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NAME

perlre - Perl regular expressions

DESCRIPTION

This page describes the syntax of regular expressions in Perl.

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

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

New in v5.22, "use re 'strict'" applies stricter rules than otherwise when compiling
regular expression patterns. It can find things that, while legal, may not be what you
intended.

Modifiers
Overview

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

m Treat string as multiple lines. That is, change "^" and "$" from matching the start
of the string's first line and the end of its last line to matching the start and end
of each line within the string.

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

Used together, as "/ms", they let the "." match any character whatsoever, while still
allowing "^" and "$" to match, respectively, just after and just before newlines
within the string.

i Do case-insensitive pattern matching.

If locale matching rules are in effect, the case map is taken from the current locale
for code points less than 255, and from Unicode rules for larger code points.
However, matches that would cross the Unicode rules/non-Unicode rules boundary (ords
255/256) will not succeed. See perllocale.

There are a number of Unicode characters that match multiple characters under "/i".
For example, "LATIN SMALL LIGATURE FI" should match the sequence "fi". Perl is not
currently able to do this when the multiple characters are in the pattern and are
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 a bracketed character class unless the
character that maps to them is explicitly mentioned, and it doesn't match them at all
if the character class is inverted, which otherwise could be highly confusing. See
"Bracketed Character Classes" in perlrecharclass, and "Negation" in perlrecharclass.

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.

In Perl 5.20 and higher this is ignored. Due to a new copy-on-write mechanism,
${^PREMATCH}, ${^MATCH}, and ${^POSTMATCH} will be available after the match
regardless of the modifier.

a, d, l and u
These modifiers, all new in 5.14, affect which character-set rules (Unicode, etc.) are
used, as described below in "Character set modifiers".

n Prevent the grouping metacharacters "()" from capturing. This modifier, new in 5.22,
will stop $1, $2, etc... from being filled in.

"hello" =~ /(hi|hello)/; # $1 is "hello"
"hello" =~ /(hi|hello)/n; # $1 is undef

This is equivalent to putting "?:" at the beginning of every capturing group:

"hello" =~ /(?:hi|hello)/; # $1 is undef

"/n" can be negated on a per-group basis. Alternatively, named captures may still be
used.

"hello" =~ /(?-n:(hi|hello))/n; # $1 is "hello"
"hello" =~ /(?<greet>hi|hello)/n; # $1 is "hello", $+{greet} is
# "hello"

Other Modifiers
There are a number of flags that can be found at the end of regular expression
constructs that are not generic regular expression flags, but apply to the operation
being performed, like matching or substitution ("m//" or "s///" respectively).

Flags described further in "Using regular expressions in Perl" in perlretut are:

c - keep the current position during repeated matching
g - globally match the pattern repeatedly in the string

Substitution-specific modifiers described in

"s/PATTERN/REPLACEMENT/msixpodualngcer" in perlop are:

e - evaluate the right-hand side as an expression
ee - evaluate the right side as a string then eval the result
o - pretend to optimize your code, but actually introduce bugs
r - perform non-destructive substitution and return the new value

Regular expression modifiers are usually written in documentation as e.g., "the "/x"
modifier", even though the delimiter in question might not really be a slash. The
modifiers "/imnsxadlup" may also be embedded within the regular expression itself using
the "(?...)" construct, see "Extended Patterns" below.

Details on some modifiers

Some of the modifiers require more explanation than given in the "Overview" above.

/x

"/x" tells the regular expression parser to ignore most whitespace that is neither
backslashed nor within a bracketed character class. You can use this to break up your
regular expression into (slightly) more readable parts. Also, the "#" character is
treated as a metacharacter introducing a comment that runs up to the pattern's closing
delimiter, or to the end of the current line if the pattern extends onto the next line.
Hence, this is very much like an ordinary Perl code comment. (You can include the closing
delimiter within the comment only if you precede it with a backslash, so be careful!)

Use of "/x" means that if you want real whitespace or "#" characters in the pattern
(outside a bracketed character class, which is unaffected by "/x"), then you'll either
have to escape them (using backslashes or "\Q...\E") or encode them using octal, hex, or
"\N{}" escapes. It is ineffective to try to continue a comment onto the next line by
escaping the "\n" with a backslash or "\Q".

You can use "(?#text)" to create a comment that ends earlier than the end of the current
line, but "text" also can't contain the closing delimiter unless escaped with a backslash.

Taken together, these features go a long way towards making Perl's regular expressions
more readable. Here's an example:

# Delete (most) C comments.
$program =~ s {
/\* # Match the opening delimiter.
.*? # Match a minimal number of characters.
\*/ # Match the closing delimiter.
} []gsx;

Note that anything inside a "\Q...\E" stays unaffected by "/x". And note that "/x"
doesn't affect space interpretation within a single multi-character construct. For
example in "\x{...}", regardless of the "/x" modifier, there can be no spaces. Same for a
quantifier such as "{3}" or "{5,}". Similarly, "(?:...)" can't have a space between the
"(", "?", and ":". Within any delimiters for such a construct, allowed spaces are not
affected by "/x", and depend on the construct. For example, "\x{...}" can't have spaces
because hexadecimal numbers don't have spaces in them. But, Unicode properties can have
spaces, so in "\p{...}" there can be spaces that follow the Unicode rules, for which see
"Properties accessible through \p{} and \P{}" in perluniprops.

