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PERLRE(1)			 Perl Programmers Reference Guide			PERLRE(1)

NAME
       perlre - Perl regular expressions

DESCRIPTION
       This page describes the syntax of regular expressions in Perl.

       If you haven't used regular expressions before, a quick-start introduction is available in
       perlrequick, and a longer tutorial introduction is available in perlretut.

       For reference on how regular expressions are used in matching operations, plus various
       examples of the same, see discussions of "m//", "s///", "qr//" and "??" in "Regexp Quote-
       Like Operators" in perlop.

       Modifiers

       Matching operations can have various modifiers.	Modifiers that relate to the interpreta-
       tion of the regular expression inside are listed below.	Modifiers that alter the way a
       regular expression is used by Perl are detailed in "Regexp Quote-Like Operators" in perlop
       and "Gory details of parsing quoted constructs" in perlop.

       m   Treat string as multiple lines.  That is, change "^" and "$" from matching the start
	   or end of the string to matching the start or end of any line anywhere within the
	   string.

       s   Treat string as single line.  That is, change "." to match any character whatsoever,
	   even a newline, which normally it would not match.

	   The "/s" and "/m" modifiers both override the $* setting.  That is, no matter what $*
	   contains, "/s" without "/m" will force "^" to match only at the beginning of the
	   string and "$" to match only at the end (or just before a newline at the end) of the
	   string.  Together, as /ms, they let the "." match any character whatsoever, while
	   still allowing "^" and "$" to match, respectively, just after and just before newlines
	   within the string.

       i   Do case-insensitive pattern matching.

	   If "use locale" is in effect, the case map is taken from the current locale.  See per-
	   llocale.

       x   Extend your pattern's legibility by permitting whitespace and comments.

       g and c
	   Global matching, and keep the Current position after failed matching.  Unlike i, m, s
	   and x, these two flags affect the way the regex is used rather than the regex itself.
	   See "Using regular expressions in Perl" in perlretut for further explanation of the g
	   and c modifiers.

       These are usually written as "the "/x" modifier", even though the delimiter in question
       might not really be a slash.  Any of these modifiers may also be embedded within the regu-
       lar expression itself using the "(?...)" construct.  See below.

       The "/x" modifier itself needs a little more explanation.  It tells the regular expression
       parser to ignore whitespace that is neither backslashed nor within a character class.  You
       can use this to break up your regular expression into (slightly) more readable parts.  The
       "#" character is also treated as a metacharacter introducing a comment, just as in ordi-
       nary Perl code.	This also means that if you want real whitespace or "#" characters in the
       pattern (outside a character class, where they are unaffected by "/x"), then you'll either
       have to escape them (using backslashes or "\Q...\E") or encode them using octal or hex
       escapes.  Taken together, these features go a long way towards making Perl's regular
       expressions more readable.  Note that you have to be careful not to include the pattern
       delimiter in the comment--perl has no way of knowing you did not intend to close the pat-
       tern early.  See the C-comment deletion code in perlop.	Also note that anything inside a
       "\Q...\E" stays unaffected by "/x".

       Regular Expressions

       The patterns used in Perl pattern matching evolved from those supplied in the Version 8
       regex routines.	(The routines are derived (distantly) from Henry Spencer's freely redis-
       tributable reimplementation of the V8 routines.)  See "Version 8 Regular Expressions" for
       details.

       In particular the following metacharacters have their standard egrep-ish meanings:

	   \   Quote the next metacharacter
	   ^   Match the beginning of the line
	   .   Match any character (except newline)
	   $   Match the end of the line (or before newline at the end)
	   |   Alternation
	   ()  Grouping
	   []  Character class

       By default, the "^" character is guaranteed to match only the beginning of the string, the
       "$" character only the end (or before the newline at the end), and Perl does certain opti-
       mizations with the assumption that the string contains only one line.  Embedded newlines
       will not be matched by "^" or "$".  You may, however, wish to treat a string as a multi-
       line buffer, such that the "^" will match after any newline within the string (except if
       the newline is the last character in the string), and "$" will match before any newline.
       At the cost of a little more overhead, you can do this by using the /m modifier on the
       pattern match operator.	(Older programs did this by setting $*, but this practice is now
       deprecated.)

       To simplify multi-line substitutions, the "." character never matches a newline unless you
       use the "/s" modifier, which in effect tells Perl to pretend the string is a single
       line--even if it isn't.	The "/s" modifier also overrides the setting of $*, in case you
       have some (badly behaved) older code that sets it in another module.

       The following standard quantifiers are recognized:

	   *	  Match 0 or more times
	   +	  Match 1 or more times
	   ?	  Match 1 or 0 times
	   {n}	  Match exactly n times
	   {n,}   Match at least n times
	   {n,m}  Match at least n but not more than m times

       (If a curly bracket occurs in any other context, it is treated as a regular character.  In
       particular, the lower bound is not optional.)  The "*" quantifier is equivalent to "{0,}",
       the "+" quantifier to "{1,}", and the "?" quantifier to "{0,1}".  n and m are limited to
       integral values less than a preset limit defined when perl is built.  This is usually
       32766 on the most common platforms.  The actual limit can be seen in the error message
       generated by code such as this:

	   $_ **= $_ , / {$_} / for 2 .. 42;

       By default, a quantified subpattern is "greedy", that is, it will match as many times as
       possible (given a particular starting location) while still allowing the rest of the pat-
       tern to match.  If you want it to match the minimum number of times possible, follow the
       quantifier with a "?".  Note that the meanings don't change, just the "greediness":

	   *?	  Match 0 or more times, not greedily
	   +?	  Match 1 or more times, not greedily
	   ??	  Match 0 or 1 time, not greedily
	   {n}?   Match exactly n times, not greedily
	   {n,}?  Match at least n times, not greedily
	   {n,m}? Match at least n but not more than m times, not greedily

       Because patterns are processed as double quoted strings, the following also work:

	   \t	       tab		     (HT, TAB)
	   \n	       newline		     (LF, NL)
	   \r	       return		     (CR)
	   \f	       form feed	     (FF)
	   \a	       alarm (bell)	     (BEL)
	   \e	       escape (think troff)  (ESC)
	   \033        octal char	     (example: ESC)
	   \x1B        hex char 	     (example: ESC)
	   \x{263a}    long hex char	     (example: Unicode SMILEY)
	   \cK	       control char	     (example: VT)
	   \N{name}    named Unicode character
	   \l	       lowercase next char (think vi)
	   \u	       uppercase next char (think vi)
	   \L	       lowercase till \E (think vi)
	   \U	       uppercase till \E (think vi)
	   \E	       end case modification (think vi)
	   \Q	       quote (disable) pattern metacharacters till \E

       If "use locale" is in effect, the case map used by "\l", "\L", "\u" and "\U" is taken from
       the current locale.  See perllocale.  For documentation of "\N{name}", see charnames.

