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 | dup-
names | 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 sub-
jects 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 regular 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 equivalent 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 dollar_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 newline. Without it, a dot does not match when the
current position is at a newline. 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 new-
line, 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 terminating 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 subject string, respectively, as well as at the very start and end. This is equivalent 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 subpatterns 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 separator, 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} | {capture, 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 regular 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
{newline, 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 Subject 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 specification 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 powerful. 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 contents, it cannot be made unanchored at matching time, hence there is no unanchored 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 subexpression 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 position, 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 posi-
tion 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 alternatives match the empty string, the entire match fails. For example, if the pattern
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 occurrences 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 metachar-
acter 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 compile time) causes dol-
lar 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 separator, 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 com-
pilation 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, beginning 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 sub-
patterns are discarded.
all_but_first :
All but the first matching subpattern, i.e. all explicitly captured subpatterns, 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 subpatterns
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 behavior.
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 corresponding 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 regular 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:char-
acters_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 unicode 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 . Unas-
signed subpatterns are returned as the empty binary or list, respectively, for other return types. Consider the regular expres-
sion:
".*((?<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 subpattern 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 necessary, 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} | CompileOpt
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 compila-
tion is done implicitly and the unicode 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 spe-
cial sequence N (where N is an integer > 0), resulting in the subexpression number N will be inserted in the result. If no subex-
pression with that number is generated by the regular expression, 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 regular expression supplied.
The splitting is done basically by running a global regexp match and dividing the initial string wherever a match occurs. The match-
ing 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 compila-
tion is done implicitly and the unicode 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 concatenating 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 subexpressions 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 subexpression 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 circum-
stances. 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 inte-
ger corresponds exactly to the Perl behaviour with a positive numerical third parameter and the default behaviour of re:split/3 cor-
responds 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 num-
ber 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 corre-
sponds 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 documentation, 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 (line-
feed) 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 pattern:
(*CR)a.b
changes the convention to CR. That pattern matches "a
b" because LF is no longer a newline. 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 com-
patibility. However, this can be changed; see the description of R in the section entitled "Newline sequences" below. A change of R set-
ting 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 repetitions 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 brackets. 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 backslash 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 escaping 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 # character 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 interpolation. Note
the following examples:
Pattern PCRE matches Perl matches
Qabc$xyzE abc$xyz abc followed by the contents of $xyz
Qabc$xyzE abc$xyz abc$xyz
QabcE$QxyzE abc$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)
:
linefeed (hex 0A)
:
carriage return (hex 0D)
:
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 recog-
nized. 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 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 x 7 specifies two binary zeros followed 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 parentheses 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:
40 :
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
11 :
is always a tab
113 :
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 charac-
ter class, the sequence is interpreted as the backspace character (hex 08), and the sequences R and X are interpreted as the charac-
ters "R" and "X", respectively. Outside a character class, these sequences have different 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 char-
acter. 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 con-
trolled 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 following:
(?>
|
|x0b|f|
|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 property 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 pattern 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 overridden 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 combined 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 proper-
ties 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 current 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 abbreviation. 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 properties 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
(?>PMpM*)
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 preceding 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 structure 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 characters not to be included in the final matched
sequence. For example, the pattern:
fooKbar
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 condition that has to be met at a particular point in
a match, without consuming any characters from the subject string. The use of subpatterns for more complicated assertions is described
below. The backslashed assertions are:
:
matches at a word boundary
B :
matches when not at a word boundary
A :
matches at the start of the subject
:
matches at the end of the subject also matches before a newline at the end of the subject
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 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, respectively.
The A, , 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 argument 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 and z is that 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 argu-
ment 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 argu-
ments, 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 expression.
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 multi-
line option is unset. Inside a character class, circumflex has an entirely different 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 circumflex,
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 alternatives
are involved, but it should be the last item in any branch in which it appears. Dollar 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 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
abc" (where
represents a newline) in multiline mode, but not other-
wise. Consequently, patterns that are anchored in single line mode because all branches start with ^ are not anchored in multiline 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 mul-
tiline is set.
Note that the sequences A, , and z can be used to match the start and end of the subject in both modes, and if all branches of a pat-
tern 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 signi-
fies 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 excep-
tion. 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 char-
acters 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 calcu-
late 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 charac-
ter 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 circumflex is actually required as a member of the class,
ensure it is not the first character, 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{ escap-
ing 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 character 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 character 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 charac-
ters. 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
[ 00- 37]. 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 equiv-
alent 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 example,
[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 character classes.
