matches all characters (including isolated CRs and  LFs).
       When  any Unicode line endings are being recognized, dot does not match CR or LF or any of
       the other line ending characters.

       The behaviour of dot with regard to newlines can be changed. If the dotall option is  set,
       a  dot matches any one character, without exception. If the two-character sequence CRLF is
       present in the subject string, it takes two dots to match it.

       The handling of dot is entirely independent of the handling of circumflex and dollar,  the
       only  relationship  being that they both involve newlines. Dot has no special meaning in a
       character class.

MATCHING A SINGLE BYTE
       Outside a character class, the escape sequence \C matches any one byte, both in and out of
       UTF-8  mode.  Unlike  a	dot, it always matches any line-ending characters. The feature is
       provided in Perl in order to match individual bytes in UTF-8 mode. Because  it  breaks  up
       UTF-8  characters  into	individual  bytes,  what remains in the string may be a malformed
       UTF-8 string. For this reason, the \C escape sequence is best avoided.

       PCRE does not allow \C to appear in lookbehind assertions (described  below),  because  in
       UTF-8 mode this would make it impossible to calculate the length of the lookbehind.

SQUARE BRACKETS AND CHARACTER CLASSES
       An  opening  square  bracket  introduces a character class, terminated by a closing square
       bracket. A closing square bracket on its own is not special. If a closing  square  bracket
       is  required  as a member of the class, it should be the first data character in the class
       (after an initial circumflex, if present) or escaped with a backslash.

       A character class matches a single character in the subject. In UTF-8 mode, the	character
       may  occupy  more  than	one  byte.  A  matched character must be in the set of characters
       defined by the class, unless the first character in the class definition is a  circumflex,
       in which case the subject character must not be in the set defined by the class. If a cir-
       cumflex is actually required as a member of the class, ensure it is not the first  charac-
       ter, or escape it with a backslash.

       For  example,  the  character  class  [aeiou] matches any lower case vowel, while [^aeiou]
       matches any character that is not a lower case vowel. Note that a  circumflex  is  just	a
       convenient  notation  for  specifying  the characters that are in the class by enumerating
       those that are not. A class that starts with a circumflex is not an  assertion:	it  still
       consumes  a  character  from  the  subject  string,  and therefore it fails if the current
       pointer is at the end of the string.

       In UTF-8 mode, characters with values greater than 255 can be included in  a  class  as	a
       literal string of bytes, or by using the \x{ escaping mechanism.

       When  caseless matching is set, any letters in a class represent both their upper case and
       lower case versions, so for example, a caseless [aeiou] matches "A" as well as "a", and	a
       caseless [^aeiou] does not match "A", whereas a caseful version would. In UTF-8 mode, PCRE
       always understands the concept of case for characters whose values are less than  128,  so
       caseless  matching  is  always possible. For characters with higher values, the concept of
       case is supported if PCRE is compiled with Unicode property support, but not otherwise. If
       you  want to use caseless matching for characters 128 and above, you must ensure that PCRE
       is compiled with Unicode property support as well as with UTF-8 support.

       Characters that might indicate line breaks are never  treated  in  any  special	way  when
       matching  character classes, whatever line-ending sequence is in use, and whatever setting
       of the dotall and multiline options is used. A class such as [^a] always  matches  one  of
       these characters.

       The  minus  (hyphen) character can be used to specify a range of characters in a character
       class. For example, [d-m] matches any letter between d and m, inclusive. If a minus  char-
       acter  is required in a class, it must be escaped with a backslash or appear in a position
       where it cannot be interpreted as indicating a range, typically as the first or last char-
       acter in the class.

       It  is  not  possible to have the literal character "]" as the end character of a range. A
       pattern such as [W-]46] is interpreted as a class of two characters ("W" and "-") followed
       by  a  literal  string  "46]",  so it would match "W46]" or "-46]". However, if the "]" is
       escaped with a backslash it is interpreted as the end of range, so [W-\]46] is interpreted
       as  a  class containing a range followed by two other characters. The octal or hexadecimal
       representation of "]" can also be used to end a range.

       Ranges operate in the collating sequence of character values. They can also  be	used  for
       characters  specified  numerically,  for  example  [\000-\037].	In UTF-8 mode, ranges can
       include characters whose values are greater than 255, for example [\x{100}-\x{2ff}].

       If a range that includes letters is used when caseless matching is  set,  it  matches  the
       letters	in  either  case.  For	example, [W-c] is equivalent to [][\\^_`wxyzabc], matched
       caselessly , and in non-UTF-8 mode, if character tables for a French locale  are  in  use,
       [\xc8-\xcb]  matches accented E characters in both cases. In UTF-8 mode, PCRE supports the
       concept of case for characters with values greater than 128 only when it is compiled  with
       Unicode property support.

       The  character  types  \d,  \D,	\p, \P, \s, \S, \w, and \W may also appear in a character
       class, and add the characters that they	match  to  the	class.	For  example,  [\dABCDEF]
       matches	any  hexadecimal digit. A circumflex can conveniently be used with the upper case
       character types to specify a more restricted set of characters  than  the  matching  lower
       case type. For example, the class [^\W_] matches any letter or digit, but not underscore.

       The  only  metacharacters  that	are recognized in character classes are backslash, hyphen
       (only where it can be interpreted as specifying a range), circumflex (only at the  start),
       opening	square bracket (only when it can be interpreted as introducing a POSIX class name
       - see the next section), and the terminating closing  square  bracket.  However,  escaping
       other non-alphanumeric characters does no harm.

POSIX CHARACTER CLASSES
       Perl supports the POSIX notation for character classes. This uses names enclosed by [: and
       :] within the enclosing square brackets. PCRE also supports this notation. For example,

       [01[:alpha:]%]

       matches "0", "1", any alphabetic character, or "%". The supported class names are

	 alnum :
	   letters and digits

	 alpha :
	   letters

	 ascii :
	   character codes 0 - 127

	 blank :
	   space or tab only

	 cntrl :
	   control characters

	 digit :
	   decimal digits (same as \d)

	 graph :
	   printing characters, excluding space

	 lower :
	   lower case letters

	 print :
	   printing characters, including space

	 punct :
	   printing characters, excluding letters and digits

	 space :
	   whitespace (not quite the same as \s)

	 upper :
	   upper case letters

	 word :
	   "word" characters (same as \w)

	 xdigit :
	   hexadecimal digits

       The "space" characters are HT(9), LF(10), VT(11), FF(12), CR(13),	and  space(32).
       Notice that this list includes the VT character (code 11). This makes "space" different to
       \s, which does not include VT (for Perl compatibility).