The set of characters that are deemed whitespace are those that Unicode calls "Pattern
White Space", namely:

U+0009 CHARACTER TABULATION
U+000A LINE FEED
U+000B LINE TABULATION
U+000C FORM FEED
U+000D CARRIAGE RETURN
U+0020 SPACE
U+0085 NEXT LINE
U+200E LEFT-TO-RIGHT MARK
U+200F RIGHT-TO-LEFT MARK
U+2028 LINE SEPARATOR
U+2029 PARAGRAPH SEPARATOR

Character set modifiers

"/d", "/u", "/a", and "/l", available starting in 5.14, are called the character set
modifiers; they affect the character set rules used for the regular expression.

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

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

Briefly, "/l" sets the character set to that of whatever Locale is in effect 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 several restrictions for ASCII-safe
matching.

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

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

The "/l" and "/u" modifiers are automatically selected for regular expressions compiled
within the scope of various pragmas, and we recommend that in general, you use those
pragmas instead of specifying these modifiers explicitly. For one thing, the modifiers
affect only pattern matching, and do not extend to even any replacement done, whereas
using the pragmas give consistent results for all appropriate 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 you want both of them to use locale rules.
To do this, instead compile the regular expression within the scope of "use locale". This
both implicitly adds the "/l", and applies locale rules to the "\U". The lesson 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 necessarily the latter) would
also use Unicode rules.

More detail on each of the modifiers follows. Most likely you don't need to know this
detail for "/l", "/u", and "/d", and can skip ahead to /a.

/l

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

The only non-single-byte locale Perl supports is (starting in v5.20) UTF-8. This means
that code points above 255 are treated as Unicode no matter what locale is in effect
(since UTF-8 implies Unicode).

Under Unicode rules, there are a few case-insensitive matches that cross the 255/256
boundary. Except for UTF-8 locales in Perls v5.20 and later, these are disallowed under
"/l". For example, 0xFF (on ASCII platforms) does not caselessly match the character at
0x178, "LATIN CAPITAL LETTER Y WITH DIAERESIS", because 0xFF may not be "LATIN SMALL
LETTER Y WITH DIAERESIS" in the current locale, and Perl has no way of knowing if that
character even exists in the locale, much less what code point it is.

In a UTF-8 locale in v5.20 and later, the only visible difference between locale and non-
locale in regular expressions should be tainting (see perlsec).

This modifier may be specified to be the default by "use locale", but see "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 their Latin-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 Unicode platform; and hence,
for example, "\w" will match any of the more than 100_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 in the world as "\w". For example, your
locale might not think that "LATIN SMALL LETTER ETH" is a letter (unless you happen to
speak Icelandic), but Unicode does. Similarly, all the characters that are decimal digits
somewhere in the world will match "\d"; this is hundreds, not 10, possible matches. And
some of those digits look like some of the 10 ASCII digits, but mean a different number,
so a human could easily think a number is a different quantity than it really is. For
example, "BENGALI DIGIT FOUR" (U+09EA) looks very much like an "ASCII DIGIT EIGHT"
(U+0038). And, "\d+", may match strings of digits that are a mixture from different
writing systems, creating a security issue. "num()" in Unicode::UCD can be used to sort
this out. Or the "/a" modifier can be used to force "\d" to match just the ASCII 0
through 9.

Also, under this modifier, case-insensitive matching works on the full set of Unicode
characters. 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. See
<http://unicode.org/reports/tr36> for a detailed discussion of Unicode security issues.

This modifier may be specified to be the default by "use feature 'unicode_strings", "use
locale ':not_characters'", or "use 5.012" (or higher), but see "Which character set
modifier is in effect?".

/d

This modifier means to use the "Default" native rules of the platform except 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{...}" or "\P{...}"); or

6. the pattern uses a Unicode break ("\b{...}" or "\B{...}"); or

7. the pattern uses ""(?[ ])""

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

Unless the pattern or string are encoded in UTF-8, only ASCII characters can match
positively.

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 the others are, so yet
another name for it is "Default".

Because of the unexpected behaviors associated with this modifier, you probably 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", and the Posix character
classes to match only in the ASCII range. They thus revert to their pre-5.6, pre-Unicode
meanings. Under "/a", "\d" always means precisely the digits "0" to "9"; "\s" means the
five characters "[ \f\n\r\t]", and starting in Perl v5.18, the vertical tab; "\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 security concerns.