       You cannot include a literal "$" or "@" within a "\Q" sequence.	An unescaped "$" or "@"
       interpolates the corresponding variable, while escaping will cause the literal string "\$"
       to be matched.  You'll need to write something like "m/\Quser\E\@\Qhost/".

       In addition, Perl defines the following:

	   \w  Match a "word" character (alphanumeric plus "_")
	   \W  Match a non-"word" character
	   \s  Match a whitespace character
	   \S  Match a non-whitespace character
	   \d  Match a digit character
	   \D  Match a non-digit character
	   \pP Match P, named property.  Use \p{Prop} for longer names.
	   \PP Match non-P
	   \X  Match eXtended Unicode "combining character sequence",
	       equivalent to (?>\PM\pM*)
	   \C  Match a single C char (octet) even under Unicode.
	       NOTE: breaks up characters into their UTF-8 bytes,
	       so you may end up with malformed pieces of UTF-8.
	       Unsupported in lookbehind.

       A "\w" matches a single alphanumeric character (an alphabetic character, or a decimal
       digit) or "_", not a whole word.  Use "\w+" to match a string of Perl-identifier charac-
       ters (which isn't the same as matching an English word).  If "use locale" is in effect,
       the list of alphabetic characters generated by "\w" is taken from the current locale.  See
       perllocale.  You may use "\w", "\W", "\s", "\S", "\d", and "\D" within character classes,
       but they aren't usable as either end of a range. If any of them precedes or follows a "-",
       the "-" is understood literally. If Unicode is in effect, "\s" matches also "\x{85}",
       "\x{2028}, and "\x{2029}". See perlunicode for more details about "\pP", "\PP", "\X" and
       the possibility of defining your own "\p" and "\P" properties, and perluniintro about Uni-
       code in general.

       The POSIX character class syntax

	   [:class:]

       is also available.  Note that the "[" and "]" brackets are literal; they must always be
       used within a character class expression.

	   # this is correct:
	   $string =~ /[[:alpha:]]/;

	   # this is not, and will generate a warning:
	   $string =~ /[:alpha:]/;

       The available classes and their backslash equivalents (if available) are as follows:

	   alpha
	   alnum
	   ascii
	   blank	       [1]
	   cntrl
	   digit       \d
	   graph
	   lower
	   print
	   punct
	   space       \s      [2]
	   upper
	   word        \w      [3]
	   xdigit

       [1] A GNU extension equivalent to "[ \t]", "all horizontal whitespace".

       [2] Not exactly equivalent to "\s" since the "[[:space:]]" includes also the (very rare)
	   "vertical tabulator", "\cK" or chr(11) in ASCII.

       [3] A Perl extension, see above.

       For example use "[:upper:]" to match all the uppercase characters.  Note that the "[]" are
       part of the "[::]" construct, not part of the whole character class.  For example:

	   [01[:alpha:]%]

       matches zero, one, any alphabetic character, and the percent sign.

       The following equivalences to Unicode \p{} constructs and equivalent backslash character
       classes (if available), will hold:

	   [[:...:]]   \p{...}	       backslash

	   alpha       IsAlpha
	   alnum       IsAlnum
	   ascii       IsASCII
	   blank
	   cntrl       IsCntrl
	   digit       IsDigit	      \d
	   graph       IsGraph
	   lower       IsLower
	   print       IsPrint
	   punct       IsPunct
	   space       IsSpace
		       IsSpacePerl    \s
	   upper       IsUpper
	   word        IsWord
	   xdigit      IsXDigit

       For example "[[:lower:]]" and "\p{IsLower}" are equivalent.

       If the "utf8" pragma is not used but the "locale" pragma is, the classes correlate with
       the usual isalpha(3) interface (except for "word" and "blank").

       The other named classes are:

       cntrl
	   Any control character.  Usually characters that don't produce output as such but
	   instead control the terminal somehow: for example newline and backspace are control
	   characters.	All characters with ord() less than 32 are usually classified as control
	   characters (assuming ASCII, the ISO Latin character sets, and Unicode), as is the
	   character with the ord() value of 127 ("DEL").

       graph
	   Any alphanumeric or punctuation (special) character.

       print
	   Any alphanumeric or punctuation (special) character or the space character.

       punct
	   Any punctuation (special) character.

       xdigit
	   Any hexadecimal digit.  Though this may feel silly ([0-9A-Fa-f] would work just fine)
	   it is included for completeness.

       You can negate the [::] character classes by prefixing the class name with a '^'. This is
       a Perl extension.  For example:

	   POSIX	 traditional  Unicode

	   [[:^digit:]]    \D	      \P{IsDigit}
	   [[:^space:]]    \S	      \P{IsSpace}
	   [[:^word:]]	   \W	      \P{IsWord}

       Perl respects the POSIX standard in that POSIX character classes are only supported within
       a character class.  The POSIX character classes [.cc.] and [=cc=] are recognized but not
       supported and trying to use them will cause an error.

       Perl defines the following zero-width assertions:

	   \b  Match a word boundary
	   \B  Match except at a word boundary
	   \A  Match only at beginning of string
	   \Z  Match only at end of string, or before newline at the end
	   \z  Match only at end of string
	   \G  Match only at pos() (e.g. at the end-of-match position
	       of prior m//g)

       A word boundary ("\b") is a spot between two characters that has a "\w" on one side of it
       and a "\W" on the other side of it (in either order), counting the imaginary characters
       off the beginning and end of the string as matching a "\W".  (Within character classes
       "\b" represents backspace rather than a word boundary, just as it normally does in any
       double-quoted string.)  The "\A" and "\Z" are just like "^" and "$", except that they
       won't match multiple times when the "/m" modifier is used, while "^" and "$" will match at
       every internal line boundary.  To match the actual end of the string and not ignore an
       optional trailing newline, use "\z".