       The name "word" is a Perl extension, and "blank" is a GNU extension from Perl 5.8. Another
       Perl extension is negation, which is indicated by a ^ character after the colon. For exam-
       ple,

       [12[:^digit:]]

       matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX syntax [.ch.]
       and  [=ch=] where "ch" is a "collating element", but these are not supported, and an error
       is given if they are encountered.

       In UTF-8 mode, characters with values greater than 128 do not match any of the POSIX char-
       acter classes.

VERTICAL BAR
       Vertical  bar  characters are used to separate alternative patterns. For example, the pat-
       tern

       gilbert|sullivan

       matches either "gilbert" or "sullivan". Any number of  alternatives  may  appear,  and  an
       empty  alternative  is  permitted  (matching the empty string). The matching process tries
       each alternative in turn, from left to right, and the first one that succeeds is used.  If
       the  alternatives  are  within a subpattern (defined below), "succeeds" means matching the
       rest of the main pattern as well as the alternative in the subpattern.

INTERNAL OPTION SETTING
       The settings of the caseless , multiline , dotall , and extended options (which are  Perl-
       compatible)  can  be  changed from within the pattern by a sequence of Perl option letters
       enclosed between "(?" and ")". The option letters are

	 i :
	   for caseless

	 m :
	   for multiline

	 s :
	   for dotall

	 x :
	   for extended

       For example, (?im) sets caseless, multiline matching. It is also possible to  unset  these
       options	by  preceding the letter with a hyphen, and a combined setting and unsetting such
       as (?im-sx), which sets caseless and multiline while unsetting dotall and  extended  ,  is
       also permitted. If a letter appears both before and after the hyphen, the option is unset.

       The  PCRE-specific  options dupnames , ungreedy , and extra can be changed in the same way
       as the Perl-compatible options by using the characters J, U and X respectively.

       When an option change occurs at top level (that is, not	inside	subpattern  parentheses),
       the  change  applies to the remainder of the pattern that follows. If the change is placed
       right at the start of a pattern, PCRE extracts it into the global options

       An option change within a subpattern (see below for a description of subpatterns)  affects
       only that part of the current pattern that follows it, so

       (a(?i)b)c

       matches	abc  and aBc and no other strings (assuming caseless is not used). By this means,
       options can be made to have different settings in different  parts  of  the  pattern.  Any
       changes	made in one alternative do carry on into subsequent branches within the same sub-
       pattern. For example,

       (a(?i)b|c)

       matches "ab", "aB", "c", and "C", even though when matching "C" the first branch is  aban-
       doned  before the option setting. This is because the effects of option settings happen at
       compile time. There would be some very weird behaviour otherwise.

       Note: There are other PCRE-specific options that can be set by the  application	when  the
       compile or match functions are called. In some cases the pattern can contain special lead-
       ing sequences to override what the application has set or what has been defaulted. Details
       are given in the section entitled "Newline sequences" above.

SUBPATTERNS
       Subpatterns  are  delimited  by parentheses (round brackets), which can be nested. Turning
       part of a pattern into a subpattern does two things:

       1. It localizes a set of alternatives. For example, the pattern

       cat(aract|erpillar|)

       matches one of the words "cat", "cataract", or "caterpillar". Without the parentheses,  it
       would match "cataract", "erpillar" or an empty string.

       2. It sets up the subpattern as a capturing subpattern. This means that, when the complete
       pattern matches, that portion of the subject string that matched the subpattern is  passed
       back to the caller via the return value of re:run/3 . Opening parentheses are counted from
       left to right (starting from 1) to obtain numbers for the capturing subpatterns.

       For example, if the string "the red king" is matched against the pattern

       the ((red|white) (king|queen))

       the captured substrings are "red king", "red", and "king", and are numbered 1, 2,  and  3,
       respectively.

       The  fact  that	plain  parentheses  fulfil two functions is not always helpful. There are
       often times when a grouping subpattern is required without a capturing requirement. If  an
       opening parenthesis is followed by a question mark and a colon, the subpattern does not do
       any capturing, and is not counted when computing the number of  any  subsequent	capturing
       subpatterns. For example, if the string "the white queen" is matched against the pattern

       the ((?:red|white) (king|queen))

       the captured substrings are "white queen" and "queen", and are numbered 1 and 2. The maxi-
       mum number of capturing subpatterns is 65535.

       As a convenient shorthand, if any option settings are required at the start of a  non-cap-
       turing subpattern, the option letters may appear between the "?" and the ":". Thus the two
       patterns

	 * (?i:saturday|sunday)

	 * (?:(?i)saturday|sunday)

       match exactly the same set of strings. Because alternative branches are tried from left to
       right,  and  options  are  not reset until the end of the subpattern is reached, an option
       setting in one branch does affect subsequent branches, so the above patterns  match  "SUN-
       DAY" as well as "Saturday".

DUPLICATE SUBPATTERN NUMBERS
       Perl 5.10 introduced a feature whereby each alternative in a subpattern uses the same num-
       bers for its capturing parentheses. Such a subpattern starts with (?| and is itself a non-
       capturing subpattern. For example, consider this pattern:

       (?|(Sat)ur|(Sun))day

       Because	the  two  alternatives are inside a (?| group, both sets of capturing parentheses
       are numbered one. Thus, when the pattern matches, you can look at captured substring  num-
       ber  one, whichever alternative matched. This construct is useful when you want to capture
       part, but not all, of one of a number of alternatives. Inside a (?| group, parentheses are
       numbered as usual, but the number is reset at the start of each branch. The numbers of any
       capturing buffers that follow the subpattern start after the highest number  used  in  any
       branch. The following example is taken from the Perl documentation. The numbers underneath
       show in which buffer the captured content will be stored.