With "/a", one can write "\d" with confidence that it will only match ASCII characters,
and should the need arise to match beyond ASCII, you can instead use "\p{Digit}" (or
"\p{Word}" for "\w"). There are similar "\p{...}" constructs that can match beyond ASCII
both white space (see "Whitespace" in perlrecharclass), and Posix classes (see "POSIX
Character Classes" in perlrecharclass). Thus, this modifier doesn't mean you can't use
Unicode, it means that to get Unicode matching you must explicitly use a construct
("\p{}", "\P{}") that signals Unicode.

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

Otherwise, "/a" behaves like the "/u" modifier, in that case-insensitive matching uses
Unicode rules; for example, "k" will match the Unicode "\N{KELVIN SIGN}" under "/i"
matching, and code points in the Latin1 range, above ASCII will have Unicode rules when it
comes 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 first occurrence of "a" restricts the "\d",
etc., and the second occurrence adds the "/i" restrictions.) But, note that code points
outside the ASCII range will use Unicode rules for "/i" matching, so the modifier doesn't
really restrict things to just ASCII; it just forbids the intermixing of ASCII and non-
ASCII.

To summarize, this modifier provides protection for applications that don't wish to be
exposed to all of Unicode. Specifying it twice gives 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 use the "/u" modifier explicitly if there are
a few regular expressions where you do want full Unicode rules (but even here, it's best
if everything were under feature "unicode_strings", along with the "use 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 regular expression depends
on a fairly complex set of interactions. These have been designed so that in general you
don't have to worry about it, but this section gives the gory details. As explained below
in "Extended Patterns" it is possible to explicitly specify modifiers that apply only to
portions of a regular expression. The innermost always has priority over any outer ones,
and one applying to the whole expression has priority over any of the default settings
that are described in the remainder of this section.

The "use re '/foo'" pragma can be used to set default modifiers (including these) for
regular expressions compiled within its scope. This pragma has precedence over the other
pragmas listed below that also change the defaults.

Otherwise, "use locale" sets the default modifier to "/l"; and "use feature
'unicode_strings", or "use 5.012" (or higher) set the default to "/u" when not in the same
scope as either "use locale" or "use bytes". ("use locale ':not_characters'" also sets
the default to "/u", overriding any plain "use locale".) Unlike the mechanisms mentioned
above, these affect operations besides regular expressions pattern matching, and so give
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 to unexpected results, it is best to specify
which other rule set should be used.

Character set modifier behavior prior to Perl 5.14

Prior to 5.14, there were no explicit modifiers, but "/l" was implied for regexes compiled
within the scope of "use locale", and "/d" was implied otherwise. However, interpolating
a regex into a larger regex would ignore the original compilation in favor of whatever was
in effect at the time of the second compilation. There were a number of inconsistencies
(bugs) with the "/d" modifier, where Unicode rules would be used when inappropriate, and
vice versa. "\p{}" did not imply Unicode 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 in the Version 8
regex routines. (The routines are derived (distantly) from Henry Spencer's freely
redistributable reimplementation of the V8 routines.) See "Version 8 Regular Expressions"
for details.

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

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

By default, the "^" character is guaranteed to match only the beginning of the string, the
"$" character only the end (or before the newline at the end), and Perl does certain
optimizations with the assumption that the string contains only one line. Embedded
newlines will not be matched by "^" or "$". You may, however, wish to treat a string as a
multi-line buffer, such that the "^" will match after any newline within the string
(except if the newline is the last character in the string), and "$" will match before any
newline. At the cost of a little more overhead, you can do this by using the /m modifier
on the pattern match operator. (Older programs did this by setting $*, but this option
was removed in perl 5.10.)

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

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 of a backslashed
sequence like "\x{...}", it is treated as a regular character. However, a deprecation
warning is raised for all such occurrences, and in Perl v5.26, literal uses of a curly
bracket will be required to be escaped, say by preceding them with a backslash ("\{") or
enclosing them within square brackets ("[{]"). This change will allow for future syntax
extensions (like making the lower bound of a quantifier optional), and better error
checking of quantifiers.)

The "*" quantifier is equivalent to "{0,}", the "+" quantifier to "{1,}", and the "?"
quantifier to "{0,1}". n and m are limited to 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 can be seen in the error message generated by code such as
this:

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

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

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

Normally when a quantified subpattern does not allow the rest of the overall pattern to
match, Perl will backtrack. However, this behaviour is sometimes undesirable. Thus Perl
provides the "possessive" quantifier form as 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 the string and won't leave
any for the remaining part of the pattern. This feature can be extremely useful to give
perl hints about where it shouldn't backtrack. For instance, the typical "match a double-
quoted string" problem can be most efficiently performed when written as:

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

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

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

Note that the possessive quantifier modifier can not be be combined with the non-greedy
modifier. This is because it would make no sense. Consider the follow equivalency table:

Illegal Legal
------------ ------
X??+ X{0}
X+?+ X{1}
X{min,max}?+ X{min}

Escape sequences

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

\t tab (HT, TAB)
\n newline (LF, NL)
\r return (CR)
\f form feed (FF)
\a alarm (bell) (BEL)
\e escape (think troff) (ESC)
\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 until \E (think vi)
\U uppercase until \E (think vi)
\Q quote (disable) pattern metacharacters until \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.
(?[...]) [8] Extended bracketed character class
\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. (Deprecated.)
\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. 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 the
character or character sequence whose name is "NAME"; and similarly when of the form
"\N{U+hex}", it matches the character whose Unicode code point is hex. Otherwise it
matches any character but "\n".