       The "\G" assertion can be used to chain global matches (using "m//g"), as described in
       "Regexp Quote-Like Operators" in perlop.  It is also useful when writing "lex"-like scan-
       ners, when you have several patterns that you want to match against consequent substrings
       of your string, see the previous reference.  The actual location where "\G" will match can
       also be influenced by using "pos()" as an lvalue: see "pos" in perlfunc. Currently "\G" is
       only fully supported when anchored to the start of the pattern; while it is permitted to
       use it elsewhere, as in "/(?<=\G..)./g", some such uses ("/.\G/g", for example) currently
       cause problems, and it is recommended that you avoid such usage for now.

       The bracketing construct "( ... )" creates capture buffers. To refer to the current con-
       tents of a buffer later on, within the same pattern, use \1 for the first, \2 for the sec-
       ond, and so on.	Outside the match use "$" instead of "\".  (The \<digit> notation works
       in certain circumstances outside the match.  See the warning below about \1 vs $1 for
       details.)  Referring back to another part of the match is called a backreference.

       There is no limit to the number of captured substrings that you may use.  However Perl
       also uses \10, \11, etc. as aliases for \010, \011, etc.  (Recall that 0 means octal, so
       \011 is the character at number 9 in your coded character set; which would be the 10th
       character, a horizontal tab under ASCII.)  Perl resolves this ambiguity by interpreting
       \10 as a backreference only if at least 10 left parentheses have opened before it.  Like-
       wise \11 is a backreference only if at least 11 left parentheses have opened before it.
       And so on.  \1 through \9 are always interpreted as backreferences.

       Examples:

	   s/^([^ ]*) *([^ ]*)/$2 $1/;	   # swap first two words

	    if (/(.)\1/) {		   # find first doubled char
		print "'$1' is the first doubled character\n";
	    }

	   if (/Time: (..):(..):(..)/) {   # parse out values
	       $hours = $1;
	       $minutes = $2;
	       $seconds = $3;
	   }

       Several special variables also refer back to portions of the previous match.  $+ returns
       whatever the last bracket match matched.  $& returns the entire matched string.	(At one
       point $0 did also, but now it returns the name of the program.)	$` returns everything
       before the matched string.  $' returns everything after the matched string. And $^N con-
       tains whatever was matched by the most-recently closed group (submatch). $^N can be used
       in extended patterns (see below), for example to assign a submatch to a variable.

       The numbered match variables ($1, $2, $3, etc.) and the related punctuation set ($+, $&,
       $`, $', and $^N) are all dynamically scoped until the end of the enclosing block or until
       the next successful match, whichever comes first.  (See "Compound Statements" in perlsyn.)

       NOTE: Failed matches in Perl do not reset the match variables, which makes it easier to
       write code that tests for a series of more specific cases and remembers the best match.

       WARNING: Once Perl sees that you need one of $&, $`, or $' anywhere in the program, it has
       to provide them for every pattern match.  This may substantially slow your program.  Perl
       uses the same mechanism to produce $1, $2, etc, so you also pay a price for each pattern
       that contains capturing parentheses.  (To avoid this cost while retaining the grouping be-
       haviour, use the extended regular expression "(?: ... )" instead.)  But if you never use
       $&, $` or $', then patterns without capturing parentheses will not be penalized.  So avoid
       $&, $', and $` if you can, but if you can't (and some algorithms really appreciate them),
       once you've used them once, use them at will, because you've already paid the price.  As
       of 5.005, $& is not so costly as the other two.

       Backslashed metacharacters in Perl are alphanumeric, such as "\b", "\w", "\n".  Unlike
       some other regular expression languages, there are no backslashed symbols that aren't
       alphanumeric.  So anything that looks like \\, \(, \), \<, \>, \{, or \} is always inter-
       preted as a literal character, not a metacharacter.  This was once used in a common idiom
       to disable or quote the special meanings of regular expression metacharacters in a string
       that you want to use for a pattern. Simply quote all non-"word" characters:

	   $pattern =~ s/(\W)/\\$1/g;

       (If "use locale" is set, then this depends on the current locale.)  Today it is more com-
       mon to use the quotemeta() function or the "\Q" metaquoting escape sequence to disable all
       metacharacters' special meanings like this:

	   /$unquoted\Q$quoted\E$unquoted/

       Beware that if you put literal backslashes (those not inside interpolated variables)
       between "\Q" and "\E", double-quotish backslash interpolation may lead to confusing
       results.  If you need to use literal backslashes within "\Q...\E", consult "Gory details
       of parsing quoted constructs" in perlop.

       Extended Patterns

       Perl also defines a consistent extension syntax for features not found in standard tools
       like awk and lex.  The syntax is a pair of parentheses with a question mark as the first
       thing within the parentheses.  The character after the question mark indicates the exten-
       sion.

       The stability of these extensions varies widely.  Some have been part of the core language
       for many years.	Others are experimental and may change without warning or be completely
       removed.  Check the documentation on an individual feature to verify its current status.

       A question mark was chosen for this and for the minimal-matching construct because 1)
       question marks are rare in older regular expressions, and 2) whenever you see one, you
       should stop and "question" exactly what is going on.  That's psychology...

       "(?#text)"
		 A comment.  The text is ignored.  If the "/x" modifier enables whitespace for-
		 matting, a simple "#" will suffice.  Note that Perl closes the comment as soon
		 as it sees a ")", so there is no way to put a literal ")" in the comment.

       "(?imsx-imsx)"
		 One or more embedded pattern-match modifiers, to be turned on (or turned off, if
		 preceded by "-") for the remainder of the pattern or the remainder of the
		 enclosing pattern group (if any). This is particularly useful for dynamic pat-
		 terns, such as those read in from a configuration file, taken from an argument,
		 or specified in a table somewhere.  Consider the case where some patterns want
		 to be case sensitive and some do not:	The case insensitive ones merely need to
		 include "(?i)" at the front of the pattern.  For example:

		     $pattern = "foobar";
		     if ( /$pattern/i ) { }

		     # more flexible:

		     $pattern = "(?i)foobar";
		     if ( /$pattern/ ) { }

		 These modifiers are restored at the end of the enclosing group. For example,

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

		 will match "blah" in any case, some spaces, and an exact (including the case!)
		 repetition of the previous word, assuming the "/x" modifier, and no "/i" modi-
		 fier outside this group.