	 # before  ---------------branch-reset----------- after
	 / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
	 # 1		2	  2  3	      2     3	  4

       A backreference or a recursive call to a numbered subpattern always refers  to  the  first
       one in the pattern with the given number.

       An  alternative	approach  to  using this "branch reset" feature is to use duplicate named
       subpatterns, as described in the next section.

NAMED SUBPATTERNS
       Identifying capturing parentheses by number is simple, but it can be  very  hard  to  keep
       track  of the numbers in complicated regular expressions. Furthermore, if an expression is
       modified, the numbers may change. To help with this difficulty, PCRE supports  the  naming
       of subpatterns. This feature was not added to Perl until release 5.10. Python had the fea-
       ture earlier, and PCRE introduced it at release 4.0, using the  Python  syntax.	PCRE  now
       supports both the Perl and the Python syntax.

       In  PCRE,  a subpattern can be named in one of three ways: (?<name>...) or (?'name'...) as
       in Perl, or (?P<name>...) as in Python. References to  capturing  parentheses  from  other
       parts  of  the  pattern, such as backreferences, recursion, and conditions, can be made by
       name as well as by number.

       Names consist of up to 32 alphanumeric characters and underscores. Named capturing  paren-
       theses  are  still  allocated  numbers  as well as names, exactly as if the names were not
       present. The capture specification to re:run/3 can use named values if they are present in
       the regular expression.

       By  default, a name must be unique within a pattern, but it is possible to relax this con-
       straint by setting the dupnames option at compile time. This can be  useful  for  patterns
       where  only one instance of the named parentheses can match. Suppose you want to match the
       name of a weekday, either as a 3-letter abbreviation or as the  full  name,  and  in  both
       cases  you  want to extract the abbreviation. This pattern (ignoring the line breaks) does
       the job:

	 (?<DN>Mon|Fri|Sun)(?:day)?|
	 (?<DN>Tue)(?:sday)?|
	 (?<DN>Wed)(?:nesday)?|
	 (?<DN>Thu)(?:rsday)?|
	 (?<DN>Sat)(?:urday)?

       There are five capturing substrings, but only one is ever set after a match. (An  alterna-
       tive  way  of  solving this problem is to use a "branch reset" subpattern, as described in
       the previous section.)

       In case of capturing named subpatterns which are  not  unique,  the  first  occurrence  is
       returned  from  re:exec/3  ,  if  the name is specified int the values part of the capture
       statement.

REPETITION
       Repetition is specified by quantifiers, which can follow any of the following items:

	 * a literal data character

	 * the dot metacharacter

	 * the \C escape sequence

	 * the \X escape sequence (in UTF-8 mode with Unicode properties)

	 * the \R escape sequence

	 * an escape such as \d that matches a single character

	 * a character class

	 * a back reference (see next section)

	 * a parenthesized subpattern (unless it is an assertion)

       The general repetition quantifier specifies a minimum  and  maximum  number  of	permitted
       matches,  by  giving the two numbers in curly brackets (braces), separated by a comma. The
       numbers must be less than 65536, and the first must be less than or equal to  the  second.
       For example:

       z{2,4}

       matches	"zz", "zzz", or "zzzz". A closing brace on its own is not a special character. If
       the second number is omitted, but the comma is present, there is no upper  limit;  if  the
       second  number and the comma are both omitted, the quantifier specifies an exact number of
       required matches. Thus

       [aeiou]{3,}

       matches at least 3 successive vowels, but may match many more, while

       \d{8}

       matches exactly 8 digits. An opening curly bracket that appears	in  a  position  where	a
       quantifier is not allowed, or one that does not match the syntax of a quantifier, is taken
       as a literal character. For example, {,6} is not a quantifier, but  a  literal  string  of
       four characters.

       In  UTF-8  mode,  quantifiers  apply  to UTF-8 characters rather than to individual bytes.
       Thus, for example, \x{100}{2} matches two UTF-8 characters, each of which  is  represented
       by  a  two-byte	sequence.  Similarly,  when  Unicode property support is available, \X{3}
       matches three Unicode extended sequences, each of which may be  several	bytes  long  (and
       they may be of different lengths).

       The  quantifier {0} is permitted, causing the expression to behave as if the previous item
       and the quantifier were not present.

       For convenience, the three most common quantifiers have single-character abbreviations:

	 * :
	   is equivalent to {0,}

	 + :
	   is equivalent to {1,}

	 ? :
	   is equivalent to {0,1}

       It is possible to construct infinite loops by following a subpattern  that  can	match  no
       characters with a quantifier that has no upper limit, for example:

       (a?)*

       Earlier versions of Perl and PCRE used to give an error at compile time for such patterns.
       However, because there are cases where this can be useful, such patterns are now accepted,
       but  if	any  repetition  of  the subpattern does in fact match no characters, the loop is
       forcibly broken.

       By default, the quantifiers are "greedy", that is, they match as much as possible  (up  to
       the  maximum  number of permitted times), without causing the rest of the pattern to fail.
       The classic example of where this gives problems is in trying to match comments in C  pro-
       grams.  These  appear between /* and */ and within the comment, individual * and / charac-
       ters may appear. An attempt to match C comments by applying the pattern

       /\*.*\*/

       to the string

       /* first comment */ not comment /* second comment */

       fails, because it matches the entire string owing to the greediness of the .* item.

       However, if a quantifier is followed by a question mark,  it  ceases  to  be  greedy,  and
       instead matches the minimum number of times possible, so the pattern

       /\*.*?\*/

       does  the  right  thing with the C comments. The meaning of the various quantifiers is not
       otherwise changed, just the preferred number of matches. Do not confuse this use of  ques-
       tion  mark  with its use as a quantifier in its own right. Because it has two uses, it can
       sometimes appear doubled, as in

       \d??\d

       which matches one digit by preference, but can match two if that is the only way the  rest
       of the pattern matches.

