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

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

Modifiers

Matching operations can have various modifiers. Modifiersthat relate to the interpretation of the regular expression insideare listed below. Modifiers that alter the way a regular expressionis used by Perl are detailed in Regexp Quote-Like Operators in perlop andGory details of parsing quoted constructs in perlop.

m
Treat string as multiple lines. That is, change "^" and "$" from matchingthe start or end of the string to matching the start or end of anyline anywhere within the string.
s
Treat string as single line. That is, change "." to match any characterwhatsoever, even a newline, which normally it would not match.
Used together, as /ms, they let the "." match any character whatsoever,while still allowing "^" and "$" to match, respectively, just afterand just before newlines within the string.
i
Do case-insensitive pattern matching.
If locale matching rules are in effect, the case map is taken from thecurrentlocale for code points less than 255, and from Unicode rules for largercode points. However, matches that would cross the Unicoderules/non-Unicode rules boundary (ords 255/256) will not succeed. Seeperllocale.
There are a number of Unicode characters that match multiple charactersunder /i. For example, LATIN SMALL LIGATURE FIshould match the sequence fi. Perl is notcurrently able to do this when the multiple characters are in the pattern andare split between groupings, or when one or more are quantified. Thus
```
 "\N{LATIN SMALL LIGATURE FI}" =~ /fi/i;  # Matches
  "\N{LATIN SMALL LIGATURE FI}" =~ /[fi][fi]/i; # Doesn't match!
  "\N{LATIN SMALL LIGATURE FI}" =~ /fi*/i; # Doesn't match!
 
  # The below doesn't match, and it isn't clear what $1 and $2 would
  # be even if it did!!
  "\N{LATIN SMALL LIGATURE FI}" =~ /(f)(i)/i;  # Doesn't match!
 
```
Perl doesn't match multiple characters in an inverted bracketedcharacter class, which otherwise could be highly confusing. SeeNegation in perlrecharclass.
Another bug involves character classes that match both a sequence ofmultiple characters, and an initial sub-string of that sequence. Forexample,
```
 /[s\xDF]/i
 
```
should match both a single and a double "s", since \xDF (on ASCIIplatforms) matches "ss". However, this bug([perl #89774])causes it to only match a single "s", even if the final larger matchfails, and matching the double "ss" would have succeeded.
Also, Perl matching doesn't fully conform to the current Unicode /irecommendations, which ask that the matching be made upon the NFD(Normalization Form Decomposed) of the text. However, Unicode isin the process of reconsidering and revising their recommendations.
x
Extend your pattern's legibility by permitting whitespace and comments.Details in /x
p
Preserve the string matched such that ${^PREMATCH}, ${^MATCH}, and${^POSTMATCH} are available for use after matching.
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 usedrather than the regex itself. SeeUsing regular expressions in Perl in perlretut for further explanationof the g and c modifiers.
a, d, l and u
These modifiers, all new in 5.14, affect which character-set semantics(Unicode, etc.) are used, as described below inCharacter set modifiers.

Regular expression modifiers are usually written in documentationas e.g., "the /x modifier", even though the delimiterin question might not really be a slash. The modifiers /imsxadlupmay also be embedded within the regular expression itself usingthe (?...) construct, see Extended Patterns below.

/x

/x tellsthe regular expression parser to ignore most whitespace that is neitherbackslashed nor within a character class. You can use this to break upyour regular expression into (slightly) more readable parts. The #character is also treated as a metacharacter introducing a comment,just as in ordinary Perl code. This also means that if you want realwhitespace or # characters in the pattern (outside a characterclass, where they are unaffected by /x), then you'll either have toescape them (using backslashes or \Q...\E) or encode them using octal,hex, or \N{} escapes. Taken together, these features go a long way towardsmaking Perl's regular expressions more readable. Note that you have tobe careful not to include the pattern delimiter in the comment--perl hasno way of knowing you did not intend to close the pattern early. Seethe C-comment deletion code in perlop. Also note that anything insidea \Q...\E stays unaffected by /x. And note that /x doesn't affectspace interpretation within a single multi-character construct. Forexample in \x{...}, regardless of the /x modifier, there can be nospaces. Same for a quantifier such as {3} or{5,}. Similarly, (?:...) can't have a space between the ? and :,but can between the ( and ?. Within any delimiters for such aconstruct, allowed spaces are not affected by /x, and depend on theconstruct. For example, \x{...} can't have spaces because hexadecimalnumbers don't have spaces in them. But, Unicode properties can have spaces, soin \p{...} there can be spaces that follow the Unicode rules, for which seeProperties accessible through \p{} and \P{} in perluniprops.

Character set modifiers

/d, /u, /a, and /l, available starting in 5.14, are calledthe character set modifiers; they affect the character set semanticsused for the regular expression.

The /d, /u, and /l modifiers are not likely to be of much useto you, and so you need not worry about them very much. They exist forPerl's internal use, so that complex regular expression data structurescan be automatically serialized and later exactly reconstituted,including all their nuances. But, since Perl can't keep a secret, andthere may be rare instances where they are useful, they are documentedhere.

The /a modifier, on the other hand, may be useful. Its purpose is toallow code that is to work mostly on ASCII data to not have to concernitself with Unicode.

Briefly, /l sets the character set to that of whatever Locale is ineffect at the time of the execution of the pattern match.

/u sets the character set to Unicode.

/a also sets the character set to Unicode, BUT adds severalrestrictions for ASCII-safe matching.

/d is the old, problematic, pre-5.14 Default character setbehavior. Its only use is to force that old behavior.

At any given time, exactly one of these modifiers is in effect. Theirexistence allows Perl to keep the originally compiled behavior of aregular expression, regardless of what rules are in effect when it isactually executed. And if it is interpolated into a larger regex, theoriginal's rules continue to apply to it, and only it.

The /l and /u modifiers are automatically selected forregular expressions compiled within the scope of various pragmas,and we recommend that in general, you use those pragmas instead ofspecifying these modifiers explicitly. For one thing, the modifiersaffect only pattern matching, and do not extend to even any replacementdone, whereas using the pragmas give consistent results for allappropriate operations within their scopes. For example,

 s/foo/\Ubar/il

will match "foo" using the locale's rules for case-insensitive matching,but the /l does not affect how the \U operates. Most likely youwant both of them to use locale rules. To do this, instead compile theregular expression within the scope of use locale. This bothimplicitly adds the /l and applies locale rules to the \U. Thelesson is to use locale and not /l explicitly.

Similarly, it would be better to use use feature 'unicode_strings'instead of,

 s/foo/\Lbar/iu

to get Unicode rules, as the \L in the former (but not necessarilythe latter) would also use Unicode rules.

More detail on each of the modifiers follows. Most likely you don'tneed to know this detail for /l, /u, and /d, and can skip aheadto /a.

/l

means to use the current locale's rules (see perllocale) when patternmatching. For example, \w will match the "word" characters of thatlocale, and "/i" case-insensitive matching will match according tothe locale's case folding rules. The locale used will be the one ineffect at the time of execution of the pattern match. This may not bethe same as the compilation-time locale, and can differ from one matchto another if there is an intervening call of thesetlocale() function.

Perl only supports single-byte locales. This means that code pointsabove 255 are treated as Unicode no matter what locale is in effect.Under Unicode rules, there are a few case-insensitive matches that crossthe 255/256 boundary. These are disallowed under /l. For example,0xFF (on ASCII platforms) does not caselessly match the character at0x178, LATIN CAPITAL LETTER Y WITH DIAERESIS, because 0xFF may not beLATIN SMALL LETTER Y WITH DIAERESIS in the current locale, and Perlhas no way of knowing if that character even exists in the locale, muchless what code point it is.

This modifier may be specified to be the default by use locale, butsee Which character set modifier is in effect?.

/u

means to use Unicode rules when pattern matching. On ASCII platforms,this means that the code points between 128 and 255 take on theirLatin-1 (ISO-8859-1) meanings (which are the same as Unicode's).(Otherwise Perl considers their meanings to be undefined.) Thus,under this modifier, the ASCII platform effectively becomes a Unicodeplatform; and hence, for example, \w will match any of the more than100_000 word characters in Unicode.

Unlike most locales, which are specific to a language and country pair,Unicode classifies all the characters that are letters somewhere inthe world as\w. For example, your locale might not think that LATIN SMALLLETTER ETH is a letter (unless you happen to speak Icelandic), butUnicode does. Similarly, all the characters that are decimal digitssomewhere in the world will match \d; this is hundreds, not 10,possible matches. And some of those digits look like some of the 10ASCII digits, but mean a different number, so a human could easily thinka number is a different quantity than it really is. For example,BENGALI DIGIT FOUR (U+09EA) looks very much like anASCII DIGIT EIGHT (U+0038). And, \d+, may match strings of digitsthat are a mixture from different writing systems, creating a securityissue. num() in Unicode::UCD can be used to sortthis out. Or the /a modifier can be used to force \d to matchjust the ASCII 0 through 9.