[8] See "Extended Bracketed Character Classes" in perlrecharclass for details.

Assertions

Perl defines the following zero-width assertions:

\b{} Match at Unicode boundary of specified type
\B{} Match where corresponding \b{} doesn't match
\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 Unicode boundary ("\b{}"), available starting in v5.22, is a spot between two
characters, or before the first character in the string, or after the final character in
the string where certain criteria defined by Unicode are met. See "\b{}, \b, \B{}, \B" in
perlrebackslash for details.

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

The "\G" assertion can be used to chain global matches (using "m//g"), as described in
"Regexp Quote-Like Operators" in perlop. It is also useful when writing "lex"-like
scanners, when you have several patterns that you want to match against consequent
substrings of your string; see the previous reference. The actual location where "\G"
will match can also be influenced by using "pos()" as an lvalue: see "pos" in perlfunc.
Note that the rule for zero-length matches (see "Repeated Patterns Matching a Zero-length
Substring") is modified somewhat, in that contents to the left of "\G" are not counted
when determining the length of the match. Thus the following will 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 to be zero-width, and thus
will not match at the same position twice in a row.

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

Note also that "s///" will refuse to overwrite part of a substitution that has already
been replaced; so for example this will stop after the first iteration, rather than
iterating its way backwards through the string:

$_ = "123456789";
pos = 6;
s/.(?=.\G)/X/g;
print; # prints 1234X6789, not XXXXX6789

Capture groups

The bracketing construct "( ... )" creates capture groups (also referred to as capture
buffers). To refer to the current contents of a group later on, within the 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. If a group did not
match, the associated backreference won't match either. (This can happen if the group is
optional, or in a different branch of an alternation.) You can omit the "g", and write
"\1", etc, but there are some issues with this form, described below.

You can also refer to capture groups relatively, by using a negative number, so that
"\g-1" and "\g{-1}" both refer to the immediately preceding capture group, and "\g-2" and
"\g{-2}" both refer to the group before it. For example:

/
(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 to interpolate regexes
into larger regexes and not have to worry about the capture groups being renumbered.

You can dispense with numbers altogether and create named capture groups. The notation is
"(?<name>...)" to declare and "\g{name}" to reference. (To be compatible with .Net
regular expressions, "\g{name}" may also 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 reference to that name assumes the
leftmost defined group. Named groups count in absolute and relative numbering, and so can
also be referred to by those numbers. (It's possible to do things with named capture
groups that would otherwise require "(??{})".)

Capture group contents are dynamically scoped and available to you outside the pattern
until the end of the enclosing block or until the next successful match, 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 for absolute or
relative numbered ones. Braces are safer when creating a regex by concatenating smaller
strings. For example if you have "qr/$a$b/", and $a contained "\g1", and $b contained
"37", you would get "/\g137/" which is probably not what you intended.

The "\g" and "\k" notations were introduced in Perl 5.10.0. Prior to that there were no
named nor relative numbered capture groups. Absolute numbered groups were referred to
using "\1", "\2", etc., and this notation is still accepted (and likely always will be).
But it leads to some ambiguities if there are more than 9 capture groups, as "\10" could
mean either the tenth capture group, or the character whose ordinal in octal is 010 (a
backspace in ASCII). Perl resolves this ambiguity by interpreting "\10" as a
backreference only if at least 10 left parentheses have opened before it. Likewise "\11"
is a backreference only if at least 11 left parentheses have opened before it. And so on.
"\1" through "\9" are always interpreted as backreferences. There are several examples
below that illustrate these perils. You can avoid the 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 3 digits padded with leading zeros, since a leading zero implies
an octal constant.

The "\digit" notation also works in certain circumstances outside the 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 previous match. $+ returns
whatever the last bracket match matched. $& returns the entire matched string. (At one
point $0 did also, but now it returns the name of the program.) "$`" returns everything
before the matched string. "$'" returns everything after the matched string. And $^N
contains whatever was matched by the most-recently closed group (submatch). $^N can be
used in extended patterns (see below), for example to assign a submatch to a variable.

These special variables, like the "%+" hash and the numbered match variables ($1, $2, $3,
etc.) are dynamically scoped until the end of the enclosing block or until the next
successful match, whichever comes first. (See "Compound Statements" in perlsyn.)

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

WARNING: If your code is to run on Perl 5.16 or earlier, beware that once Perl sees that
you need one of $&, "$`", or "$'" anywhere in the program, it has to provide them for
every pattern match. This may substantially slow your program.