       If  the	ungreedy option is set (an option that is not available in Perl), the quantifiers
       are not greedy by default, but individual ones can be made greedy by following them with a
       question mark. In other words, it inverts the default behaviour.

       When  a parenthesized subpattern is quantified with a minimum repeat count that is greater
       than 1 or with a limited maximum, more memory is required for  the  compiled  pattern,  in
       proportion to the size of the minimum or maximum.

       If  a  pattern  starts with .* or .{0,} and the dotall option (equivalent to Perl's /s) is
       set, thus allowing the dot to match newlines, the pattern is implicitly anchored,  because
       whatever  follows will be tried against every character position in the subject string, so
       there is no point in retrying the overall match at any position after the first. PCRE nor-
       mally treats such a pattern as though it were preceded by \A.

       In  cases where it is known that the subject string contains no newlines, it is worth set-
       ting dotall in order to obtain this optimization, or alternatively  using  ^  to  indicate
       anchoring explicitly.

       However,  there	is one situation where the optimization cannot be used. When .* is inside
       capturing parentheses that are the subject of a backreference elsewhere in the pattern,	a
       match at the start may fail where a later one succeeds. Consider, for example:

       (.*)abc\1

       If the subject is "xyz123abc123" the match point is the fourth character. For this reason,
       such a pattern is not implicitly anchored.

       When a capturing subpattern is repeated, the value captured is the substring that  matched
       the final iteration. For example, after

       (tweedle[dume]{3}\s*)+

       has  matched  "tweedledum tweedledee" the value of the captured substring is "tweedledee".
       However, if there are nested capturing subpatterns, the corresponding captured values  may
       have been set in previous iterations. For example, after

       /(a|(b))+/

       matches "aba" the value of the second captured substring is "b".

ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
       With  both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") repetition, failure
       of what follows normally causes the repeated item to be re-evaluated to see if a different
       number  of repeats allows the rest of the pattern to match. Sometimes it is useful to pre-
       vent this, either to change the nature of the match, or to cause it fail earlier  than  it
       otherwise might, when the author of the pattern knows there is no point in carrying on.

       Consider, for example, the pattern \d+foo when applied to the subject line

       123456bar

       After  matching	all  6	digits	and then failing to match "foo", the normal action of the
       matcher is to try again with only 5 digits matching the \d+ item, and then with 4, and  so
       on, before ultimately failing. "Atomic grouping" (a term taken from Jeffrey Friedl's book)
       provides the means for specifying that once a subpattern has matched, it is not to be  re-
       evaluated in this way.

       If  we  use  atomic grouping for the previous example, the matcher gives up immediately on
       failing to match "foo" the first time. The notation is  a  kind	of  special  parenthesis,
       starting with (?> as in this example:

       (?>\d+)foo

       This  kind  of  parenthesis  "locks  up"  the  part of the pattern it contains once it has
       matched, and a failure further into the pattern is prevented from  backtracking	into  it.
       Backtracking past it to previous items, however, works as normal.

       An alternative description is that a subpattern of this type matches the string of charac-
       ters that an identical standalone pattern would match, if anchored at the current point in
       the subject string.

       Atomic  grouping subpatterns are not capturing subpatterns. Simple cases such as the above
       example can be thought of as a maximizing repeat that must swallow everything it can.  So,
       while both \d+ and \d+? are prepared to adjust the number of digits they match in order to
       make the rest of the pattern match, (?>\d+) can only match an entire sequence of digits.

       Atomic groups in general can of course contain arbitrarily  complicated	subpatterns,  and
       can  be nested. However, when the subpattern for an atomic group is just a single repeated
       item, as in the example above, a simpler notation, called a "possessive quantifier" can be
       used.  This consists of an additional + character following a quantifier. Using this nota-
       tion, the previous example can be rewritten as

       \d++foo

       Note that a possessive quantifier can be used with an entire group, for example:

       (abc|xyz){2,3}+

       Possessive quantifiers are always greedy; the setting of the ungreedy option  is  ignored.
       They are a convenient notation for the simpler forms of atomic group. However, there is no
       difference in the meaning of a possessive quantifier  and  the  equivalent  atomic  group,
       though  there  may  be a performance difference; possessive quantifiers should be slightly
       faster.

       The possessive quantifier syntax is an extension to the Perl 5.8  syntax.  Jeffrey  Friedl
       originated  the idea (and the name) in the first edition of his book. Mike McCloskey liked
       it, so implemented it when he built Sun's Java package, and PCRE copied it from there.  It
       ultimately found its way into Perl at release 5.10.

       PCRE  has  an  optimization  that automatically "possessifies" certain simple pattern con-
       structs. For example, the sequence A+B is treated as A++B because there	is  no	point  in
       backtracking into a sequence of A's when B must follow.

       When  a	pattern  contains  an  unlimited  repeat  inside  a subpattern that can itself be
       repeated an unlimited number of times, the use of an atomic group is the only way to avoid
       some failing matches taking a very long time indeed. The pattern

       (\D+|<\d+>)*[!?]

       matches	an  unlimited  number  of substrings that either consist of non-digits, or digits
       enclosed in <>, followed by either ! or ?. When it matches, it runs quickly.  However,  if
       it is applied to

       aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

       it  takes  a long time before reporting failure. This is because the string can be divided
       between the internal \D+ repeat and the external * repeat in a large number of  ways,  and
       all  have  to  be tried. (The example uses [!?] rather than a single character at the end,
       because both PCRE and Perl have an optimization that allows for fast failure when a single
       character  is  used. They remember the last single character that is required for a match,
       and fail early if it is not present in the string.) If the pattern is changed so  that  it
       uses an atomic group, like this:

       ((?>\D+)|<\d+>)*[!?]

       sequences of non-digits cannot be broken, and failure happens quickly.

BACK REFERENCES
       Outside	a  character  class, a backslash followed by a digit greater than 0 (and possibly
       further digits) is a back reference to a capturing subpattern earlier  (that  is,  to  its
       left) in the pattern, provided there have been that many previous capturing left parenthe-
       ses.