       "(?:pattern)"
       "(?imsx-imsx:pattern)"
		 This is for clustering, not capturing; it groups subexpressions like "()", but
		 doesn't make backreferences as "()" does.  So

		     @fields = split(/\b(?:a|b|c)\b/)

		 is like

		     @fields = split(/\b(a|b|c)\b/)

		 but doesn't spit out extra fields.  It's also cheaper not to capture characters
		 if you don't need to.

		 Any letters between "?" and ":" act as flags modifiers as with "(?imsx-imsx)".
		 For example,

		     /(?s-i:more.*than).*million/i

		 is equivalent to the more verbose

		     /(?:(?s-i)more.*than).*million/i

       "(?=pattern)"
		 A zero-width positive look-ahead assertion.  For example, "/\w+(?=\t)/" matches
		 a word followed by a tab, without including the tab in $&.

       "(?!pattern)"
		 A zero-width negative look-ahead assertion.  For example "/foo(?!bar)/" matches
		 any occurrence of "foo" that isn't followed by "bar".	Note however that look-
		 ahead and look-behind are NOT the same thing.	You cannot use this for
		 look-behind.

		 If you are looking for a "bar" that isn't preceded by a "foo", "/(?!foo)bar/"
		 will not do what you want.  That's because the "(?!foo)" is just saying that the
		 next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will match.
		 You would have to do something like "/(?!foo)...bar/" for that.   We say "like"
		 because there's the case of your "bar" not having three characters before it.
		 You could cover that this way: "/(?:(?!foo)...|^.{0,2})bar/".	Sometimes it's
		 still easier just to say:

		     if (/bar/ && $` !~ /foo$/)

		 For look-behind see below.

       "(?<=pattern)"
		 A zero-width positive look-behind assertion.  For example, "/(?<=\t)\w+/"
		 matches a word that follows a tab, without including the tab in $&.  Works only
		 for fixed-width look-behind.

       "(?<!pattern)"
		 A zero-width negative look-behind assertion.  For example "/(?<!bar)foo/"
		 matches any occurrence of "foo" that does not follow "bar".  Works only for
		 fixed-width look-behind.

       "(?{ code })"
		 WARNING: This extended regular expression feature is considered highly experi-
		 mental, and may be changed or deleted without notice.

		 This zero-width assertion evaluates any embedded Perl code.  It always succeeds,
		 and its "code" is not interpolated.  Currently, the rules to determine where the
		 "code" ends are somewhat convoluted.

		 This feature can be used together with the special variable $^N to capture the
		 results of submatches in variables without having to keep track of the number of
		 nested parentheses. For example:

		   $_ = "The brown fox jumps over the lazy dog";
		   /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i;
		   print "color = $color, animal = $animal\n";

		 Inside the "(?{...})" block, $_ refers to the string the regular expression is
		 matching against. You can also use "pos()" to know what is the current position
		 of matching within this string.

		 The "code" is properly scoped in the following sense: If the assertion is back-
		 tracked (compare "Backtracking"), all changes introduced after "local"ization
		 are undone, so that

		   $_ = 'a' x 8;
		   m<
		      (?{ $cnt = 0 })			 # Initialize $cnt.
		      (
			a
			(?{
			    local $cnt = $cnt + 1;	 # Update $cnt, backtracking-safe.
			})
		      )*
		      aaaa
		      (?{ $res = $cnt })		 # On success copy to non-localized
							 # location.
		    >x;

		 will set "$res = 4".  Note that after the match, $cnt returns to the globally
		 introduced value, because the scopes that restrict "local" operators are
		 unwound.

		 This assertion may be used as a "(?(condition)yes-pattern|no-pattern)" switch.
		 If not used in this way, the result of evaluation of "code" is put into the spe-
		 cial variable $^R.  This happens immediately, so $^R can be used from other "(?{
		 code })" assertions inside the same regular expression.

		 The assignment to $^R above is properly localized, so the old value of $^R is
		 restored if the assertion is backtracked; compare "Backtracking".

		 Due to an unfortunate implementation issue, the Perl code contained in these
		 blocks is treated as a compile time closure that can have seemingly bizarre con-
		 sequences when used with lexically scoped variables inside of subroutines or
		 loops.  There are various workarounds for this, including simply using global
		 variables instead.  If you are using this construct and strange results occur
		 then check for the use of lexically scoped variables.

		 For reasons of security, this construct is forbidden if the regular expression
		 involves run-time interpolation of variables, unless the perilous "use re
		 'eval'" pragma has been used (see re), or the variables contain results of
		 "qr//" operator (see "qr/STRING/imosx" in perlop).

		 This restriction is due to the wide-spread and remarkably convenient custom of
		 using run-time determined strings as patterns.  For example:

		     $re = <>;
		     chomp $re;
		     $string =~ /$re/;

		 Before Perl knew how to execute interpolated code within a pattern, this opera-
		 tion was completely safe from a security point of view, although it could raise
		 an exception from an illegal pattern.	If you turn on the "use re 'eval'",
		 though, it is no longer secure, so you should only do so if you are also using
		 taint checking.  Better yet, use the carefully constrained evaluation within a
		 Safe compartment.  See perlsec for details about both these mechanisms.

		 Because Perl's regex engine is currently not re-entrant, interpolated code may
		 not invoke the regex engine either directly with "m//" or "s///"), or indirectly
		 with functions such as "split".

       "(??{ code })"
		 WARNING: This extended regular expression feature is considered highly experi-
		 mental, and may be changed or deleted without notice.	A simplified version of
		 the syntax may be introduced for commonly used idioms.

		 This is a "postponed" regular subexpression.  The "code" is evaluated at run
		 time, at the moment this subexpression may match.  The result of evaluation is
		 considered as a regular expression and matched as if it were inserted instead of
		 this construct.

		 The "code" is not interpolated.  As before, the rules to determine where the
		 "code" ends are currently somewhat convoluted.