Also, under this modifier, case-insensitive matching works on the fullset of Unicodecharacters. The KELVIN SIGN, for example matches the letters "k" and"K"; and LATIN SMALL LIGATURE FF matches the sequence "ff", which,if you're not prepared, might make it look like a hexadecimal constant,presenting another potential security issue. Seehttp://unicode.org/reports/tr36 for a detailed discussion of Unicodesecurity issues.

On the EBCDIC platforms that Perl handles, the native character set isequivalent to Latin-1. Thus this modifier changes behavior only whenthe "/i" modifier is also specified, and it turns out it affects onlytwo characters, giving them full Unicode semantics: the MICRO SIGNwill match the Greek capital and small letters MU, otherwise not; andthe LATIN CAPITAL LETTER SHARP S will match any of SS, Ss,sS, and ss, otherwise not.

This modifier may be specified to be the default by use feature'unicode_strings, use locale ':not_characters', oruse VERSION (or higher),but see Which character set modifier is in effect?.

/d

This modifier means to use the "Default" native rules of the platformexcept when there is cause to use Unicode rules instead, as follows:

1: the target string is encoded in UTF-8; or
2: the pattern is encoded in UTF-8; or
3: the pattern explicitly mentions a code point that is above 255 (say by\x{100}); or
4: the pattern uses a Unicode name (\N{...}); or
5: the pattern uses a Unicode property (\p{...})

Another mnemonic for this modifier is "Depends", as the rules actuallyused depend on various things, and as a result you can get unexpectedresults. See The Unicode Bug in perlunicode. The Unicode Bug hasbecome rather infamous, leading to yet another (printable) name for thismodifier, "Dodgy".

On ASCII platforms, the native rules are ASCII, and on EBCDIC platforms(at least the ones that Perl handles), they are Latin-1.

Here are some examples of how that works on an ASCII platform:

 $str =  "\xDF";  # $str is not in UTF-8 format.
  $str =~ /^\w/;   # No match, as $str isn't in UTF-8 format.
  $str .= "\x{0e0b}";  # Now $str is in UTF-8 format.
  $str =~ /^\w/;   # Match! $str is now in UTF-8 format.
  chop $str;
  $str =~ /^\w/;   # Still a match! $str remains in UTF-8 format.

This modifier is automatically selected by default when none of theothers are, so yet another name for it is "Default".

Because of the unexpected behaviors associated with this modifier, youprobably should only use it to maintain weird backward compatibilities.

/a (and /aa)

This modifier stands for ASCII-restrict (or ASCII-safe). This modifier,unlike the others, may be doubled-up to increase its effect.

When it appears singly, it causes the sequences \d, \s, \w, andthe Posix character classes to match only in the ASCII range. They thusrevert to their pre-5.6, pre-Unicode meanings. Under /a, \dalways means precisely the digits "0" to "9"; \s means the fivecharacters [ \f\n\r\t]; \w means the 63 characters[A-Za-z0-9_]; and likewise, all the Posix classes such as[[:print:]] match only the appropriate ASCII-range characters.

This modifier is useful for people who only incidentally use Unicode,and who do not wish to be burdened with its complexities and securityconcerns.

With /a, one can write \d with confidence that it will only matchASCII characters, and should the need arise to match beyond ASCII, youcan instead use \p{Digit} (or \p{Word} for \w). There aresimilar \p{...} constructs that can match beyond ASCII both whitespace (see Whitespace in perlrecharclass), and Posix classes (seePOSIX Character Classes in perlrecharclass). Thus, this modifierdoesn't mean you can't use Unicode, it means that to get Unicodematching you must explicitly use a construct (\p{}, \P{}) thatsignals Unicode.

As you would expect, this modifier causes, for example, \D to meanthe same thing as [^0-9]; in fact, all non-ASCII characters match\D, \S, and \W. \b still means to match at the boundarybetween \w and \W, using the /a definitions of them (similarlyfor \B).

Otherwise, /a behaves like the /u modifier, in thatcase-insensitive matching uses Unicode semantics; for example, "k" willmatch the Unicode \N{KELVIN SIGN} under /i matching, and codepoints in the Latin1 range, above ASCII will have Unicode rules when itcomes to case-insensitive matching.

To forbid ASCII/non-ASCII matches (like "k" with \N{KELVIN SIGN}),specify the "a" twice, for example /aai or /aia. (The firstoccurrence of "a" restricts the \d, etc., and the second occurrenceadds the /i restrictions.) But, note that code points outside theASCII range will use Unicode rules for /i matching, so the modifierdoesn't really restrict things to just ASCII; it just forbids theintermixing of ASCII and non-ASCII.

To summarize, this modifier provides protection for applications thatdon't wish to be exposed to all of Unicode. Specifying it twicegives added protection.

This modifier may be specified to be the default by use re '/a'or use re '/aa'. If you do so, you may actually have occasion to usethe /u modifier explictly if there are a few regular expressionswhere you do want full Unicode rules (but even here, it's best ifeverything were under feature "unicode_strings", along with theuse re '/aa'). Also see Which character set modifier is in effect?.

Which character set modifier is in effect?

Which of these modifiers is in effect at any given point in a regularexpression depends on a fairly complex set of interactions. These havebeen designed so that in general you don't have to worry about it, butthis section gives the gory details. Asexplained below in Extended Patterns it is possible to explicitlyspecify modifiers that apply only to portions of a regular expression.The innermost always has priority over any outer ones, and one applyingto the whole expression has priority over any of the default settings that aredescribed in the remainder of this section.

The use re '/foo' pragma can be used to setdefault modifiers (including these) for regular expressions compiledwithin its scope. This pragma has precedence over the other pragmaslisted below that also change the defaults.

Otherwise, use locale sets the default modifier to /l;and use feature 'unicode_strings, oruse VERSION (or higher) set the default to/u when not in the same scope as either use localeor use bytes.(use locale ':not_characters' alsosets the default to /u, overriding any plain use locale.)Unlike the mechanisms mentioned above, theseaffect operations besides regular expressions pattern matching, and sogive more consistent results with other operators, including using\U, \l, etc. in substitution replacements.

If none of the above apply, for backwards compatibility reasons, the/d modifier is the one in effect by default. As this can lead tounexpected results, it is best to specify which other rule set should beused.

Character set modifier behavior prior to Perl 5.14

Prior to 5.14, there were no explicit modifiers, but /l was impliedfor regexes compiled within the scope of use locale, and /d wasimplied otherwise. However, interpolating a regex into a larger regexwould ignore the original compilation in favor of whatever was in effectat the time of the second compilation. There were a number ofinconsistencies (bugs) with the /d modifier, where Unicode ruleswould be used when inappropriate, and vice versa. \p{} did not implyUnicode rules, and neither did all occurrences of \N{}, until 5.12.

Regular Expressions

Metacharacters

The patterns used in Perl pattern matching evolved from those supplied inthe Version 8 regex routines. (The routines are derived(distantly) from Henry Spencer's freely redistributable reimplementationof the V8 routines.) See Version 8 Regular Expressions fordetails.

In particular the following metacharacters have their standard egrep-ishmeanings:

 \ 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
  []   Bracketed Character class

By default, the "^" character is guaranteed to match only thebeginning of the string, the "$" character only the end (or before thenewline at the end), and Perl does certain optimizations with theassumption that the string contains only one line. Embedded newlineswill not be matched by "^" or "$". You may, however, wish to treat astring as a multi-line buffer, such that the "^" will match after anynewline within the string (except if the newline is the last character inthe string), and "$" will match before any newline. At thecost of a little more overhead, you can do this by using the /m modifieron the pattern match operator. (Older programs did this by setting $*,but this option was removed in perl 5.9.)

To simplify multi-line substitutions, the "." character never matches anewline unless you use the /s modifier, which in effect tells Perl to pretendthe string is a single line--even if it isn't.

Quantifiers

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 and does not form part ofa backslashed sequence like \x{...}, it is treatedas a regular character. In particular, the lower quantifier boundis not optional. However, in Perl v5.18, it is planned to issue adeprecation warning for all such occurrences, and in Perl v5.20 torequire literal uses of a curly bracket to be escaped, say by precedingthem with a backslash or enclosing them within square brackets, ("\{"or "[{]"). This change will allow for future syntax extensions (likemaking the lower bound of a quantifier optional), and better errorchecking of quantifiers. Now, a typo in a quantifier silently causesit to be treated as the literal characters. For example,

 /o{4,3}/

looks like a quantifier that matches 0 times, since 4 is greater than 3,but it really means to match the sequence of six characters"o { 4 , 3 }".)