       However, if the decimal number following the backslash is less than 10, it is always taken
       as  a  back  reference, and causes an error only if there are not that many capturing left
       parentheses in the entire pattern. In other words, the  parentheses  that  are  referenced
       need  not be to the left of the reference for numbers less than 10. A "forward back refer-
       ence" of this type can make sense when a repetition is involved and the subpattern to  the
       right has participated in an earlier iteration.

       It is not possible to have a numerical "forward back reference" to a subpattern whose num-
       ber is 10 or more using this syntax because a sequence such as \50  is  interpreted  as	a
       character  defined  in  octal. See the subsection entitled "Non-printing characters" above
       for further details of the handling of digits following a  backslash.  There  is  no  such
       problem	when  named  parentheses are used. A back reference to any subpattern is possible
       using named parentheses (see below).

       Another way of avoiding the ambiguity inherent in the use of digits following a	backslash
       is  to use the \g escape sequence, which is a feature introduced in Perl 5.10. This escape
       must be followed by an unsigned number  or  a  negative	number,  optionally  enclosed  in
       braces. These examples are all identical:

	 * (ring), \1

	 * (ring), \g1

	 * (ring), \g{1}

       An  unsigned  number specifies an absolute reference without the ambiguity that is present
       in the older syntax. It is also useful when literal digits follow the reference.  A  nega-
       tive number is a relative reference. Consider this example:

       (abc(def)ghi)\g{-1}

       The  sequence  \g{-1}  is  a  reference	to the most recently started capturing subpattern
       before \g, that is, is it equivalent to \2. Similarly, \g{-2} would be equivalent  to  \1.
       The  use of relative references can be helpful in long patterns, and also in patterns that
       are created by joining together fragments that contain references within themselves.

       A back reference matches whatever actually matched the capturing subpattern in the current
       subject	string,  rather than anything matching the subpattern itself (see "Subpatterns as
       subroutines" below for a way of doing that). So the pattern

       (sens|respons)e and \1ibility

       matches "sense and sensibility" and "response and  responsibility",  but  not  "sense  and
       responsibility".  If  caseful  matching is in force at the time of the back reference, the
       case of letters is relevant. For example,

       ((?i)rah)\s+\1

       matches "rah rah" and "RAH RAH", but not "RAH rah", even  though  the  original	capturing
       subpattern is matched caselessly.

       There are several different ways of writing back references to named subpatterns. The .NET
       syntax \k{name} and the Perl syntax \k<name> or \k'name' are supported, as is  the  Python
       syntax  (?P=name).  Perl 5.10's unified back reference syntax, in which \g can be used for
       both numeric and named references, is also supported. We could rewrite the  above  example
       in any of the following ways:

	 * (?<p1>(?i)rah)\s+\k<p1>

	 * (?'p1'(?i)rah)\s+\k{p1}

	 * (?P<p1>(?i)rah)\s+(?P=p1)

	 * (?<p1>(?i)rah)\s+\g{p1}

       A subpattern that is referenced by name may appear in the pattern before or after the ref-
       erence.

       There may be more than one back reference to the same subpattern. If a subpattern has  not
       actually been used in a particular match, any back references to it always fail. For exam-
       ple, the pattern

       (a|(bc))\2

       always fails if it starts to match "a" rather than "bc". Because there may be many captur-
       ing  parentheses  in  a pattern, all digits following the backslash are taken as part of a
       potential back reference number. If the pattern continues with  a  digit  character,  some
       delimiter  must	be  used  to terminate the back reference. If the extended option is set,
       this can be whitespace. Otherwise an empty comment (see "Comments" below) can be used.

       A back reference that occurs inside the parentheses to which it refers fails when the sub-
       pattern	is first used, so, for example, (a\1) never matches. However, such references can
       be useful inside repeated subpatterns. For example, the pattern

       (a|b\1)+

       matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of the subpat-
       tern, the back reference matches the character string corresponding to the previous itera-
       tion. In order for this to work, the pattern must be such that the  first  iteration  does
       not  need to match the back reference. This can be done using alternation, as in the exam-
       ple above, or by a quantifier with a minimum of zero.

ASSERTIONS
       An assertion is a test on the characters following or preceding the current matching point
       that  does not actually consume any characters. The simple assertions coded as \b, \B, \A,
       \G, \Z, \z, ^ and $ are described above.

       More complicated assertions are coded as subpatterns. There are two kinds: those that look
       ahead  of  the  current	position in the subject string, and those that look behind it. An
       assertion subpattern is matched in the normal way, except that it does not cause the  cur-
       rent matching position to be changed.

       Assertion  subpatterns  are not capturing subpatterns, and may not be repeated, because it
       makes no sense to assert the same thing several times. If any kind of  assertion  contains
       capturing  subpatterns within it, these are counted for the purposes of numbering the cap-
       turing subpatterns in the whole pattern. However, substring capturing is carried out  only
       for positive assertions, because it does not make sense for negative assertions.

       Lookahead assertions

       Lookahead  assertions  start  with (?= for positive assertions and (?! for negative asser-
       tions. For example,

       \w+(?=;)

       matches a word followed by a semicolon, but does not include the semicolon in  the  match,
       and

       foo(?!bar)

       matches	any  occurrence  of "foo" that is not followed by "bar". Note that the apparently
       similar pattern

       (?!foo)bar

       does not find an occurrence of "bar" that is preceded by something other  than  "foo";  it
       finds  any  occurrence  of  "bar" whatsoever, because the assertion (?!foo) is always true
       when the next three characters are "bar". A lookbehind assertion is needed to achieve  the
       other effect.

       If  you	want  to force a matching failure at some point in a pattern, the most convenient
       way to do it is with (?!) because an empty string always matches,  so  an  assertion  that
       requires there not to be an empty string must always fail.