		 The following pattern matches a parenthesized group:

		   $re = qr{
			      \(
			      (?:
				 (?> [^()]+ )	 # Non-parens without backtracking
			       |
				 (??{ $re })	 # Group with matching parens
			      )*
			      \)
			   }x;

		 Because perl's regex engine is not currently re-entrant, delayed code may not
		 invoke the regex engine either directly with "m//" or "s///"), or indirectly
		 with functions such as "split".

       "(?>pattern)"
		 WARNING: This extended regular expression feature is considered highly experi-
		 mental, and may be changed or deleted without notice.

		 An "independent" subexpression, one which matches the substring that a stand-
		 alone "pattern" would match if anchored at the given position, and it matches
		 nothing other than this substring.  This construct is useful for optimizations
		 of what would otherwise be "eternal" matches, because it will not backtrack (see
		 "Backtracking").  It may also be useful in places where the "grab all you can,
		 and do not give anything back" semantic is desirable.

		 For example: "^(?>a*)ab" will never match, since "(?>a*)" (anchored at the
		 beginning of string, as above) will match all characters "a" at the beginning of
		 string, leaving no "a" for "ab" to match.  In contrast, "a*ab" will match the
		 same as "a+b", since the match of the subgroup "a*" is influenced by the follow-
		 ing group "ab" (see "Backtracking").  In particular, "a*" inside "a*ab" will
		 match fewer characters than a standalone "a*", since this makes the tail match.

		 An effect similar to "(?>pattern)" may be achieved by writing "(?=(pattern))\1".
		 This matches the same substring as a standalone "a+", and the following "\1"
		 eats the matched string; it therefore makes a zero-length assertion into an ana-
		 logue of "(?>...)".  (The difference between these two constructs is that the
		 second one uses a capturing group, thus shifting ordinals of backreferences in
		 the rest of a regular expression.)

		 Consider this pattern:

		     m{ \(
			   (
			     [^()]+		 # x+
			   |
			     \( [^()]* \)
			   )+
			\)
		      }x

		 That will efficiently match a nonempty group with matching parentheses two lev-
		 els deep or less.  However, if there is no such group, it will take virtually
		 forever on a long string.  That's because there are so many different ways to
		 split a long string into several substrings.  This is what "(.+)+" is doing, and
		 "(.+)+" is similar to a subpattern of the above pattern.  Consider how the pat-
		 tern above detects no-match on "((()aaaaaaaaaaaaaaaaaa" in several seconds, but
		 that each extra letter doubles this time.  This exponential performance will
		 make it appear that your program has hung.  However, a tiny change to this pat-
		 tern

		     m{ \(
			   (
			     (?> [^()]+ )	 # change x+ above to (?> x+ )
			   |
			     \( [^()]* \)
			   )+
			\)
		      }x

		 which uses "(?>...)" matches exactly when the one above does (verifying this
		 yourself would be a productive exercise), but finishes in a fourth the time when
		 used on a similar string with 1000000 "a"s.  Be aware, however, that this pat-
		 tern currently triggers a warning message under the "use warnings" pragma or -w
		 switch saying it "matches null string many times in regex".

		 On simple groups, such as the pattern "(?> [^()]+ )", a comparable effect may be
		 achieved by negative look-ahead, as in "[^()]+ (?! [^()] )".  This was only 4
		 times slower on a string with 1000000 "a"s.

		 The "grab all you can, and do not give anything back" semantic is desirable in
		 many situations where on the first sight a simple "()*" looks like the correct
		 solution.  Suppose we parse text with comments being delimited by "#" followed
		 by some optional (horizontal) whitespace.  Contrary to its appearance, "#[ \t]*"
		 is not the correct subexpression to match the comment delimiter, because it may
		 "give up" some whitespace if the remainder of the pattern can be made to match
		 that way.  The correct answer is either one of these:

		     (?>#[ \t]*)
		     #[ \t]*(?![ \t])

		 For example, to grab non-empty comments into $1, one should use either one of
		 these:

		     / (?> \# [ \t]* ) (	.+ ) /x;
		     /	   \# [ \t]*   ( [^ \t] .* ) /x;

		 Which one you pick depends on which of these expressions better reflects the
		 above specification of comments.

       "(?(condition)yes-pattern|no-pattern)"
       "(?(condition)yes-pattern)"
		 WARNING: This extended regular expression feature is considered highly experi-
		 mental, and may be changed or deleted without notice.

		 Conditional expression.  "(condition)" should be either an integer in parenthe-
		 ses (which is valid if the corresponding pair of parentheses matched), or
		 look-ahead/look-behind/evaluate zero-width assertion.

		 For example:

		     m{ ( \( )?
			[^()]+
			(?(1) \) )
		      }x

		 matches a chunk of non-parentheses, possibly included in parentheses themselves.

       Backtracking

       NOTE: This section presents an abstract approximation of regular expression behavior.  For
       a more rigorous (and complicated) view of the rules involved in selecting a match among
       possible alternatives, see "Combining RE Pieces".

       A fundamental feature of regular expression matching involves the notion called backtrack-
       ing, which is currently used (when needed) by all regular non-possessive expression quan-
       tifiers, namely "*", "*?", "+", "+?", "{n,m}", and "{n,m}?".  Backtracking is often opti-
       mized internally, but the general principle outlined here is valid.

       For a regular expression to match, the entire regular expression must match, not just part
       of it.  So if the beginning of a pattern containing a quantifier succeeds in a way that
       causes later parts in the pattern to fail, the matching engine backs up and recalculates
       the beginning part--that's why it's called backtracking.

       Here is an example of backtracking:  Let's say you want to find the word following "foo"
       in the string "Food is on the foo table.":

	   $_ = "Food is on the foo table.";
	   if ( /\b(foo)\s+(\w+)/i ) {
	       print "$2 follows $1.\n";
	   }

       When the match runs, the first part of the regular expression ("\b(foo)") finds a possible
       match right at the beginning of the string, and loads up $1 with "Foo".	However, as soon
       as the matching engine sees that there's no whitespace following the "Foo" that it had
       saved in $1, it realizes its mistake and starts over again one character after where it
       had the tentative match.  This time it goes all the way until the next occurrence of
       "foo". The complete regular expression matches this time, and you get the expected output
       of "table follows foo."