The "*" quantifier is equivalent to {0,}, the "+"quantifier to {1,}, and the "?" quantifier to {0,1}. n and m are limitedto non-negative integral values less than a preset limit defined when perl is built.This is usually 32766 on the most common platforms. The actual limit canbe 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 asmany times as possible (given a particular starting location) while stillallowing the rest of the pattern to match. If you want it to match theminimum number of times possible, follow the quantifier with a "?". Notethat 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 (redundant)
   {n,}? Match at least n times, not greedily
  {n,m}? Match at least n but not more than m times, not greedily

By default, when a quantified subpattern does not allow the rest of theoverall pattern to match, Perl will backtrack. However, this behaviour issometimes undesirable. Thus Perl provides the "possessive" quantifier formas well.

 *+ Match 0 or more times and give nothing back
  ++ Match 1 or more times and give nothing back
  ?+ Match 0 or 1 time and give nothing back
  {n}+   Match exactly n times and give nothing back (redundant)
  {n,}+  Match at least n times and give nothing back
  {n,m}+ Match at least n but not more than m times and give nothing back

For instance,

   'aaaa' =~ /a++a/

will never match, as the a++ will gobble up all the a's in thestring and won't leave any for the remaining part of the pattern. Thisfeature can be extremely useful to give perl hints about where itshouldn't backtrack. For instance, the typical "match a double-quotedstring" problem can be most efficiently performed when written as:

   /"(?:[^"\]++|\.)*+"/

as we know that if the final quote does not match, backtracking will nothelp. See the independent subexpression(?>pattern) for more details;possessive quantifiers are just syntactic sugar for that construct. Forinstance the above example could also be written as follows:

   /"(?>(?:(?>[^"\]+)|\.)*)"/

Escape sequences

Because patterns are processed as double-quoted strings, the followingalso 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)
  \cK control char  (example: VT)
  \x{}, \x00  character whose ordinal is the given hexadecimal number
  \N{name} named Unicode character or character sequence
  \N{U+263D}  Unicode character (example: FIRST QUARTER MOON)
  \o{}, \000  character whose ordinal is the given octal number
  \l  lowercase next char (think vi)
  \u  uppercase next char (think vi)
  \L  lowercase till \E (think vi)
  \U  uppercase till \E (think vi)
  \Q  quote (disable) pattern metacharacters till \E
  \E  end either case modification or quoted section, think vi

Details are in Quote and Quote-like Operators in perlop.

Character Classes and other Special Escapes

In addition, Perl defines the following:

 Sequence   Note Description
   [...] [1]  Match a character according to the rules of the
    bracketed character class defined by the "...".
    Example: [a-z] matches "a" or "b" or "c" ... or "z"
   [[:...:]] [2]  Match a character according to the rules of the POSIX
    character class "..." within the outer bracketed
    character class.  Example: [[:upper:]] matches any
    uppercase character.
   \w [3]  Match a "word" character (alphanumeric plus "_", plus
    other connector punctuation chars plus Unicode
    marks)
   \W [3]  Match a non-"word" character
   \s [3]  Match a whitespace character
     \S [3]  Match a non-whitespace character
   \d [3]  Match a decimal digit character
   \D [3]  Match a non-digit character
   \pP   [3]  Match P, named property.  Use \p{Prop} for longer names
   \PP   [3]  Match non-P
   \X [4]  Match Unicode "eXtended grapheme cluster"
   \C Match a single C-language char (octet) even if that is
    part of a larger UTF-8 character.  Thus it breaks up
    characters into their UTF-8 bytes, so you may end up
    with malformed pieces of UTF-8.  Unsupported in
    lookbehind.
   \1 [5]  Backreference to a specific capture group or buffer.
    '1' may actually be any positive integer.
   \g1   [5]  Backreference to a specific or previous group,
   \g{-1} [5]  The number may be negative indicating a relative
    previous group and may optionally be wrapped in
    curly brackets for safer parsing.
   \g{name}  [5]  Named backreference
   \k<name>  [5]  Named backreference
   \K [6]  Keep the stuff left of the \K, don't include it in $&
   \N [7]  Any character but \n (experimental).  Not affected by
    /s modifier
     \v [3]  Vertical whitespace
     \V [3]  Not vertical whitespace
     \h [3]  Horizontal whitespace
     \H [3]  Not horizontal whitespace
     \R [4]  Linebreak

[1]
See Bracketed Character Classes in perlrecharclass for details.
[2]
See POSIX Character Classes in perlrecharclass for details.
[3]
See Backslash sequences in perlrecharclass for details.
[4]
See Misc in perlrebackslash for details.
[5]
See Capture groups below for details.
[6]
See Extended Patterns below for details.
[7]
Note that \N has two meanings. When of the form \N{NAME}, it matches thecharacter or character sequence whose name is NAME; and similarlywhen of the form \N{U+hex}, it matches the character whose Unicodecode point is hex. Otherwise it matches any character but \n.

Assertions

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

The \G assertion can be used to chain global matches (usingm//g), as described in Regexp Quote-Like Operators in perlop.It is also useful when writing lex-like scanners, when you haveseveral patterns that you want to match against consequent substringsof your string; see the previous reference. The actual locationwhere \G will match can also be influenced by using pos() asan lvalue: see pos. Note that the rule for zero-lengthmatches (see Repeated Patterns Matching a Zero-length Substring)is modified somewhat, in that contents to the left of \G arenot counted when determining the length of the match. Thus the followingwill not match forever:

 my $string = 'ABC';
  pos($string) = 1;
  while ($string =~ /(.\G)/g) {
  print $1;
  }

It will print 'A' and then terminate, as it considers the match tobe zero-width, and thus will not match at the same position twice in arow.

It is worth noting that \G improperly used can result in an infiniteloop. Take care when using patterns that include \G in an alternation.

Capture groups

The bracketing construct ( ... ) creates capture groups (also referred to ascapture buffers). To refer to the current contents of a group later on, withinthe same pattern, use \g1 (or \g{1}) for the first, \g2 (or \g{2})for the second, and so on.This is called a backreference. There is no limit to the number of captured substrings that you may use.Groups are numbered with the leftmost open parenthesis being number 1, etc. Ifa group did not match, the associated backreference won't match either. (Thiscan happen if the group is optional, or in a different branch of analternation.)You can omit the "g", and write "\1", etc, but there are some issues withthis form, described below.

You can also refer to capture groups relatively, by using a negative number, sothat \g-1 and \g{-1} both refer to the immediately preceding capturegroup, and \g-2 and \g{-2} both refer to the group before it. Forexample:

 /
   (Y) # group 1
   (  # group 2
   (X) # group 3
   \g{-1}  # backref to group 3
   \g{-3}  # backref to group 1
   )
   /x

would match the same as /(Y) ( (X) \g3 \g1 )/x. This allows you tointerpolate regexes into larger regexes and not have to worry about thecapture groups being renumbered.

You can dispense with numbers altogether and create named capture groups.The notation is (?<name>...) to declare and \g{name} toreference. (To be compatible with .Net regular expressions, \g{name} mayalso be written as \k{name}, \k<name> or \k'name'.)name must not begin with a number, nor contain hyphens.When different groups within the same pattern have the same name, any referenceto that name assumes the leftmost defined group. Named groups count inabsolute and relative numbering, and so can also be referred to by thosenumbers.(It's possible to do things with named capture groups that would otherwiserequire (??{}).)

Capture group contents are dynamically scoped and available to you outside thepattern until the end of the enclosing block or until the next successfulmatch, whichever comes first. (See Compound Statements in perlsyn.)You can refer to them by absolute number (using "$1" instead of "\g1",etc); or by name via the %+ hash, using "$+{name}".

Braces are required in referring to named capture groups, but are optional forabsolute or relative numbered ones. Braces are safer when creating a regex byconcatenating smaller strings. For example if you have qr/$a$b/, and $acontained "\g1", and $b contained "37", you would get /\g137/ whichis probably not what you intended.