Perl uses the same mechanism to produce $1, $2, etc, so you also pay a price for each
pattern that contains capturing parentheses. (To avoid this cost while retaining the
grouping behaviour, use the extended regular expression "(?: ... )" instead.) But if you
never use $&, "$`" or "$'", then patterns without capturing parentheses will not be
penalized. So avoid $&, "$'", and "$`" if you can, but if you can't (and some algorithms
really appreciate them), once you've used them once, use them at will, because you've
already paid the price.

Perl 5.16 introduced a slightly more efficient mechanism that notes separately whether
each of "$`", $&, and "$'" have been seen, and thus may only need to copy part of the
string. Perl 5.20 introduced a much more efficient copy-on-write mechanism which
eliminates any slowdown.

As another workaround for this problem, Perl 5.10.0 introduced "${^PREMATCH}", "${^MATCH}"
and "${^POSTMATCH}", which are equivalent to "$`", $& and "$'", except that they are only
guaranteed to be defined after a successful match that was executed with the "/p"
(preserve) modifier. The use of these variables incurs no global performance penalty,
unlike their punctuation char equivalents, however at the trade-off that you have to tell
perl when you want to use them. As of Perl 5.20, these three variables are equivalent to
"$`", $& and "$'", and "/p" is ignored.

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

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

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

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

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

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

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

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

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

"(?#text)"
A comment. The text is ignored. Note that Perl closes the comment as soon as it sees
a ")", so there is no way to put a literal ")" in the comment. The pattern's closing
delimiter must be escaped by a backslash if it appears in the comment.

See "/x" for another way to have comments in patterns.

"(?adlupimnsx-imnsx)"
"(?^alupimnsx)"
One or more embedded pattern-match modifiers, to be turned on (or turned off, if
preceded by "-") for the remainder of the pattern or the remainder of the enclosing
pattern group (if any).

This is particularly useful for dynamic patterns, such as those read in from a
configuration file, taken from an argument, or specified in a table somewhere.
Consider the case where some patterns want to be case-sensitive and some do not: The
case-insensitive ones merely need to include "(?i)" at the front of the pattern. For
example:

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

# more flexible:

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

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

( (?i) blah ) \s+ \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 "/i" modifier
outside this group.

These modifiers do not carry over into named subpatterns called in the enclosing
group. In other words, a pattern such as "((?i)(?&NAME))" does not change the case-
sensitivity of the "NAME" pattern.

Any of these modifiers can be set to apply globally to all regular expressions
compiled within the scope of a "use re". See "'/flags' mode" in re.

Starting in Perl 5.14, a "^" (caret or circumflex accent) immediately after the "?" is
a shorthand equivalent to "d-imnsx". 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 in that they can only
be enabled, not disabled, and the "a", "d", "l", and "u" modifiers are mutually
exclusive: specifying one de-specifies the others, and a maximum of one (or two "a"'s)
may appear in the construct. Thus, for example, "(?-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 presence anywhere in a pattern
has a global effect.

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

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

is like

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

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

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

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

is equivalent to the more verbose

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

Note that any "(...)" constructs enclosed within this one will still capture unless
the "/n" modifier is in effect.

Starting in Perl 5.14, a "^" (caret or circumflex accent) immediately after the "?" is
a shorthand equivalent to "d-imnsx". Any positive flags (except "d") may follow the
caret, so

(?^x:foo)

is equivalent to

(?x-imns:foo)

The caret tells Perl that this cluster doesn't inherit the flags of any surrounding
pattern, but uses the system defaults ("d-imnsx"), modified by any flags specified.

The caret allows for simpler stringification of compiled regular expressions. 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 the system default flags hard-
coded in it, just the caret. If new flags are added to Perl, the meaning of the
caret's expansion will change to include the default for those flags, so the test will
still work, unchanged.

Specifying a negative flag after the caret is an error, as the flag is redundant.

Mnemonic for "(?^...)": A fresh beginning since the usual use of a caret is to match
at the beginning.

"(?|pattern)"
This is the "branch reset" pattern, which has the special property that the capture
groups are numbered from the same starting point in each alternation branch. It is
available starting from perl 5.10.0.

Capture groups are numbered from left to right, but inside this construct the
numbering is restarted for each branch.

The numbering within each branch will be as normal, and any groups following this
construct will be numbered as though the construct contained only one branch, that
being the one with the most capture groups in it.

This construct is useful when you want to capture one of a number of alternative
matches.

Consider the following pattern. The numbers underneath show in which 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 the implementation of the branch reset pattern. If
you are using named captures 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 group named "b" are aliases
for the group belonging to $1.

Look-Around Assertions
Look-around assertions are zero-width patterns which match a specific pattern without
including it in $&. Positive assertions match when their subpattern matches, negative
assertions match when their subpattern fails. 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". Note however that look-
ahead and look-behind are NOT the same thing. You cannot use this for look-
behind.

If you are looking for a "bar" that isn't preceded by a "foo", "/(?!foo)bar/" will
not do what you want. That's because the "(?!foo)" is just saying that the next
thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will match. 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" (available since Perl
5.10.0), which causes the regex engine to "keep" everything it had matched prior
to the "\K" and not include it in $&. This effectively provides variable-length
look-behind. The use of "\K" inside of another look-around assertion is allowed,
but the behaviour is currently not well defined.