       Sometimes minimal matching can help a lot.  Imagine you'd like to match everything between
       "foo" and "bar".  Initially, you write something like this:

	   $_ =  "The food is under the bar in the barn.";
	   if ( /foo(.*)bar/ ) {
	       print "got <$1>\n";
	   }

       Which perhaps unexpectedly yields:

	 got <d is under the bar in the >

       That's because ".*" was greedy, so you get everything between the first "foo" and the last
       "bar".  Here it's more effective to use minimal matching to make sure you get the text
       between a "foo" and the first "bar" thereafter.

	   if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
	 got <d is under the >

       Here's another example. Let's say you'd like to match a number at the end of a string, and
       you also want to keep the preceding part of the match.  So you write this:

	   $_ = "I have 2 numbers: 53147";
	   if ( /(.*)(\d*)/ ) { 			       # Wrong!
	       print "Beginning is <$1>, number is <$2>.\n";
	   }

       That won't work at all, because ".*" was greedy and gobbled up the whole string. As "\d*"
       can match on an empty string the complete regular expression matched successfully.

	   Beginning is <I have 2 numbers: 53147>, number is <>.

       Here are some variants, most of which don't work:

	   $_ = "I have 2 numbers: 53147";
	   @pats = qw{
	       (.*)(\d*)
	       (.*)(\d+)
	       (.*?)(\d*)
	       (.*?)(\d+)
	       (.*)(\d+)$
	       (.*?)(\d+)$
	       (.*)\b(\d+)$
	       (.*\D)(\d+)$
	   };

	   for $pat (@pats) {
	       printf "%-12s ", $pat;
	       if ( /$pat/ ) {
		   print "<$1> <$2>\n";
	       } else {
		   print "FAIL\n";
	       }
	   }

       That will print out:

	   (.*)(\d*)	<I have 2 numbers: 53147> <>
	   (.*)(\d+)	<I have 2 numbers: 5314> <7>
	   (.*?)(\d*)	<> <>
	   (.*?)(\d+)	<I have > <2>
	   (.*)(\d+)$	<I have 2 numbers: 5314> <7>
	   (.*?)(\d+)$	<I have 2 numbers: > <53147>
	   (.*)\b(\d+)$ <I have 2 numbers: > <53147>
	   (.*\D)(\d+)$ <I have 2 numbers: > <53147>

       As you see, this can be a bit tricky.  It's important to realize that a regular expression
       is merely a set of assertions that gives a definition of success.  There may be 0, 1, or
       several different ways that the definition might succeed against a particular string.  And
       if there are multiple ways it might succeed, you need to understand backtracking to know
       which variety of success you will achieve.

       When using look-ahead assertions and negations, this can all get even trickier.	Imagine
       you'd like to find a sequence of non-digits not followed by "123".  You might try to write
       that as

	   $_ = "ABC123";
	   if ( /^\D*(?!123)/ ) {	       # Wrong!
	       print "Yup, no 123 in $_\n";
	   }

       But that isn't going to match; at least, not the way you're hoping.  It claims that there
       is no 123 in the string.  Here's a clearer picture of why that pattern matches, contrary
       to popular expectations:

	   $x = 'ABC123';
	   $y = 'ABC445';

	   print "1: got $1\n" if $x =~ /^(ABC)(?!123)/;
	   print "2: got $1\n" if $y =~ /^(ABC)(?!123)/;

	   print "3: got $1\n" if $x =~ /^(\D*)(?!123)/;
	   print "4: got $1\n" if $y =~ /^(\D*)(?!123)/;

       This prints

	   2: got ABC
	   3: got AB
	   4: got ABC

       You might have expected test 3 to fail because it seems to a more general purpose version
       of test 1.  The important difference between them is that test 3 contains a quantifier
       ("\D*") and so can use backtracking, whereas test 1 will not.  What's happening is that
       you've asked "Is it true that at the start of $x, following 0 or more non-digits, you have
       something that's not 123?"  If the pattern matcher had let "\D*" expand to "ABC", this
       would have caused the whole pattern to fail.

       The search engine will initially match "\D*" with "ABC".  Then it will try to match
       "(?!123" with "123", which fails.  But because a quantifier ("\D*") has been used in the
       regular expression, the search engine can backtrack and retry the match differently in the
       hope of matching the complete regular expression.

       The pattern really, really wants to succeed, so it uses the standard pattern back-off-and-
       retry and lets "\D*" expand to just "AB" this time.  Now there's indeed something follow-
       ing "AB" that is not "123".  It's "C123", which suffices.

       We can deal with this by using both an assertion and a negation.  We'll say that the first
       part in $1 must be followed both by a digit and by something that's not "123".  Remember
       that the look-aheads are zero-width expressions--they only look, but don't consume any of
       the string in their match.  So rewriting this way produces what you'd expect; that is,
       case 5 will fail, but case 6 succeeds:

	   print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/;
	   print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/;

	   6: got ABC

       In other words, the two zero-width assertions next to each other work as though they're
       ANDed together, just as you'd use any built-in assertions:  "/^$/" matches only if you're
       at the beginning of the line AND the end of the line simultaneously.  The deeper underly-
       ing truth is that juxtaposition in regular expressions always means AND, except when you
       write an explicit OR using the vertical bar.  "/ab/" means match "a" AND (then) match "b",
       although the attempted matches are made at different positions because "a" is not a zero-
       width assertion, but a one-width assertion.

       WARNING: Particularly complicated regular expressions can take exponential time to solve
       because of the immense number of possible ways they can use backtracking to try for a
       match.  For example, without internal optimizations done by the regular expression engine,
       this will take a painfully long time to run:

	   'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/

       And if you used "*"'s in the internal groups instead of limiting them to 0 through 5
       matches, then it would take forever--or until you ran out of stack space.  Moreover, these
       internal optimizations are not always applicable.  For example, if you put "{0,5}" instead
       of "*" on the external group, no current optimization is applicable, and the match takes a
       long time to finish.

       A powerful tool for optimizing such beasts is what is known as an "independent group",
       which does not backtrack (see ""(?>pattern)"").	Note also that zero-length
       look-ahead/look-behind assertions will not backtrack to make the tail match, since they
       are in "logical" context: only whether they match is considered relevant.  For an example
       where side-effects of look-ahead might have influenced the following match, see ""(?>pat-
       tern)"".

       Version 8 Regular Expressions

       In case you're not familiar with the "regular" Version 8 regex routines, here are the pat-
       tern-matching rules not described above.