The \g and \k notations were introduced in Perl 5.10.0. Prior to thatthere were no named nor relative numbered capture groups. Absolute numberedgroups were referred to using \1,\2, etc., and this notation is stillaccepted (and likely always will be). But it leads to some ambiguities ifthere are more than 9 capture groups, as \10 could mean either the tenthcapture group, or the character whose ordinal in octal is 010 (a backspace inASCII). Perl resolves this ambiguity by interpreting \10 as a backreferenceonly if at least 10 left parentheses have opened before it. Likewise \11 isa backreference only if at least 11 left parentheses have opened before it.And so on. \1 through \9 are always interpreted as backreferences.There are several examples below that illustrate these perils. You can avoidthe ambiguity by always using \g{} or \g if you mean capturing groups;and for octal constants always using \o{}, or for \077 and below, using 3digits padded with leading zeros, since a leading zero implies an octalconstant.

The \digit notation also works in certain circumstances outsidethe pattern. See Warning on \1 Instead of $1 below for details.

Examples:

 s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
 
  /(.)\g1/ # find first doubled char
  and print "'$1' is the first doubled character\n";
 
  /(?<char>.)\k<char>/ # ... a different way
  and print "'$+{char}' is the first doubled character\n";
 
  /(?'char'.)\g1/ # ... mix and match
  and print "'$1' is the first doubled character\n";
 
  if (/Time: (..):(..):(..)/) {   # parse out values
  $hours = $1;
  $minutes = $2;
  $seconds = $3;
  }
 
  /(.)(.)(.)(.)(.)(.)(.)(.)(.)\g10/   # \g10 is a backreference
  /(.)(.)(.)(.)(.)(.)(.)(.)(.)\10/ # \10 is octal
  /((.)(.)(.)(.)(.)(.)(.)(.)(.))\10/  # \10 is a backreference
  /((.)(.)(.)(.)(.)(.)(.)(.)(.))\010/ # \010 is octal
 
  $a = '(.)\1'; # Creates problems when concatenated.
  $b = '(.)\g{1}'; # Avoids the problems.
  "aa" =~ /${a}/;  # True
  "aa" =~ /${b}/;  # True
  "aa0" =~ /${a}0/; # False!
  "aa0" =~ /${b}0/; # True
  "aa\x08" =~ /${a}0/;  # True!
  "aa\x08" =~ /${b}0/;  # False

Several special variables also refer back to portions of the previousmatch. $+ returns whatever the last bracket match matched.$& returns the entire matched string. (At one point $0 didalso, but now it returns the name of the program.) $` returnseverything before the matched string. $' returns everythingafter the matched string. And $^N contains whatever was matched bythe most-recently closed group (submatch). $^N can be used inextended patterns (see below), for example to assign a submatch to avariable.

These special variables, like the %+ hash and the numbered match variables($1, $2, $3, etc.) are dynamically scopeduntil the end of the enclosing block or until the next successfulmatch, 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 morespecific 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 everypattern match. This may substantially slow your program. Perluses the same mechanism to produce $1, $2, etc, so you also pay aprice for each pattern that contains capturing parentheses. (Toavoid this cost while retaining the grouping behaviour, use theextended regular expression (?: ... ) instead.) But if you neveruse $&, $` or $', then patterns without capturingparentheses will not be penalized. So avoid $&, $', and $`if you can, but if you can't (and some algorithms really appreciatethem), once you've used them once, use them at will, because you'vealready paid the price. As of 5.005, $& is not so costly as theother two.

As a workaround for this problem, Perl 5.10.0 introduces ${^PREMATCH},${^MATCH} and ${^POSTMATCH}, which are equivalent to $`, $&and $', except that they are only guaranteed to be defined after asuccessful match that was executed with the /p (preserve) modifier.The use of these variables incurs no global performance penalty, unliketheir punctuation char equivalents, however at the trade-off that youhave to tell perl when you want to use them.

Quoting metacharacters

Backslashed metacharacters in Perl are alphanumeric, such as \b,\w, \n. Unlike some other regular expression languages, thereare no backslashed symbols that aren't alphanumeric. So anythingthat looks like \, $, $, \<, \>, \{, or \} is alwaysinterpreted as a literal character, not a metacharacter. This wasonce used in a common idiom to disable or quote the special meaningsof regular expression metacharacters in a string that you want touse 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 common to use the quotemeta() function or the \Qmetaquoting escape sequence to disable all metacharacters' specialmeanings like this:

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

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

quotemeta() and \Q are fully described in quotemeta.

Extended Patterns

Perl also defines a consistent extension syntax for features notfound in standard tools like awk andlex. The syntax for most of these is apair of parentheses with a question mark as the first thing withinthe parentheses. The character after the question mark indicatesthe extension.

The stability of these extensions varies widely. Some have beenpart of the core language for many years. Others are experimentaland may change without warning or be completely removed. Checkthe documentation on an individual feature to verify its currentstatus.

A question mark was chosen for this and for the minimal-matchingconstruct because 1) question marks are rare in older regularexpressions, 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 enableswhitespace formatting, a simple # will suffice. Note that Perl closesthe comment as soon as it sees a ), so there is no way to put a literal) in the comment.
(?adlupimsx-imsx)
(?^alupimsx)
One or more embedded pattern-match modifiers, to be turned on (orturned off, if preceded by -) for the remainder of the pattern orthe remainder of the enclosing pattern group (if any).
This is particularly useful for dynamic patterns, such as those read in from aconfiguration file, taken from an argument, or specified in a tablesomewhere. Consider the case where some patterns want to becase-sensitive and some do not: The case-insensitive ones merely need toinclude (?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+ \g1
 
```
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 /imodifier outside this group.
These modifiers do not carry over into named subpatterns called in theenclosing group. In other words, a pattern such as ((?i)(?&NAME)) does notchange the case-sensitivity of the "NAME" pattern.
Any of these modifiers can be set to apply globally to all regularexpressions compiled within the scope of a use re. See'/flags' mode in re.
Starting in Perl 5.14, a "^" (caret or circumflex accent) immediatelyafter the "?" is a shorthand equivalent to d-imsx. Flags (except"d") may follow the caret to override it.But a minus sign is not legal with it.
Note that the a, d, l, p, and u modifiers are special inthat they can only be enabled, not disabled, and the a, d, l, andu modifiers are mutually exclusive: specifying one de-specifies theothers, and a maximum of one (or two a's) may appear in theconstruct. Thus, forexample, (?-p) will warn when compiled under use warnings;(?-d:...) and (?dl:...) are fatal errors.
Note also that the p modifier is special in that its presenceanywhere in a pattern has a global effect.
(?:pattern)
(?adluimsx-imsx:pattern)
(?^aluimsx: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 capturecharacters if you don't need to.
Any letters between ? and : act as flags modifiers as with(?adluimsx-imsx). For example,
```
 /(?s-i:more.*than).*million/i
 
```
is equivalent to the more verbose
```
 /(?:(?s-i)more.*than).*million/i
 
```
Starting in Perl 5.14, a "^" (caret or circumflex accent) immediatelyafter the "?" is a shorthand equivalent to d-imsx. Any positiveflags (except "d") may follow the caret, so
```
 (?^x:foo)
 
```
is equivalent to
```
 (?x-ims:foo)
 
```
The caret tells Perl that this cluster doesn't inherit the flags of anysurrounding pattern, but uses the system defaults (d-imsx),modified by any flags specified.
The caret allows for simpler stringification of compiled regularexpressions. These look like
```
 (?^:pattern)
 
```
with any non-default flags appearing between the caret and the colon.A test that looks at such stringification thus doesn't need to have thesystem default flags hard-coded in it, just the caret. If new flags areadded to Perl, the meaning of the caret's expansion will change to includethe default for those flags, so the test will still work, unchanged.
Specifying a negative flag after the caret is an error, as the flag isredundant.
Mnemonic for (?^...): A fresh beginning since the usual use of a caret isto match at the beginning.
(?|pattern)
This is the "branch reset" pattern, which has the special propertythat the capture groups are numbered from the same starting pointin each alternation branch. It is available starting from perl 5.10.0.
Capture groups are numbered from left to right, but inside thisconstruct the numbering is restarted for each branch.
The numbering within each branch will be as normal, and any groupsfollowing this construct will be numbered as though the constructcontained only one branch, that being the one with the most capturegroups in it.
This construct is useful when you want to capture one of anumber of alternative matches.
Consider the following pattern. The numbers underneath show inwhich group 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
 
```
Be careful when using the branch reset pattern in combination with named captures. Named captures are implemented as being aliases to numbered groups holding the captures, and that interferes with theimplementation of the branch reset pattern. If you are using namedcaptures in a branch reset pattern, it's best to use the same names,in the same order, in each of the alternations:
```
   /(?|  (?<a> x ) (?<b> y )
     |  (?<a> z ) (?<b> w )) /x
 
```
Not doing so may lead to surprises:
```
  "12" =~ /(?| (?<a> \d+ ) | (?<b> \D+))/x;
   say $+ {a};   # Prints '12'
   say $+ {b};   # *Also* prints '12'.
 