For various reasons "\K" may be significantly more efficient than the equivalent
"(?<=...)" construct, and it is especially useful in situations where you want to
efficiently remove something following something 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". Works only for fixed-width
look-behind.

"(?'NAME'pattern)"
"(?<NAME>pattern)"
A named capture group. Identical in every respect to normal capturing parentheses "()"
but for the additional fact that the group can be referred to by name in various
regular expression constructs (like "\g{NAME}") and can be accessed by name after a
successful match via "%+" or "%-". See perlvar for 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 similar function in .NET
regexes, the behavior is not. In Perl the groups are numbered sequentially regardless
of being named or not. Thus in the pattern

/(x)(?<foo>y)(z)/

$+{foo} will be the same as $2, and $3 will contain 'z' instead of the 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/" or its Unicode extension (see utf8), though it
isn't extended by the locale (see perllocale).

NOTE: In order to make things easier for programmers with experience with the Python
or PCRE regex engines, the pattern "(?P<NAME>pattern)" may be used instead of
"(?<NAME>pattern)"; however this form does not support the use of single quotes as a
delimiter for the name.

"\k<NAME>"
"\k'NAME'"
Named backreference. Similar to numeric backreferences, except that the group is
designated by name and not number. If multiple groups have the same name then it
refers to the leftmost defined group in the 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 experience with the Python
or PCRE regex engines, the pattern "(?P=NAME)" may be used instead of "\k<NAME>".

"(?{ code })"
WARNING: Using this feature safely requires that you understand its limitations. Code
executed that has side effects may not perform identically from version to version due
to the effect of future optimisations in the regex engine. For more information on
this, see "Embedded Code Execution Frequency".

This zero-width assertion executes any embedded Perl code. It always succeeds, and
its return value is set as $^R.

In literal patterns, the code is parsed at the same time as the surrounding code.
While within the pattern, control is passed temporarily back to the perl parser, until
the logically-balancing closing brace is encountered. This is similar to the way that
an array index expression in a literal string is handled, for example

"abc$array[ 1 + f('[') + g()]def"

In particular, braces do not need to be balanced:

s/abc(?{ f('{'); })/def/

Even in a pattern that is interpolated and compiled at run-time, literal code blocks
will be compiled once, at perl compile time; the following prints "ABCD":

print "D";
my $qr = qr/(?{ BEGIN { print "A" } })/;
my $foo = "foo";
/$foo$qr(?{ BEGIN { print "B" } })/;
BEGIN { print "C" }

In patterns where the text of the code is derived from run-time information rather
than appearing literally in a source code /pattern/, the code is compiled at the same
time that the pattern is compiled, and for reasons of security, "use re 'eval'" must
be in scope. This is to stop user-supplied patterns containing code snippets from
being executable.

In situations where you need to enable this with "use re 'eval'", you should also have
taint checking enabled. Better yet, use the carefully constrained evaluation within a
Safe compartment. See perlsec for details about both these mechanisms.

From the viewpoint of parsing, lexical variable scope and closures,

/AAA(?{ BBB })CCC/

behaves approximately like

/AAA/ && do { BBB } && /CCC/

Similarly,

qr/AAA(?{ BBB })CCC/

behaves approximately like

sub { /AAA/ && do { BBB } && /CCC/ }

In particular:

{ my $i = 1; $r = qr/(?{ print $i })/ }
my $i = 2;
/$r/; # prints "1"

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

The code block introduces a new scope from the perspective of lexical variable
declarations, but not from the perspective of "local" and similar localizing
behaviours. So later code blocks within the same pattern will still see the values
which were localized in earlier blocks. These accumulated localizations are undone
either at the end of a successful match, or if the assertion is backtracked (compare
"Backtracking"). For example,

$_ = '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 initially increment $cnt up to 8; then during backtracking, its value will be
unwound back to 4, which is the value assigned to $res. At the end of the regex
execution, $cnt will be wound back to its initial value of 0.

This assertion may be used as the condition in a

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

switch. If not used in this way, the result of evaluation of "code" is put into the
special variable $^R. This happens immediately, so $^R can be used from other "(?{
code })" assertions inside the same regular expression.

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

Note that the special variable $^N is particularly useful with code blocks to capture
the results of submatches in variables without having to keep track of the number of
nested parentheses. For example:

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

"(??{ code })"
WARNING: Using this feature safely requires that you understand its limitations. Code
executed that has side effects may not perform identically from version to version due
to the effect of future optimisations in the regex engine. For more information on
this, see "Embedded Code Execution Frequency".

This is a "postponed" regular subexpression. It behaves in exactly the same way as a
"(?{ code })" code block as described above, except that its return value, rather than
being assigned to $^R, is treated as a pattern, compiled if it's a string (or used as-
is if its a qr// object), then matched as if it were inserted instead of this
construct.