       Any single character matches itself, unless it is a metacharacter with a special meaning
       described here or above.  You can cause characters that normally function as metacharac-
       ters to be interpreted literally by prefixing them with a "\" (e.g., "\." matches a ".",
       not any character; "\\" matches a "\"). This escape mechanism is also required for the
       character used as the pattern delimiter.

       A series of characters matches that series of characters in the target string, so the pat-
       tern  "blurfl" would match "blurfl" in the target string.

       You can specify a character class, by enclosing a list of characters in "[]", which will
       match any one character from the list.  If the first character after the "[" is "^", the
       class matches any character not in the list.  Within a list, the "-" character specifies a
       range, so that "a-z" represents all characters between "a" and "z", inclusive.  If you
       want either "-" or "]" itself to be a member of a class, put it at the start of the list
       (possibly after a "^"), or escape it with a backslash.  "-" is also taken literally when
       it is at the end of the list, just before the closing "]".  (The following all specify the
       same class of three characters: "[-az]", "[az-]", and "[a\-z]".	All are different from
       "[a-z]", which specifies a class containing twenty-six characters, even on EBCDIC based
       coded character sets.)  Also, if you try to use the character classes "\w", "\W", "\s",
       "\S", "\d", or "\D" as endpoints of a range, that's not a range, the "-" is understood
       literally.

       Note also that the whole range idea is rather unportable between character sets--and even
       within character sets they may cause results you probably didn't expect.  A sound princi-
       ple is to use only ranges that begin from and end at either alphabetics of equal case
       ([a-e], [A-E]), or digits ([0-9]).  Anything else is unsafe.  If in doubt, spell out the
       character sets in full.

       Characters may be specified using a metacharacter syntax much like that used in C: "\n"
       matches a newline, "\t" a tab, "\r" a carriage return, "\f" a form feed, etc.  More gener-
       ally, \nnn, where nnn is a string of octal digits, matches the character whose coded char-
       acter set value is nnn.	Similarly, \xnn, where nn are hexadecimal digits, matches the
       character whose numeric value is nn. The expression \cx matches the character control-x.
       Finally, the "." metacharacter matches any character except "\n" (unless you use "/s").

       You can specify a series of alternatives for a pattern using "|" to separate them, so that
       "fee|fie|foe" will match any of "fee", "fie", or "foe" in the target string (as would
       "f(e|i|o)e").  The first alternative includes everything from the last pattern delimiter
       ("(", "[", or the beginning of the pattern) up to the first "|", and the last alternative
       contains everything from the last "|" to the next pattern delimiter.  That's why it's com-
       mon practice to include alternatives in parentheses: to minimize confusion about where
       they start and end.

       Alternatives are tried from left to right, so the first alternative found for which the
       entire expression matches, is the one that is chosen. This means that alternatives are not
       necessarily greedy. For example: when matching "foo|foot" against "barefoot", only the
       "foo" part will match, as that is the first alternative tried, and it successfully matches
       the target string. (This might not seem important, but it is important when you are cap-
       turing matched text using parentheses.)

       Also remember that "|" is interpreted as a literal within square brackets, so if you write
       "[fee|fie|foe]" you're really only matching "[feio|]".

       Within a pattern, you may designate subpatterns for later reference by enclosing them in
       parentheses, and you may refer back to the nth subpattern later in the pattern using the
       metacharacter \n.  Subpatterns are numbered based on the left to right order of their
       opening parenthesis.  A backreference matches whatever actually matched the subpattern in
       the string being examined, not the rules for that subpattern.  Therefore,
       "(0|0x)\d*\s\1\d*" will match "0x1234 0x4321", but not "0x1234 01234", because subpattern
       1 matched "0x", even though the rule "0|0x" could potentially match the leading 0 in the
       second number.

       Warning on \1 Instead of $1

       Some people get too used to writing things like:

	   $pattern =~ s/(\W)/\\\1/g;

       This is grandfathered for the RHS of a substitute to avoid shocking the sed addicts, but
       it's a dirty habit to get into.	That's because in PerlThink, the righthand side of an
       "s///" is a double-quoted string.  "\1" in the usual double-quoted string means a con-
       trol-A.	The customary Unix meaning of "\1" is kludged in for "s///".  However, if you get
       into the habit of doing that, you get yourself into trouble if you then add an "/e" modi-
       fier.

	   s/(\d+)/ \1 + 1 /eg;        # causes warning under -w

       Or if you try to do

	   s/(\d+)/\1000/;

       You can't disambiguate that by saying "\{1}000", whereas you can fix it with "${1}000".
       The operation of interpolation should not be confused with the operation of matching a
       backreference.  Certainly they mean two different things on the left side of the "s///".

       Repeated Patterns Matching a Zero-length Substring

       WARNING: Difficult material (and prose) ahead.  This section needs a rewrite.

       Regular expressions provide a terse and powerful programming language.  As with most other
       power tools, power comes together with the ability to wreak havoc.

       A common abuse of this power stems from the ability to make infinite loops using regular
       expressions, with something as innocuous as:

	   'foo' =~ m{ ( o? )* }x;

       The "o?" matches at the beginning of 'foo', and since the position in the string is not
       moved by the match, "o?" would match again and again because of the "*" quantifier.
       Another common way to create a similar cycle is with the looping modifier "//g":

	   @matches = ( 'foo' =~ m{ o? }xg );

       or

	   print "match: <$&>\n" while 'foo' =~ m{ o? }xg;

       or the loop implied by split().

       However, long experience has shown that many programming tasks may be significantly sim-
       plified by using repeated subexpressions that may match zero-length substrings.	Here's a
       simple example being:

	   @chars = split //, $string;		 # // is not magic in split
	   ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// /

       Thus Perl allows such constructs, by forcefully breaking the infinite loop.  The rules for
       this are different for lower-level loops given by the greedy quantifiers "*+{}", and for
       higher-level ones like the "/g" modifier or split() operator.

       The lower-level loops are interrupted (that is, the loop is broken) when Perl detects that
       a repeated expression matched a zero-length substring.	Thus

	  m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;

       is made equivalent to

	  m{   (?: NON_ZERO_LENGTH )*
	     |
	       (?: ZERO_LENGTH )?
	   }x;

       The higher level-loops preserve an additional state between iterations: whether the last
       match was zero-length.  To break the loop, the following match after a zero-length match
       is prohibited to have a length of zero.	This prohibition interacts with backtracking (see
       "Backtracking"), and so the second best match is chosen if the best match is of zero
       length.