```
The problem here is that both the group named a and the groupnamed b are aliases for the group belonging to $1.
Look-Around Assertions
Look-around assertions are zero-width patterns which match a specificpattern without including it in $&. Positive assertions match whentheir subpattern matches, negative assertions match when their subpatternfails. Look-behind matches text up to the current match position,look-ahead matches text following the current match position.
- (?=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". Notehowever that look-ahead and look-behind are NOT the same thing. You cannotuse 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 thatthe next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" willmatch. Use look-behind instead (see below).
- (?<=pattern) \K
  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.
  There is a special form of this construct, called \K, which causes theregex engine to "keep" everything it had matched prior to the \K andnot include it in $&. This effectively provides variable-lengthlook-behind. The use of \K inside of another look-around assertionis allowed, but the behaviour is currently not well defined.
  For various reasons \K may be significantly more efficient than theequivalent (?<=...) construct, and it is especially useful insituations where you want to efficiently remove something followingsomething else in a string. For instance
```
  s/(foo)bar/$1/g;
 
```
  can be rewritten as the much more efficient
```
  s/foo\Kbar//g;
 
```
- (?<!pattern)
  A zero-width negative look-behind assertion. For example /(?<!bar)foo/matches any occurrence of "foo" that does not follow "bar". Worksonly for fixed-width look-behind.
(?'NAME'pattern)
(?<NAME>pattern)
A named capture group. Identical in every respect to normal capturingparentheses () but for the additional fact that the groupcan be referred to by name in various regular expressionconstructs (like \g{NAME}) and can be accessed by nameafter a successful match via %+ or %-. See perlvarfor more details on the %+ and %- hashes.
If multiple distinct capture groups have the same name then the$+{NAME} will refer to the leftmost defined group in the match.
The forms (?'NAME'pattern) and (?<NAME>pattern) are equivalent.
NOTE: While the notation of this construct is the same as the similarfunction in .NET regexes, the behavior is not. In Perl the groups arenumbered sequentially regardless of being named or not. Thus in thepattern
```
  /(x)(?<foo>y)(z)/
 
```
$+{foo} will be the same as $2, and $3 will contain 'z' instead ofthe opposite which is what a .NET regex hacker might expect.
Currently NAME is restricted to simple identifiers only.In other words, it must match /^[_A-Za-z][_A-Za-z0-9]*\z/ orits Unicode extension (see utf8),though it isn't extended by the locale (see perllocale).
NOTE: In order to make things easier for programmers with experiencewith the Python or PCRE regex engines, the pattern (?P<NAME>pattern)may be used instead of (?<NAME>pattern); however this form does notsupport the use of single quotes as a delimiter for the name.
\k<NAME>
\k'NAME'
Named backreference. Similar to numeric backreferences, except thatthe group is designated by name and not number. If multiple groupshave the same name then it refers to the leftmost defined group inthe current match.
It is an error to refer to a name not defined by a (?<NAME>)earlier in the pattern.
Both forms are equivalent.
NOTE: In order to make things easier for programmers with experiencewith the Python or PCRE regex engines, the pattern (?P=NAME)may be used instead of \k<NAME>.
(?{ code })
WARNING: This extended regular expression feature is consideredexperimental, and may be changed without notice. Code executed thathas side effects may not perform identically from version to versiondue to the effect of future optimisations in the regex engine.
This zero-width assertion evaluates any embedded Perl code. Italways 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 tocapture the results of submatches in variables without having to keeptrack 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 regularexpression is matching against. You can also use pos() to know what isthe current position of matching within this string.
The code is properly scoped in the following sense: If the assertionis backtracked (compare Backtracking), all changes introduced afterlocalization 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 globallyintroduced value, because the scopes that restrict local operatorsare unwound.
This assertion may be used as a (?(condition)yes-pattern|no-pattern)switch. If not used in this way, the result of evaluation ofcode is put into the special variable $^R. This happensimmediately, so $^R can be used from other (?{ code }) assertionsinside the same regular expression.
The assignment to $^R above is properly localized, so the oldvalue of $^R is restored if the assertion is backtracked; compareBacktracking.
For reasons of security, this construct is forbidden if the regularexpression involves run-time interpolation of variables, unless theperilous use re 'eval' pragma has been used (see re), or thevariables contain results of the qr// operator (seeqr/STRING/msixpodual in perlop).
This restriction is due to the wide-spread and remarkably convenientcustom 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 operation was completely safe from a security point of view,although it could raise an exception from an illegal pattern. Ifyou 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 Safecompartment. See perlsec for details about both these mechanisms.
WARNING: Use of lexical (my) variables in these blocks isbroken. The result is unpredictable and will make perl unstable. Theworkaround is to use global (our) variables.
WARNING: In perl 5.12.x and earlier, the regex enginewas not re-entrant, so interpolated code could notsafely invoke the regex engine either directly withm// or s///), or indirectly with functions such assplit. Invoking the regex engine in these blocks would make perlunstable.
(??{ code })
WARNING: This extended regular expression feature is consideredexperimental, and may be changed without notice. Code executed thathas side effects may not perform identically from version to versiondue to the effect of future optimisations in the regex engine.
This is a "postponed" regular subexpression. The code is evaluatedat run time, at the moment this subexpression may match. The resultof evaluation is considered a regular expression and matched asif it were inserted instead of this construct. Note that this meansthat the contents of capture groups defined inside an eval'ed patternare not available outside of the pattern, and vice versa, there is noway for the inner pattern returned from the code block to refer to acapture group defined outside. (The code block itself can use $1, etc.,to refer to the enclosing pattern's capture groups.) Thus,
```
 ('a' x 100)=~/(??{'(.)' x 100})/
 