       Lookbehind assertions

       Lookbehind assertions start with (?<= for positive assertions and (?<! for negative asser-
       tions. For example,

       (?<!foo)bar

       does find an occurrence of "bar" that is not preceded by "foo". The contents of a  lookbe-
       hind  assertion	are  restricted  such  that  all the strings it matches must have a fixed
       length. However, if there are several top-level alternatives, they do not all have to have
       the same fixed length. Thus

       (?<=bullock|donkey)

       is permitted, but

       (?<!dogs?|cats?)

       causes  an error at compile time. Branches that match different length strings are permit-
       ted only at the top level of a lookbehind assertion. This is an	extension  compared  with
       Perl  (at  least for 5.8), which requires all branches to match the same length of string.
       An assertion such as

       (?<=ab(c|de))

       is not permitted, because its single top-level branch can match two different lengths, but
       it is acceptable if rewritten to use two top-level branches:

       (?<=abc|abde)

       In some cases, the Perl 5.10 escape sequence \K (see above) can be used instead of a look-
       behind assertion; this is not restricted to a fixed-length.

       The implementation of lookbehind assertions is, for each alternative, to temporarily  move
       the current position back by the fixed length and then try to match. If there are insuffi-
       cient characters before the current position, the assertion fails.

       PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode) to appear in
       lookbehind assertions, because it makes it impossible to calculate the length of the look-
       behind. The \X and \R escapes, which can match different numbers of bytes,  are	also  not
       permitted.

       Possessive  quantifiers	can  be used in conjunction with lookbehind assertions to specify
       efficient matching at the end of the subject string. Consider a simple pattern such as

       abcd$

       when applied to a long string that does not match. Because matching proceeds from left  to
       right, PCRE will look for each "a" in the subject and then see if what follows matches the
       rest of the pattern. If the pattern is specified as

       ^.*abcd$

       the initial .* matches the entire string at first, but when this fails (because	there  is
       no  following  "a"),  it  backtracks to match all but the last character, then all but the
       last two characters, and so on. Once again the search for "a" covers  the  entire  string,
       from right to left, so we are no better off. However, if the pattern is written as

       ^.*+(?<=abcd)

       there  can  be  no backtracking for the .*+ item; it can match only the entire string. The
       subsequent lookbehind assertion does a single test on the  last	four  characters.  If  it
       fails,  the  match  fails immediately. For long strings, this approach makes a significant
       difference to the processing time.

       Using multiple assertions

       Several assertions (of any sort) may occur in succession. For example,

       (?<=\d{3})(?<!999)foo

       matches "foo" preceded by three digits that are not "999". Notice that each of the  asser-
       tions  is  applied independently at the same point in the subject string. First there is a
       check that the previous three characters are all digits, and then there is  a  check  that
       the same three characters are not "999". This pattern does not match "foo" preceded by six
       characters, the first of which are digits and the last three of which are not  "999".  For
       example, it doesn't match "123abcfoo". A pattern to do that is

       (?<=\d{3}...)(?<!999)foo

       This  time  the	first  assertion looks at the preceding six characters, checking that the
       first three are digits, and then the second assertion  checks  that  the  preceding  three
       characters are not "999".

       Assertions can be nested in any combination. For example,

       (?<=(?<!foo)bar)baz

       matches	an occurrence of "baz" that is preceded by "bar" which in turn is not preceded by
       "foo", while

       (?<=\d{3}(?!999)...)foo

       is another pattern that matches "foo" preceded by three digits and  any	three  characters
       that are not "999".

CONDITIONAL SUBPATTERNS
       It  is  possible  to  cause  the matching process to obey a subpattern conditionally or to
       choose between two alternative subpatterns, depending on the result of  an  assertion,  or
       whether	a  previous capturing subpattern matched or not. The two possible forms of condi-
       tional subpattern are

	 * (?(condition)yes-pattern)

	 * (?(condition)yes-pattern|no-pattern)

       If the condition is satisfied, the yes-pattern  is  used;  otherwise  the  no-pattern  (if
       present)  is  used.  If there are more than two alternatives in the subpattern, a compile-
       time error occurs.

       There are four kinds of condition: references to subpatterns, references to  recursion,	a
       pseudo-condition called DEFINE, and assertions.

       Checking for a used subpattern by number

       If  the	text  between  the parentheses consists of a sequence of digits, the condition is
       true if the capturing subpattern of that number has  previously	matched.  An  alternative
       notation  is to precede the digits with a plus or minus sign. In this case, the subpattern
       number is relative rather than absolute. The most recently opened parentheses can be  ref-
       erenced by (?(-1), the next most recent by (?(-2), and so on. In looping constructs it can
       also make sense to refer to subsequent groups with constructs such as (?(+2).

       Consider the following pattern, which contains non-significant whitespace to make it  more
       readable  (assume  the extended option) and to divide it into three parts for ease of dis-
       cussion:

       ( \( )? [^()]+ (?(1) \) )

       The first part matches an optional opening parenthesis, and if that character is  present,
       sets  it  as  the first captured substring. The second part matches one or more characters
       that are not parentheses. The third part is a conditional subpattern  that  tests  whether
       the first set of parentheses matched or not. If they did, that is, if subject started with
       an opening parenthesis, the condition is true, and so the yes-pattern is  executed  and	a
       closing	parenthesis  is required. Otherwise, since no-pattern is not present, the subpat-
       tern matches nothing. In other words, this pattern matches a sequence of  non-parentheses,
       optionally enclosed in parentheses.

       If you were embedding this pattern in a larger one, you could use a relative reference:

       ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...

       This makes the fragment independent of the parentheses in the larger pattern.

       Checking for a used subpattern by name

       Perl  uses  the	syntax	(?(<name>)...) or (?('name')...) to test for a used subpattern by
       name. For compatibility with earlier versions of PCRE,  which  had  this  facility  before
       Perl,  the  syntax (?(name)...) is also recognized. However, there is a possible ambiguity
       with this syntax, because subpattern names may consist  entirely  of  digits.  PCRE  looks
       first for a named subpattern; if it cannot find one and the name consists entirely of dig-
       its, PCRE looks for a subpattern of that number, which must be greater  than  zero.  Using
       subpattern names that consist entirely of digits is not recommended.

       Rewriting the above example to use a named subpattern gives this:

       (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )

       Checking for pattern recursion

       If the condition is the string (R), and there is no subpattern with the name R, the condi-
       tion is true if a recursive call to the whole pattern or any subpattern has been made.  If
       digits or a name preceded by ampersand follow the letter R, for example:

       (?(R3)...) or (?(R&name)...)

       the  condition is true if the most recent recursion is into the subpattern whose number or
       name is given. This condition does not check the entire recursion stack.