During the matching of this sub-pattern, it has its own set of captures which are
valid during the sub-match, but are discarded once control returns to the main
pattern. For example, the following matches, with the inner pattern capturing "B" and
matching "BB", while the outer pattern captures "A";

my $inner = '(.)\1';
"ABBA" =~ /^(.)(??{ $inner })\1/;
print $1; # prints "A";

Note that this means that there is no way for the inner pattern to refer to a capture
group defined outside. (The code block itself can use $1, etc., to refer to the
enclosing pattern's capture groups.) Thus, although

('a' x 100)=~/(??{'(.)' x 100})/

will match, it will not set $1 on exit.

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 accomplish the same task.

Executing a postponed regular expression 50 times without consuming any input string
will result in a fatal error. The maximum depth is compiled into perl, so changing it
requires a custom build.

"(?PARNO)" "(?-PARNO)" "(?+PARNO)" "(?R)" "(?0)"
Recursive subpattern. Treat the contents of a given capture buffer in the current
pattern as an independent subpattern and attempt to match it at the current position
in the string. Information about capture state from the caller for things like
backreferences is available to the subpattern, but capture buffers set by the
subpattern are not visible to the caller.

Similar to "(??{ code })" except that it does not involve executing any code or
potentially compiling a returned pattern string; instead it treats the part of the
current pattern contained within a specified capture group as an independent pattern
that must match at the current position. Also different is the treatment of capture
buffers, unlike "(??{ code })" recursive patterns have access to their callers match
state, so one can use backreferences safely.

PARNO is a sequence of digits (not starting with 0) whose value reflects the paren-
number of the capture group to recurse to. "(?R)" recurses to the 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 assumed to be relative, with negative numbers indicating
preceding capture groups and positive ones following. Thus "(?-1)" refers to the most
recently declared group, and "(?+1)" indicates the next group to be declared. Note
that the counting for relative recursion differs from that of relative backreferences,
in that with recursion unclosed groups are included.

The following pattern matches a function foo() which may contain balanced 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 a fatal error.
Recursing deeper than 50 times without consuming any input string will also result in
a fatal error. The maximum depth is compiled into perl, so changing it requires a
custom build.

The following shows how using negative indexing can make it easier to embed recursive
patterns inside of a "qr//" construct for 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 equivalent PCRE or Python
construct of the same form. In Perl you can backtrack into a recursed group, in PCRE
and Python the recursed into group is treated as atomic. Also, modifiers are resolved
at compile time, so constructs like (?i:(?1)) or (?:(?i)(?1)) do not affect how the
sub-pattern will be processed.

"(?&NAME)"
Recurse to a named subpattern. Identical to "(?PARNO)" except that the parenthesis to
recurse to is determined by name. If multiple parentheses have the same name, then it
recurses to the leftmost.

It is an error to refer to a name that is not declared somewhere in the pattern.

NOTE: In order to make things easier for programmers with experience with 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" yields a true value,
matches "no-pattern" otherwise. A missing pattern always matches.

"(condition)" should be one of: 1) an integer in parentheses (which is valid if the
corresponding pair of parentheses matched); 2) a look-ahead/look-behind/evaluate zero-
width assertion; 3) a name in angle brackets or single quotes (which is valid if a
group with the given name matched); or 4) the special symbol (R) (true when evaluated
inside of recursion or eval). Additionally the R may be followed by a number, (which
will be true when evaluated when recursing inside of the appropriate group), or by
&NAME, in which case it will be 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 directly inside 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 executing directly inside
of the leftmost group with a given name (this is the same logic used by "(?&NAME)"
to disambiguate). It does not check the full stack, but only the name of the
innermost active recursion.

(DEFINE)
In this case, the yes-pattern is never directly executed, and no no-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 parentheses themselves.

A special form is the "(DEFINE)" predicate, which never executes its yes-pattern
directly, and does not allow a no-pattern. This allows one to define subpatterns which
will be executed only by the recursion mechanism. This way, you can define a set of
regular expression rules that can be bundled into any pattern you choose.

It is recommended that for this usage you put the DEFINE block at the end of the
pattern, and that you name any subpatterns defined within it.

Also, it's worth noting that patterns defined this way probably will not be as
efficient, as the optimizer is not very clever about handling them.

An example of how this might be used is as follows:

/(?<NAME>(?&NAME_PAT))(?<ADDR>(?&ADDRESS_PAT))
(?(DEFINE)
(?<NAME_PAT>....)
(?<ADDRESS_PAT>....)
)/x

Note that capture groups matched inside of recursion are not accessible after the
recursion returns, so the extra layer of capturing groups is necessary. Thus
$+{NAME_PAT} would not be defined even though $+{NAME} would be.

Finally, keep in mind that subpatterns created inside a DEFINE block count 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 to compile the
definitions with the "qr//" operator, and later interpolate them in another pattern.