       For example:

	   $_ = 'bar';
	   s/\w??/<$&>/g;

       results in "<><b><><a><><r><>".	At each position of the string the best match given by
       non-greedy "??" is the zero-length match, and the second best match is what is matched by
       "\w".  Thus zero-length matches alternate with one-character-long matches.

       Similarly, for repeated "m/()/g" the second-best match is the match at the position one
       notch further in the string.

       The additional state of being matched with zero-length is associated with the matched
       string, and is reset by each assignment to pos().  Zero-length matches at the end of the
       previous match are ignored during "split".

       Combining RE Pieces

       Each of the elementary pieces of regular expressions which were described before (such as
       "ab" or "\Z") could match at most one substring at the given position of the input string.
       However, in a typical regular expression these elementary pieces are combined into more
       complicated patterns using combining operators "ST", "S|T", "S*" etc (in these examples
       "S" and "T" are regular subexpressions).

       Such combinations can include alternatives, leading to a problem of choice: if we match a
       regular expression "a|ab" against "abc", will it match substring "a" or "ab"?  One way to
       describe which substring is actually matched is the concept of backtracking (see "Back-
       tracking").  However, this description is too low-level and makes you think in terms of a
       particular implementation.

       Another description starts with notions of "better"/"worse".  All the substrings which may
       be matched by the given regular expression can be sorted from the "best" match to the
       "worst" match, and it is the "best" match which is chosen.  This substitutes the question
       of "what is chosen?"  by the question of "which matches are better, and which are worse?".

       Again, for elementary pieces there is no such question, since at most one match at a given
       position is possible.  This section describes the notion of better/worse for combining
       operators.  In the description below "S" and "T" are regular subexpressions.

       "ST"
	   Consider two possible matches, "AB" and "A'B'", "A" and "A'" are substrings which can
	   be matched by "S", "B" and "B'" are substrings which can be matched by "T".

	   If "A" is better match for "S" than "A'", "AB" is a better match than "A'B'".

	   If "A" and "A'" coincide: "AB" is a better match than "AB'" if "B" is better match for
	   "T" than "B'".

       "S|T"
	   When "S" can match, it is a better match than when only "T" can match.

	   Ordering of two matches for "S" is the same as for "S".  Similar for two matches for
	   "T".

       "S{REPEAT_COUNT}"
	   Matches as "SSS...S" (repeated as many times as necessary).

       "S{min,max}"
	   Matches as "S{max}|S{max-1}|...|S{min+1}|S{min}".

       "S{min,max}?"
	   Matches as "S{min}|S{min+1}|...|S{max-1}|S{max}".

       "S?", "S*", "S+"
	   Same as "S{0,1}", "S{0,BIG_NUMBER}", "S{1,BIG_NUMBER}" respectively.

       "S??", "S*?", "S+?"
	   Same as "S{0,1}?", "S{0,BIG_NUMBER}?", "S{1,BIG_NUMBER}?" respectively.

       "(?>S)"
	   Matches the best match for "S" and only that.

       "(?=S)", "(?<=S)"
	   Only the best match for "S" is considered.  (This is important only if "S" has captur-
	   ing parentheses, and backreferences are used somewhere else in the whole regular
	   expression.)

       "(?!S)", "(?<!S)"
	   For this grouping operator there is no need to describe the ordering, since only
	   whether or not "S" can match is important.

       "(??{ EXPR })"
	   The ordering is the same as for the regular expression which is the result of EXPR.

       "(?(condition)yes-pattern|no-pattern)"
	   Recall that which of "yes-pattern" or "no-pattern" actually matches is already deter-
	   mined.  The ordering of the matches is the same as for the chosen subexpression.

       The above recipes describe the ordering of matches at a given position.	One more rule is
       needed to understand how a match is determined for the whole regular expression: a match
       at an earlier position is always better than a match at a later position.

       Creating Custom RE Engines

       Overloaded constants (see overload) provide a simple way to extend the functionality of
       the RE engine.

       Suppose that we want to enable a new RE escape-sequence "\Y|" which matches at a boundary
       between whitespace characters and non-whitespace characters.  Note that
       "(?=\S)(?<!\S)|(?!\S)(?<=\S)" matches exactly at these positions, so we want to have each
       "\Y|" in the place of the more complicated version.  We can create a module "customre" to
       do this:

	   package customre;
	   use overload;

	   sub import {
	     shift;
	     die "No argument to customre::import allowed" if @_;
	     overload::constant 'qr' => \&convert;
	   }

	   sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"}

	   # We must also take care of not escaping the legitimate \\Y|
	   # sequence, hence the presence of '\\' in the conversion rules.
	   my %rules = ( '\\' => '\\\\',
			 'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ );
	   sub convert {
	     my $re = shift;
	     $re =~ s{
		       \\ ( \\ | Y . )
		     }
		     { $rules{$1} or invalid($re,$1) }sgex;
	     return $re;
	   }

       Now "use customre" enables the new escape in constant regular expressions, i.e., those
       without any runtime variable interpolations.  As documented in overload, this conversion
       will work only over literal parts of regular expressions.  For "\Y|$re\Y|" the variable
       part of this regular expression needs to be converted explicitly (but only if the special
       meaning of "\Y|" should be enabled inside $re):

	   use customre;
	   $re = <>;
	   chomp $re;
	   $re = customre::convert $re;
	   /\Y|$re\Y|/;

BUGS
       This document varies from difficult to understand to completely and utterly opaque.  The
       wandering prose riddled with jargon is hard to fathom in several places.

       This document needs a rewrite that separates the tutorial content from the reference con-
       tent.

SEE ALSO
       perlrequick.

       perlretut.

       "Regexp Quote-Like Operators" in perlop.

       "Gory details of parsing quoted constructs" in perlop.

       perlfaq6.

       "pos" in perlfunc.

       perllocale.

       perlebcdic.

       Mastering Regular Expressions by Jeffrey Friedl, published by O'Reilly and Associates.

perl v5.8.9				    2007-11-17					PERLRE(1)
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