       At "top level", all these recursion test conditions  are  false.  Recursive  patterns  are
       described below.

       Defining subpatterns for use by reference only

       If  the condition is the string (DEFINE), and there is no subpattern with the name DEFINE,
       the condition is always false. In this case, there may be only one alternative in the sub-
       pattern.  It  is  always skipped if control reaches this point in the pattern; the idea of
       DEFINE is that it can be used to define "subroutines" that can be  referenced  from  else-
       where.  (The  use of "subroutines" is described below.) For example, a pattern to match an
       IPv4 address could be written like this (ignore whitespace and line breaks):

       (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) \b	(?&byte)  (\.(?&byte)){3}
       \b

       The  first part of the pattern is a DEFINE group inside which a another group named "byte"
       is defined. This matches an individual component of an IPv4 address (a  number  less  than
       256).  When  matching takes place, this part of the pattern is skipped because DEFINE acts
       like a false condition.

       The rest of the pattern uses references to the named group to match the four dot-separated
       components of an IPv4 address, insisting on a word boundary at each end.

       Assertion conditions

       If  the condition is not in any of the above formats, it must be an assertion. This may be
       a positive or negative lookahead or lookbehind assertion.  Consider  this  pattern,  again
       containing non-significant whitespace, and with the two alternatives on the second line:

	 (?(?=[^a-z]*[a-z])
	 \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

       The  condition is a positive lookahead assertion that matches an optional sequence of non-
       letters followed by a letter. In other words, it tests for the presence of  at  least  one
       letter  in  the	subject.  If  a letter is found, the subject is matched against the first
       alternative; otherwise it is matched against the second. This pattern matches  strings  in
       one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.

COMMENTS
       The sequence (?# marks the start of a comment that continues up to the next closing paren-
       thesis. Nested parentheses are not permitted. The characters that make up a  comment  play
       no part in the pattern matching at all.

       If  the	extended option is set, an unescaped # character outside a character class intro-
       duces a comment that continues to immediately after the next newline in the pattern.

RECURSIVE PATTERNS
       Consider the problem of matching a string in parentheses, allowing  for	unlimited  nested
       parentheses.  Without  the use of recursion, the best that can be done is to use a pattern
       that matches up to some fixed depth of nesting. It is not possible to handle an	arbitrary
       nesting depth.

       For  some  time,  Perl  has provided a facility that allows regular expressions to recurse
       (amongst other things). It does this by interpolating Perl code in the expression  at  run
       time,  and the code can refer to the expression itself. A Perl pattern using code interpo-
       lation to solve the parentheses problem can be created like this:

       $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;

       The (?p{...}) item interpolates Perl code at run time, and in this case refers recursively
       to the pattern in which it appears.

       Obviously,  PCRE  cannot support the interpolation of Perl code. Instead, it supports spe-
       cial syntax for recursion of the entire pattern, and also for individual subpattern recur-
       sion.  After  its  introduction	in PCRE and Python, this kind of recursion was introduced
       into Perl at release 5.10.

       A special item that consists of (? followed by a number greater than zero  and  a  closing
       parenthesis  is	a  recursive call of the subpattern of the given number, provided that it
       occurs inside that subpattern. (If not, it is a "subroutine" call, which is  described  in
       the next section.) The special item (?R) or (?0) is a recursive call of the entire regular
       expression.

       In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  treated  as
       an  atomic group. That is, once it has matched some of the subject string, it is never re-
       entered, even if it contains untried alternatives and there is a subsequent matching fail-
       ure.

       This PCRE pattern solves the nested parentheses problem (assume the extended option is set
       so that whitespace is ignored):

       \( ( (?>[^()]+) | (?R) )* \)

       First it matches an opening parenthesis. Then it matches any number  of	substrings  which
       can  either  be	a sequence of non-parentheses, or a recursive match of the pattern itself
       (that is, a correctly parenthesized substring). Finally there is a closing parenthesis.

       If this were part of a larger pattern, you would not want to recurse the  entire  pattern,
       so instead you could use this:

       ( \( ( (?>[^()]+) | (?1) )* \) )

       We  have  put  the  pattern  into  parentheses,	and caused the recursion to refer to them
       instead of the whole pattern.

       In a larger pattern, keeping track of parenthesis numbers can be tricky. This is made eas-
       ier  by the use of relative references. (A Perl 5.10 feature.) Instead of (?1) in the pat-
       tern above you can write (?-2) to refer to the second  most  recently  opened  parentheses
       preceding  the  recursion.  In other words, a negative number counts capturing parentheses
       leftwards from the point at which it is encountered.

       It is also possible to refer to subsequently opened  parentheses,  by  writing  references
       such  as (?+2). However, these cannot be recursive because the reference is not inside the
       parentheses that are referenced. They are always "subroutine" calls, as described  in  the
       next section.

       An  alternative	approach is to use named parentheses instead. The Perl syntax for this is
       (?&name); PCRE's earlier syntax (?P>name) is also supported. We could  rewrite  the  above
       example as follows:

       (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )

       If there is more than one subpattern with the same name, the earliest one is used.

       This  particular  example  pattern  that we have been looking at contains nested unlimited
       repeats, and so the use of atomic grouping for  matching  strings  of  non-parentheses  is
       important  when	applying the pattern to strings that do not match. For example, when this
       pattern is applied to

       (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

       it yields "no match" quickly. However, if atomic grouping is not used, the match runs  for
       a  very	long time indeed because there are so many different ways the + and * repeats can
       carve up the subject, and all have to be tested before failure can be reported.

       At the end of a match, the values set for any capturing subpatterns  are  those	from  the
       outermost  level  of  the  recursion  at which the subpattern value is set. If the pattern
       above is matched against

       (ab(cd)ef)

       the value for the capturing parentheses is "ef", which is the last value taken on  at  the
       top level. If additional parentheses are added, giving

	 \( ( ( (?>[^()]+) | (?R) )* ) \)
	    ^			     ^
	    ^			     ^

       the string they capture is "ab(cd)ef", the contents of the top level parentheses.