```
will match, it will not set $1.
The code is not interpolated. As before, the rules to determinewhere 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;
 
```
See also (?PARNO) for a different, more efficient way to accomplishthe same task.
For reasons of security, this construct is forbidden if the regularexpression involves run-time interpolation of variables, unless theperilous use re 'eval' pragma has been used (see re), or thevariables contain results of the qr// operator (seeqr/STRING/msixpodual in perlop).
In perl 5.12.x and earlier, because the regex engine was not re-entrant,delayed code could not safely invoke the regex engine either directly withm// or s///), or indirectly with functions such as split.
Recursing deeper than 50 times without consuming any input string willresult in a fatal error. The maximum depth is compiled into perl, sochanging it requires a custom build.
(?PARNO) (?-PARNO) (?+PARNO) (?R) (?0)
Similar to (??{ code }) except it does not involve compiling any code,instead it treats the contents of a capture group as an independentpattern that must match at the current position. Capture groupscontained by the pattern will have the value as determined by theoutermost recursion.
PARNO is a sequence of digits (not starting with 0) whose value reflectsthe paren-number of the capture group to recurse to. (?R) recurses tothe beginning of the whole pattern. (?0) is an alternate syntax for(?R). If PARNO is preceded by a plus or minus sign then it is assumedto be relative, with negative numbers indicating preceding capture groupsand positive ones following. Thus (?-1) refers to the most recentlydeclared group, and (?+1) indicates the next group to be declared.Note that the counting for relative recursion differs from that ofrelative backreferences, in that with recursion unclosed groups areincluded.
The following pattern matches a function foo() which may containbalanced parentheses as the argument.
```
  $re = qr{ (   # paren group 1 (full function)
     foo
     ( # paren group 2 (parens)
   $
     ( # paren group 3 (contents of parens)
     (?:
       (?> [^()]+ ) # Non-parens without backtracking
     |
       (?2) # Recurse to start of paren group 2
     )*
     )
   $
     )
   )
     }x;
 
```
If the pattern was used as follows
```
 'foo(bar(baz)+baz(bop))'=~/$re/
  and print "\$1 = $1\n",
   "\$2 = $2\n",
   "\$3 = $3\n";
 
```
the output produced should be the following:
```
 $1 = foo(bar(baz)+baz(bop))
  $2 = (bar(baz)+baz(bop))
  $3 = bar(baz)+baz(bop)
 
```
If there is no corresponding capture group defined, then it is afatal error. Recursing deeper than 50 times without consuming any inputstring will also result in a fatal error. The maximum depth is compiledinto perl, so changing it requires a custom build.
The following shows how using negative indexing can make iteasier to embed recursive patterns inside of a qr// constructfor later use:
```
 my $parens = qr/($(?:[^()]++|(?-1))*+$)/;
  if (/foo $parens \s+ + \s+ bar $parens/x) {
    # do something here...
  }
 
```
Note that this pattern does not behave the same way as the equivalentPCRE or Python construct of the same form. In Perl you can backtrack intoa recursed group, in PCRE and Python the recursed into group is treatedas atomic. Also, modifiers are resolved at compile time, so constructslike (?i:(?1)) or (?:(?i)(?1)) do not affect how the sub-pattern willbe processed.
(?&NAME)
Recurse to a named subpattern. Identical to (?PARNO) except that theparenthesis to recurse to is determined by name. If multiple parentheses havethe same name, then it recurses to the leftmost.
It is an error to refer to a name that is not declared somewhere in thepattern.
NOTE: In order to make things easier for programmers with experiencewith the Python or PCRE regex engines the pattern (?P>NAME)may be used instead of (?&NAME).
(?(condition)yes-pattern|no-pattern)
(?(condition)yes-pattern)
Conditional expression. Matches yes-pattern if condition yieldsa true value, matches no-pattern otherwise. A missing pattern alwaysmatches.
(condition) should be either an integer inparentheses (which is valid if the corresponding pair of parenthesesmatched), a look-ahead/look-behind/evaluate zero-width assertion, aname in angle brackets or single quotes (which is valid if a groupwith the given name matched), or the special symbol (R) (true whenevaluated inside of recursion or eval). Additionally the R may befollowed by a number, (which will be true when evaluated when recursinginside of the appropriate group), or by &NAME, in which case it willbe true only when evaluated during recursion in the named group.
Here's a summary of the possible predicates:
- (1) (2) ...
  Checks if the numbered capturing group has matched something.
- (<NAME>) ('NAME')
  Checks if a group with the given name has matched something.
- (?=...) (?!...) (?<=...) (?<!...)
  Checks whether the pattern matches (or does not match, for the '!'variants).
- (?{ CODE })
  Treats the return value of the code block as the condition.
- (R)
  Checks if the expression has been evaluated inside of recursion.
- (R1) (R2) ...
  Checks if the expression has been evaluated while executing directlyinside of the n-th capture group. This check is the regex equivalent of
```
  if ((caller(0))[3] eq 'subname') { ... }
 
```
  In other words, it does not check the full recursion stack.
- (R&NAME)
  Similar to (R1), this predicate checks to see if we're executingdirectly inside of the leftmost group with a given name (this is the samelogic used by (?&NAME) to disambiguate). It does not check the fullstack, but only the name of the innermost active recursion.
- (DEFINE)
  In this case, the yes-pattern is never directly executed, and nono-pattern is allowed. Similar in spirit to (?{0}) but more efficient.See below for details.
For example:
```
 m{ ( $ )?
       [^()]+
       (?(1) $ )
   }x
 
```
matches a chunk of non-parentheses, possibly included in parenthesesthemselves.
A special form is the (DEFINE) predicate, which never executes itsyes-pattern directly, and does not allow a no-pattern. This allows one todefine subpatterns which will be executed only by the recursion mechanism.This way, you can define a set of regular expression rules that can bebundled into any pattern you choose.
It is recommended that for this usage you put the DEFINE block at theend of the pattern, and that you name any subpatterns defined within it.
Also, it's worth noting that patterns defined this way probably willnot be as efficient, as the optimiser is not very clever abouthandling them.
An example of how this might be used is as follows:
```
  /(?<NAME>(?&NAME_PAT))(?<ADDR>(?&ADDRESS_PAT))
       (?(DEFINE)
   (?<NAME_PAT>....)
   (?<ADRESS_PAT>....)
       )/x
 
```
Note that capture groups matched inside of recursion are not accessibleafter the recursion returns, so the extra layer of capturing groups isnecessary. Thus $+{NAME_PAT} would not be defined even though$+{NAME} would be.
Finally, keep in mind that subpatterns created inside a DEFINE blockcount towards the absolute and relative number of captures, so this:
```
 my @captures = "a" =~ /(.)  # First capture
       (?(DEFINE)
       (?<EXAMPLE> 1 )  # Second capture
       )/x;
  say scalar @captures;
 
```
Will output 2, not 1. This is particularly important if you intend tocompile the definitions with the qr// operator, and laterinterpolate them in another pattern.
(?>pattern)
An "independent" subexpression, one which matches the substringthat a standalone pattern would match if anchored at the givenposition, and it matches nothing other than this substring. Thisconstruct 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 notgive anything back" semantic is desirable.
For example: ^(?>a*)ab will never match, since (?>a*)(anchored at the beginning of string, as above) will match allcharacters a at the beginning of string, leaving no a forab to match. In contrast, a*ab will match the same as a+b,since the match of the subgroup a* is influenced by the followinggroup ab (see Backtracking). In particular, a* insidea*ab will match fewer characters than a standalone a*, sincethis makes the tail match.
(?>pattern) does not disable backtracking altogether once it hasmatched. It is still possible to backtrack past the construct, but notinto it. So ((?>a*)|(?>b*))ar will still match "bar".
An effect similar to (?>pattern) may be achieved by writing(?=(pattern))\g{-1}. This matches the same substring as a standalonea+, and the following \g{-1} eats the matched string; it thereforemakes a zero-length assertion into an analogue of (?>...).(The difference between these two constructs is that the second oneuses a capturing group, thus shifting ordinals of backreferencesin the rest of a regular expression.)
Consider this pattern:
```
 m{ $
     (
   [^()]+   # x+
     |
   \( [^()]* $
     )+
       \)
   }x
 
```
That will efficiently match a nonempty group with matching parenthesestwo levels deep or less. However, if there is no such group, itwill take virtually forever on a long string. That's because thereare so many different ways to split a long string into severalsubstrings. This is what (.+)+ is doing, and (.+)+ is similarto a subpattern of the above pattern. Consider how the patternabove detects no-match on ((()aaaaaaaaaaaaaaaaaa in severalseconds, but that each extra letter doubles this time. Thisexponential performance will make it appear that your program hashung. However, a tiny change to this pattern
```
 m{ $
     (
   (?> [^()]+ ) # change x+ above to (?> x+ )
     |
   \( [^()]* $
     )+
       \)
   }x
 
```
which uses (?>...) matches exactly when the one above does (verifyingthis yourself would be a productive exercise), but finishes in a fourththe time when used on a similar string with 1000000 as. Be aware,however, that, when this construct is followed by aquantifier, it currently triggers a warning message underthe use warnings pragma or -w switch saying it"matches null string many times in regex".
On simple groups, such as the pattern (?> [^()]+ ), a comparableeffect may be achieved by negative look-ahead, as in [^()]+ (?! [^()] ).This was only 4 times slower on a string with 1000000 as.
The "grab all you can, and do not give anything back" semantic is desirablein many situations where on the first sight a simple ()* looks likethe correct solution. Suppose we parse text with comments being delimitedby # followed by some optional (horizontal) whitespace. Contrary toits appearance, #[ \t]* is not the correct subexpression to matchthe comment delimiter, because it may "give up" some whitespace ifthe remainder of the pattern can be made to match that way. The correctanswer is either one of these:
```
 (?>#[ \t]*)
   #[ \t]*(?![ \t])
 
```
For example, to grab non-empty comments into $1, one should use eitherone of these:
```
 / (?> \# [ \t]* ) ( .+ ) /x;
  / \# [ \t]*   ( [^ \t] .* ) /x;
 
```
Which one you pick depends on which of these expressions better reflectsthe above specification of comments.
In some literature this construct is called "atomic matching" or"possessive matching".
Possessive quantifiers are equivalent to putting the item they are appliedto inside of one of these constructs. The following equivalences apply:
```
 Quantifier Form Bracketing Form
  --------------- ---------------
  PAT*+   (?>PAT*)
  PAT++   (?>PAT+)
  PAT?+   (?>PAT?)