VERTICAL BAR
Vertical bar characters are used to separate alternative patterns. For example, the pattern
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 alterna-
tives 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 permit-
ted. 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 char-
acters 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 fol-
lows 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 subpattern. For
example,
(a(?i)b|c)
matches "ab", "aB", "c", and "C", even though when matching "C" the first branch is abandoned 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 leading 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 cap-
turing, 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 maximum number of capturing subpatterns is 65535.
As a convenient shorthand, if any option settings are required at the start of a non-capturing 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 "SUNDAY" as well
as "Saturday".
DUPLICATE SUBPATTERN NUMBERS
Perl 5.10 introduced a feature whereby each alternative in a subpattern uses the same numbers for its capturing parentheses. Such a subpat-
tern 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 number 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 follow-
ing 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 expres-
sions. Furthermore, if an expression is modified, the numbers may change. To help with this difficulty, PCRE supports the naming of subpat-
terns. This feature was not added to Perl until release 5.10. Python had the feature 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 parentheses 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 reg-
ular expression.
By default, a name must be unique within a pattern, but it is possible to relax this constraint 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 alternative 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 charac-
ters.
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 char-
acters, 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 programs.
These appear between /* and */ and within the comment, individual * and / characters 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 question 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 normally 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 setting 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:
(.*)abc1
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 subpat-
terns, 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 prevent 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 pre-
vented 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 characters 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 notation, 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 ulti-
mately found its way into Perl at release 5.10.
PCRE has an optimization that automatically "possessifies" certain simple pattern constructs. 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 parentheses.
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 reference" 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 number 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 fur-
ther 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 fea-
ture 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 lit-
eral digits follow the reference. A negative 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 reference.
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 example, the pattern
(a|(bc))2
always fails if it starts to match "a" rather than "bc". Because there may be many capturing 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 subpattern is first used, so, for example, (a1)
never matches. However, such references can be useful inside repeated subpatterns. For example, the pattern
(a|b1)+
matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of the subpattern, the back reference matches the character
string corresponding to the previous iteration. 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 example 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, A, G, , 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 current
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 capturing
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 assertions. 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 assertions. For example,
(?<!foo)bar
does find an occurrence of "bar" that is not preceded by "foo". The contents of a lookbehind 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 permitted only at the top level of a lookbehind asser-
tion. This is an extension compared with Perl (at least for 5.8), which requires all branches to match the same length of string. An asser-
tion 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 lookbehind 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 insufficient 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 lookbehind. The X and R escapes, which can match different numbers of bytes, are also not per-
mitted.
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 assertions 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 asser-
tion 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 conditional 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 alterna-
tives 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 rel-
ative rather than absolute. The most recently opened parentheses can be referenced 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 discussion:
( ( )? [^()]+ (?(1) ) )
The first part matches an optional opening parenthesis, and if that character is present, sets it as the first captured substring. The sec-
ond 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 subpattern 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 digits, 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 condition is true if a recursive call to the whole pat-
tern 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 subpattern. 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 elsewhere. (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] | 1dd | [1-9]?d) ) (?&byte) (.(?&byte)){3}
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 let-
ters and dd are digits.
COMMENTS
The sequence (?# marks the start of a comment that continues up to the next closing parenthesis. 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 introduces 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 interpolat-
ing Perl code in the expression at run time, and the code can refer to the expression itself. A Perl pattern using code interpolation 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 special syntax for recursion of the entire pattern, and
also for individual subpattern recursion. 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 failure.
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 easier by the use of relative references. (A Perl
5.10 feature.) Instead of (?1) in the pattern 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 sec-
tion.
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 sub-
pattern 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 brack-
ets, allowing for arbitrary nesting. Only digits are allowed in nested brackets (that is, when recursing), whereas any characters are per-
mitted at the outer level.
< (?: (?(R) d++ | [^<>]*+) | (?R)) * >
In this pattern, (?(R) is the start of a conditional subpattern, with two different alternatives 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 ref-
erence 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 can-
not 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 followed 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 backtrack 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 finding 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 starting 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, backtracking 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 alternation, it acts
exactly like (*PRUNE).
Ericsson AB stdlib 1.17.3 re(3erl)