       Do  not	confuse the (?R) item with the condition (R), which tests for recursion. Consider
       this pattern, which matches text in angle brackets, allowing for arbitrary  nesting.  Only
       digits  are  allowed  in nested brackets (that is, when recursing), whereas any characters
       are permitted at the outer level.

       < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >

       In this pattern, (?(R) is the start of a conditional subpattern, with two different alter-
       natives	for  the recursive and non-recursive cases. The (?R) item is the actual recursive
       call.

SUBPATTERNS AS SUBROUTINES
       If the syntax for a recursive subpattern reference (either by number or by name)  is  used
       outside the parentheses to which it refers, it operates like a subroutine in a programming
       language. The "called" subpattern may be defined before or after the reference. A numbered
       reference can be absolute or relative, as in these examples:

	 * (...(absolute)...)...(?2)...

	 * (...(relative)...)...(?-1)...

	 * (...(?+1)...(relative)...

       An earlier example pointed out that the pattern

       (sens|respons)e and \1ibility

       matches	"sense	and  sensibility"  and	"response and responsibility", but not "sense and
       responsibility". If instead the pattern

       (sens|respons)e and (?1)ibility

       is used, it does match "sense and responsibility"  as  well  as	the  other  two  strings.
       Another example is given in the discussion of DEFINE above.

       Like recursive subpatterns, a "subroutine" call is always treated as an atomic group. That
       is, once it has matched some of the subject string, it is never	re-entered,  even  if  it
       contains untried alternatives and there is a subsequent matching failure.

       When  a	subpattern  is used as a subroutine, processing options such as case-independence
       are fixed when the subpattern is defined. They cannot be changed for different calls.  For
       example, consider this pattern:

       (abc)(?i:(?-1))

       It  matches  "abcabc".  It does not match "abcABC" because the change of processing option
       does not affect the called subpattern.

BACKTRACKING CONTROL
       Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which are described
       in  the	Perl  documentation as "experimental and subject to change or removal in a future
       version of Perl". It goes on to say: "Their usage in production code should  be	noted  to
       avoid  problems during upgrades." The same remarks apply to the PCRE features described in
       this section.

       The new verbs make use of what was previously invalid syntax: an opening parenthesis  fol-
       lowed  by  an  asterisk. In Perl, they are generally of the form (*VERB:ARG) but PCRE does
       not support the use of arguments, so its general form is just (*VERB). Any number of these
       verbs may occur in a pattern. There are two kinds:

       Verbs that act immediately

       The following verbs act as soon as they are encountered:

       (*ACCEPT)

       This  verb  causes  the	match to end successfully, skipping the remainder of the pattern.
       When inside a recursion, only the innermost pattern is  ended  immediately.  PCRE  differs
       from  Perl  in what happens if the (*ACCEPT) is inside capturing parentheses. In Perl, the
       data so far is captured: in PCRE no data is captured. For example:

       A(A|B(*ACCEPT)|C)D

       This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is captured.

       (*FAIL) or (*F)

       This verb causes the match to fail, forcing backtracking to occur.  It  is  equivalent  to
       (?!) but easier to read. The Perl documentation notes that it is probably useful only when
       combined with (?{}) or (??{}). Those are, of course, Perl features that are not present in
       PCRE. The nearest equivalent is the callout feature, as for example in this pattern:

       a+(?C)(*FAIL)

       A  match  with  the string "aaaa" always fails, but the callout is taken before each back-
       track happens (in this example, 10 times).

       Verbs that act after backtracking

       The following verbs do nothing when they are encountered.  Matching  continues  with  what
       follows,  but  if  there  is no subsequent match, a failure is forced. The verbs differ in
       exactly what kind of failure occurs.

       (*COMMIT)

       This verb causes the whole match to fail outright if the rest  of  the  pattern	does  not
       match. Even if the pattern is unanchored, no further attempts to find a match by advancing
       the start point take place. Once (*COMMIT) has been passed, re:run/3 is committed to find-
       ing a match at the current starting point, or not at all. For example:

       a+(*COMMIT)b

       This  matches  "xxaab" but not "aacaab". It can be thought of as a kind of dynamic anchor,
       or "I've started, so I must finish."

       (*PRUNE)

       This verb causes the match to fail at the current position if the rest of the pattern does
       not match. If the pattern is unanchored, the normal "bumpalong" advance to the next start-
       ing character then happens. Backtracking can occur as usual to the left	of  (*PRUNE),  or
       when  matching to the right of (*PRUNE), but if there is no match to the right, backtrack-
       ing cannot cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an alternative  to
       an  atomic group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
       be expressed in any other way.

       (*SKIP)

       This verb is like (*PRUNE), except that if the  pattern	is  unanchored,  the  "bumpalong"
       advance is not to the next character, but to the position in the subject where (*SKIP) was
       encountered. (*SKIP) signifies that whatever text was matched leading up to it  cannot  be
       part of a successful match. Consider:

       a+(*SKIP)b

       If  the	subject is "aaaac...", after the first match attempt fails (starting at the first
       character in the string), the starting point skips on to start the next	attempt  at  "c".
       Note  that a possessive quantifier does not have the same effect in this example; although
       it would suppress backtracking during the first match attempt, the  second  attempt  would
       start at the second character instead of skipping on to "c".

       (*THEN)

       This verb causes a skip to the next alternation if the rest of the pattern does not match.
       That is, it cancels pending backtracking, but only within  the  current	alternation.  Its
       name  comes  from  the  observation  that  it can be used for a pattern-based if-then-else
       block:

       ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

       If the COND1 pattern matches, FOO is tried (and possibly further items after  the  end  of
       the  group  if  FOO  succeeds); on failure the matcher skips to the second alternative and
       tries COND2, without backtracking into COND1. If (*THEN) is used outside of  any  alterna-
       tion, it acts exactly like (*PRUNE).

Ericsson AB				  stdlib 1.17.3 				 re(3erl)
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