  PAT{min,max}+   (?>PAT{min,max})
 
```

Special Backtracking Control Verbs

WARNING: These patterns are experimental and subject to change orremoval in a future version of Perl. Their usage in production code shouldbe noted to avoid problems during upgrades.

These special patterns are generally of the form (*VERB:ARG). Unlessotherwise stated the ARG argument is optional; in some cases, it isforbidden.

Any pattern containing a special backtracking verb that allows an argumenthas the special behaviour that when executed it sets the current package's$REGERROR and $REGMARK variables. When doing so the followingrules apply:

On failure, the $REGERROR variable will be set to the ARG value of theverb pattern, if the verb was involved in the failure of the match. If theARG part of the pattern was omitted, then $REGERROR will be set to thename of the last (*MARK:NAME) pattern executed, or to TRUE if there wasnone. Also, the $REGMARK variable will be set to FALSE.

On a successful match, the $REGERROR variable will be set to FALSE, andthe $REGMARK variable will be set to the name of the last(*MARK:NAME) pattern executed. See the explanation for the(*MARK:NAME) verb below for more details.

NOTE: $REGERROR and $REGMARK are not magic variables like $1and most other regex-related variables. They are not local to a scope, norreadonly, but instead are volatile package variables similar to $AUTOLOAD.Use local to localize changes to them to a specific scope if necessary.

If a pattern does not contain a special backtracking verb that allows anargument, then $REGERROR and $REGMARK are not touched at all.

Verbs that take an argument
- (*PRUNE) (*PRUNE:NAME)
  This zero-width pattern prunes the backtracking tree at the current pointwhen backtracked into on failure. Consider the pattern A (*PRUNE) B,where A and B are complex patterns. Until the (*PRUNE) verb is reached,A may backtrack as necessary to match. Once it is reached, matchingcontinues in B, which may also backtrack as necessary; however, should Bnot match, then no further backtracking will take place, and the patternwill fail outright at the current starting position.
  The following example counts all the possible matching strings in apattern (without actually matching any of them).
```
 'aaab' =~ /a+b?(?{print "$&\n" $count++})(*FAIL)/;
  print "Count=$count\n";
 
```
  which produces:
```
 aaab
  aaa
  aa
  a
  aab
  aa
  a
  ab
  a
  Count=9
 
```
  If we add a (*PRUNE) before the count like the following
```
 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n" $count++})(*FAIL)/;
  print "Count=$count\n";
 
```
  we prevent backtracking and find the count of the longest matching stringat each matching starting point like so:
```
 aaab
  aab
  ab
  Count=3
 
```
  Any number of (*PRUNE) assertions may be used in a pattern.
  See also (?>pattern) and possessive quantifiers for other ways tocontrol backtracking. In some cases, the use of (*PRUNE) can bereplaced with a (?>pattern) with no functional difference; however,(*PRUNE) can be used to handle cases that cannot be expressed using a(?>pattern) alone.
- (*SKIP) (*SKIP:NAME)
  This zero-width pattern is similar to (*PRUNE), except that onfailure it also signifies that whatever text that was matched leading upto the (*SKIP) pattern being executed cannot be part of any matchof this pattern. This effectively means that the regex engine "skips" forwardto this position on failure and tries to match again, (assuming thatthere is sufficient room to match).
  The name of the (*SKIP:NAME) pattern has special significance. If a(*MARK:NAME) was encountered while matching, then it is that positionwhich is used as the "skip point". If no (*MARK) of that name wasencountered, then the (*SKIP) operator has no effect. When usedwithout a name the "skip point" is where the match point was whenexecuting the (*SKIP) pattern.
  Compare the following to the examples in (*PRUNE); note the stringis twice as long:
```
 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n" $count++})(*FAIL)/;
  print "Count=$count\n";
 
```
  outputs
```
 aaab
  aaab
  Count=2
 
```
  Once the 'aaab' at the start of the string has matched, and the (*SKIP)executed, the next starting point will be where the cursor was when the(*SKIP) was executed.
- (*MARK:NAME) (*:NAME)
  This zero-width pattern can be used to mark the point reached in a stringwhen a certain part of the pattern has been successfully matched. Thismark may be given a name. A later (*SKIP) pattern will then skipforward to that point if backtracked into on failure. Any number of(*MARK) patterns are allowed, and the NAME portion may be duplicated.
  In addition to interacting with the (*SKIP) pattern, (*MARK:NAME)can be used to "label" a pattern branch, so that after matching, theprogram can determine which branches of the pattern were involved in thematch.
  When a match is successful, the $REGMARK variable will be set to thename of the most recently executed (*MARK:NAME) that was involvedin the match.
  This can be used to determine which branch of a pattern was matchedwithout using a separate capture group for each branch, which in turncan result in a performance improvement, as perl cannot optimize/(?:(x)|(y)|(z))/ as efficiently as something like/(?:x(*MARK:x)|y(*MARK:y)|z(*MARK:z))/.
  When a match has failed, and unless another verb has been involved infailing the match and has provided its own name to use, the $REGERRORvariable will be set to the name of the most recently executed(*MARK:NAME).
  See (*SKIP) for more details.
  As a shortcut (*MARK:NAME) can be written (*:NAME).
- (*THEN) (*THEN:NAME)
  This is similar to the "cut group" operator :: from Perl 6. Like(*PRUNE), this verb always matches, and when backtracked into onfailure, it causes the regex engine to try the next alternation in theinnermost enclosing group (capturing or otherwise) that has alternations.The two branches of a (?(condition)yes-pattern|no-pattern) do notcount as an alternation, as far as (*THEN) is concerned.
  Its name comes from the observation that this operation combined with thealternation operator (|) can be used to create what is essentially apattern-based if/then/else block:
```
  ( COND (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ )
 
```
  Note that if this operator is used and NOT inside of an alternation thenit acts exactly like the (*PRUNE) operator.
```
  / A (*PRUNE) B /
 
```
  is the same as
```
  / A (*THEN) B /
 
```
  but
```
  / ( A (*THEN) B | C (*THEN) D ) /
 
```
  is not the same as
```
  / ( A (*PRUNE) B | C (*PRUNE) D ) /
 
```
  as after matching the A but failing on the B the (*THEN) verb willbacktrack and try C; but the (*PRUNE) verb will simply fail.
Verbs without an argument
- (*COMMIT)
  This is the Perl 6 "commit pattern" <commit> or :::. It's azero-width pattern similar to (*SKIP), except that when backtrackedinto on failure it causes the match to fail outright. No further attemptsto find a valid match by advancing the start pointer will occur again.For example,
```
 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n" $count++})(*FAIL)/;
  print "Count=$count\n";
 
```
  outputs
```
 aaab
  Count=1
 
```
  In other words, once the (*COMMIT) has been entered, and if the patterndoes not match, the regex engine will not try any further matching on therest of the string.
- (*FAIL) (*F)
  This pattern matches nothing and always fails. It can be used to force theengine to backtrack. It is equivalent to (?!), but easier to read. Infact, (?!) gets optimised into (*FAIL) internally.
  It is probably useful only when combined with (?{}) or (??{}).
- (*ACCEPT)
  WARNING: This feature is highly experimental. It is not recommendedfor production code.
  This pattern matches nothing and causes the end of successful matching atthe point at which the (*ACCEPT) pattern was encountered, regardless ofwhether there is actually more to match in the string. When inside of anested pattern, such as recursion, or in a subpattern dynamically generatedvia (??{}), only the innermost pattern is ended immediately.
  If the (*ACCEPT) is inside of capturing groups then the groups aremarked as ended at the point at which the (*ACCEPT) was encountered.For instance:
```
  'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x;
 
```
  will match, and $1 will be AB and $2 will be B, $3 will notbe set. If another branch in the inner parentheses was matched, such as in thestring 'ACDE', then the D and E would have to be matched as well.

Backtracking

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

A fundamental feature of regular expression matching involves thenotion called backtracking, which is currently used (when needed)by all regular non-possessive expression quantifiers, namely *, *?, +,+?, {n,m}, and {n,m}?. Backtracking is often optimizedinternally, but the general principle outlined here is valid.

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

Here is an example of backtracking: Let's say you want to find theword 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'sno whitespace following the "Foo" that it had saved in $1, it realizes itsmistake and starts over again one character after where it had thetentative match. This time it goes all the way until the next occurrenceof "foo". The complete regular expression matches this time, and you getthe expected output of "table follows foo."

Sometimes minimal matching can help a lot. Imagine you'd like to matcheverything between "foo" and "bar". Initially, you write somethinglike 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 thefirst "foo" and the last "bar". Here it's more effectiveto 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 endof 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 thewhole string. As \d* can match on an empty string the completeregular 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 aregular expression is merely a set of assertions that gives a definitionof success. There may be 0, 1, or several different ways that thedefinition might succeed against a particular string. And if there aremultiple ways it might succeed, you need to understand backtracking toknow which variety of success you will achieve.

When using look-ahead assertions and negations, this can all get eventrickier. Imagine you'd like to find a sequence of non-digits notfollowed 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. Itclaims that there is no 123 in the string. Here's a clearer picture ofwhy 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 moregeneral purpose version of test 1. The important difference betweenthem is that test 3 contains a quantifier (\D*) and so can usebacktracking, whereas test 1 will not. What's happening isthat you've asked "Is it true that at the start of $x, following 0 or morenon-digits, you have something that's not 123?" If the pattern matcher hadlet \D* expand to "ABC", this would have caused the whole pattern tofail.

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

The pattern really, really wants to succeed, so it uses thestandard pattern back-off-and-retry and lets \D* expand to just "AB" thistime. Now there's indeed something following "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 digitand by something that's not "123". Remember that the look-aheadsare zero-width expressions--they only look, but don't consume anyof the string in their match. So rewriting this way produces whatyou'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 thoughthey'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 theline simultaneously. The deeper underlying truth is that juxtaposition inregular expressions always means AND, except when you write an explicit ORusing 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 takeexponential time to solve because of the immense number of possibleways they can use backtracking to try for a match. For example, withoutinternal optimizations done by the regular expression engine, this willtake 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 themto 0 through 5 matches, then it would take forever--or until you ranout of stack space. Moreover, these internal optimizations are notalways applicable. For example, if you put {0,5} instead of *on the external group, no current optimization is applicable, and thematch 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 thatzero-length look-ahead/look-behind assertions will not backtrack to makethe tail match, since they are in "logical" context: onlywhether they match is considered relevant. For an examplewhere side-effects of look-ahead might have influenced thefollowing match, see (?>pattern).

Version 8 Regular Expressions

In case you're not familiar with the "regular" Version 8 regexroutines, here are the pattern-matching rules not described above.

Any single character matches itself, unless it is a metacharacterwith a special meaning described here or above. You can causecharacters that normally function as metacharacters to be interpretedliterally by prefixing them with a "\" (e.g., "\." matches a ".", not anycharacter; "\" matches a "\"). This escape mechanism is also requiredfor the character used as the pattern delimiter.

A series of characters matches that series of characters in the targetstring, so the pattern blurfl would match "blurfl" in the targetstring.

You can specify a character class, by enclosing a list of charactersin [], which will match any character from the list. If thefirst character after the "[" is "^", the class matches any character notin the list. Within a list, the "-" character specifies arange, so that a-z represents all characters between "a" and "z",inclusive. If you want either "-" or "]" itself to be a member of aclass, put it at the start of the list (possibly after a "^"), orescape it with a backslash. "-" is also taken literally when it isat the end of the list, just before the closing "]". (Thefollowing all specify the same class of three characters: [-az],[az-], and [a\-z]. All are different from [a-z], whichspecifies a class containing twenty-six characters, even on EBCDIC-basedcharacter sets.) Also, if you try to use the characterclasses \w, \W, \s, \S, \d, or \D as endpoints ofa range, the "-" is understood literally.

Note also that the whole range idea is rather unportable betweencharacter sets--and even within character sets they may cause resultsyou probably didn't expect. A sound principle is to use only rangesthat 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 thatused in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return,"\f" a form feed, etc. More generally, \nnn, where nnn is a stringof three octal digits, matches the character whose coded character set valueis nnn. Similarly, \xnn, where nn are hexadecimal digits,matches the character whose ordinal is nn. The expression \cxmatches the character control-x. Finally, the "." metacharactermatches any character except "\n" (unless you use /s).

You can specify a series of alternatives for a pattern using "|" toseparate 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). Thefirst alternative includes everything from the last pattern delimiter("(", "(?:", etc. or the beginning of the pattern) up to the first "|", andthe last alternative contains everything from the last "|" to the nextclosing pattern delimiter. That's why it's common practice to includealternatives in parentheses: to minimize confusion about where theystart and end.

Alternatives are tried from left to right, so the firstalternative found for which the entire expression matches, is the one thatis chosen. This means that alternatives are not necessarily greedy. Forexample: when matching foo|foot against "barefoot", only the "foo"part will match, as that is the first alternative tried, and it successfullymatches the target string. (This might not seem important, but it isimportant when you are capturing 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 referenceby enclosing them in parentheses, and you may refer back to thenth subpattern later in the pattern using the metacharacter\n or \gn. Subpatterns are numbered based on the left to right orderof their opening parenthesis. A backreference matches whateveractually matched the subpattern in the string being examined, notthe rules for that subpattern. Therefore, (0|0x)\d*\s\g1\d* willmatch "0x1234 0x4321", but not "0x1234 01234", because subpattern1 matched "0x", even though the rule 0|0x could potentially matchthe 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 \1 to \9) for the RHS of a substitute to avoidshocking thesed addicts, but it's a dirty habit to get into. That's because inPerlThink, the righthand side of an s/// is a double-quoted string. \1 inthe usual double-quoted string means a control-A. The customary Unixmeaning of \1 is kludged in for s///. However, if you get into the habitof doing that, you get yourself into trouble if you then add an /emodifier.

 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 confusedwith the operation of matching a backreference. Certainly they mean twodifferent 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. Aswith most other power tools, power comes together with the abilityto wreak havoc.

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

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

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

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

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

or the loop implied by split().

However, long experience has shown that many programming tasks maybe significantly simplified by using repeated subexpressions thatmay 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 breakingthe infinite loop. The rules for this are different for lower-levelloops given by the greedy quantifiers *+{}, and for higher-levelones like the /g modifier or split() operator.

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

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

is made equivalent to

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

For example, this program

   #!perl -l
    "aaaaab" =~ /
   (?:
   a # non-zero
   | # or
       (?{print "hello"}) # print hello whenever this
     # branch is tried
       (?=(b)) # zero-width assertion
   )*  # any number of times
   /x;
    print $&;
    print $1;

prints

   hello
    aaaaa
    b

Notice that "hello" is only printed once, as when Perl sees that the sixthiteration of the outermost (?:)* matches a zero-length string, it stopsthe *.

The higher-level loops preserve an additional state between iterations:whether the last match was zero-length. To break the loop, the followingmatch 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 ofzero length.

For example:

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

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

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

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

Combining RE Pieces

Each of the elementary pieces of regular expressions which were describedbefore (such as ab or \Z) could match at most one substringat the given position of the input string. However, in a typical regularexpression these elementary pieces are combined into more complicatedpatterns 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 matchsubstring "a" or "ab"? One way to describe which substring isactually matched is the concept of backtracking (see Backtracking).However, this description is too low-level and makes you thinkin terms of a particular implementation.

Another description starts with notions of "better"/"worse". All thesubstrings which may be matched by the given regular expression can besorted 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 mostone match at a given position is possible. This section describes thenotion of better/worse for combining operators. In the descriptionbelow S and T are regular subexpressions.

ST
Consider two possible matches, AB and A'B', A and A' aresubstrings which can be matched by S, B and B' are substringswhich can be matched by T.
If A is a better match for S than A', AB is a bettermatch than A'B'.
If A and A' coincide: AB is a better match than AB' ifB is a 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 fortwo 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 ifS has capturing parentheses, and backreferences are used somewhereelse in the whole regular expression.)
(?!S), (?<!S)
For this grouping operator there is no need to describe the ordering, sinceonly whether or not S can match is important.
(??{ EXPR }), (?PARNO)
The ordering is the same as for the regular expression which isthe result of EXPR, or the pattern contained by capture group PARNO.
(?(condition)yes-pattern|no-pattern)
Recall that which of yes-pattern or no-pattern actually matches isalready determined. The ordering of the matches is the same as for thechosen 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 thewhole regular expression: a match at an earlier position is always betterthan a match at a later position.

Creating Custom RE Engines

As of Perl 5.10.0, one can create custom regular expression engines. Thisis not for the faint of heart, as they have to plug in at the C level. Seeperlreapi for more details.

As an alternative, overloaded constants (see overload) provide a simpleway to extend the functionality of the RE engine, by substituting onepattern for another.

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

 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 regularexpressions, i.e., those without any runtime variable interpolations.As documented in overload, this conversion will work only overliteral parts of regular expressions. For \Y|$re\Y| the variablepart 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|/;

PCRE/Python Support

As of Perl 5.10.0, Perl supports several Python/PCRE-specific extensionsto the regex syntax. While Perl programmers are encouraged to use thePerl-specific syntax, the following are also accepted:

(?P<NAME>pattern)
Define a named capture group. Equivalent to (?<NAME>pattern).
(?P=NAME)
Backreference to a named capture group. Equivalent to \g{NAME}.
(?P>NAME)
Subroutine call to a named capture group. Equivalent to (?&NAME).

BUGS

Many regular expression constructs don't work on EBCDIC platforms.

There are a number of issues with regard to case-insensitive matchingin Unicode rules. See i under Modifiers above.

This document varies from difficult to understand to completelyand utterly opaque. The wandering prose riddled with jargon ishard to fathom in several places.

This document needs a rewrite that separates the tutorial contentfrom the reference content.

Regular expression regex regexp

Daftar Isi

NAME

DESCRIPTION