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

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

"(?>pattern)" does not disable backtracking altogether once it has matched. It is
still possible to backtrack past the construct, but not into 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 standalone "a+", and the following "\g{-1}" eats
the matched string; it therefore makes a zero-length assertion into an analogue of
"(?>...)". (The difference between these two constructs is that the second one uses a
capturing group, thus shifting ordinals of backreferences in the rest of a regular
expression.)

Consider this pattern:

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

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

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

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

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

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

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

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

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

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

In some literature this construct is called "atomic matching" or "possessive
matching".

Possessive quantifiers are equivalent to putting the item they are applied to 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})

"(?[ ])"
See "Extended Bracketed Character Classes" in perlrecharclass.

Special Backtracking Control Verbs
These special patterns are generally of the form "(*VERB:ARG)". Unless otherwise stated
the ARG argument is optional; in some cases, it is forbidden.

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

On failure, the $REGERROR variable will be set to the ARG value of the verb pattern, if
the verb was involved in the failure of the match. If the ARG part of the pattern was
omitted, then $REGERROR will be set to the name of the last "(*MARK:NAME)" pattern
executed, or to TRUE if there was none. Also, the $REGMARK variable will be set to FALSE.

On a successful match, the $REGERROR variable will be set to FALSE, and the $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 $1 and most other regex-related
variables. They are not local to a scope, nor readonly, 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 an argument, 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 point when
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, matching continues in B, which may also
backtrack as necessary; however, should B not match, then no further backtracking
will take place, and the pattern will fail outright at the current starting
position.

The following example counts all the possible matching strings in a pattern
(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 string at 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 to control
backtracking. In some cases, the use of "(*PRUNE)" can be replaced 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 on failure it also
signifies that whatever text that was matched leading up to the "(*SKIP)" pattern
being executed cannot be part of any match of this pattern. This effectively means
that the regex engine "skips" forward to this position on failure and tries to
match again, (assuming that there 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 position which is
used as the "skip point". If no "(*MARK)" of that name was encountered, then the
"(*SKIP)" operator has no effect. When used without a name the "skip point" is
where the match point was when executing the (*SKIP) pattern.

Compare the following to the examples in "(*PRUNE)"; note the string is 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 string when a
certain part of the pattern has been successfully matched. This mark may be given a
name. A later "(*SKIP)" pattern will then skip forward 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, the program can determine
which branches of the pattern were involved in the match.

When a match is successful, the $REGMARK variable will be set to the name of the
most recently executed "(*MARK:NAME)" that was involved in the match.

This can be used to determine which branch of a pattern was matched without using a
separate capture group for each branch, which in turn can 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 in failing the
match and has provided its own name to use, the $REGERROR variable 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 on failure, it causes the regex
engine to try the next alternation in the innermost enclosing group (capturing or
otherwise) that has alternations. The two branches of a
"(?(condition)yes-pattern|no-pattern)" do not count as an alternation, as far as
"(*THEN)" is concerned.

Its name comes from the observation that this operation combined with the
alternation operator ("|") can be used to create what is essentially a pattern-
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 then it acts
exactly like the "(*PRUNE)" operator.

/ A (*PRUNE) B /

is the same as

/ A (*THEN) B /

but

/ ( A (*THEN) B | C ) /

is not the same as

/ ( A (*PRUNE) B | C ) /

as after matching the A but failing on the B the "(*THEN)" verb will backtrack 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 a zero-width pattern
similar to "(*SKIP)", except that when backtracked into on failure it causes the
match to fail outright. No further attempts to 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 pattern does not
match, the regex engine will not try any further matching on the rest of the
string.

"(*FAIL)" "(*F)"
This pattern matches nothing and always fails. It can be used to force the engine
to backtrack. It is equivalent to "(?!)", but easier to read. In fact, "(?!)" gets
optimised into "(*FAIL)" internally.

It is probably useful only when combined with "(?{})" or "(??{})".

"(*ACCEPT)"
This pattern matches nothing and causes the end of successful matching at the point
at which the "(*ACCEPT)" pattern was encountered, regardless of whether there is
actually more to match in the string. When inside of a nested pattern, such as
recursion, or in a subpattern dynamically generated via "(??{})", only the
innermost pattern is ended immediately.

If the "(*ACCEPT)" is inside of capturing groups then the groups are marked 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 not be set. If another
branch in the inner parentheses was matched, such as in the string 'ACDE', then the
"D" and "E" would have to be matched as well.

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

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

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

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

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

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

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

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

Which perhaps unexpectedly yields:

got <d is under the bar in the >

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

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

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

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

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

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

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

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

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

That will print out:

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

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

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

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

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

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

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

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

This prints

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

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

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

The pattern really, really wants to succeed, so it uses the standard pattern back-off-and-
retry and lets "\D*" expand to just "AB" this time. Now there's indeed something
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 digit and by something that's not "123". Remember
that the look-aheads are zero-width expressions--they only look, but don't consume any of
the string in their match. So rewriting this way produces what you'd expect; that is,
case 5 will fail, but case 6 succeeds:

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

6: got ABC

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

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

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

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

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

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

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

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

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

Note also that the whole range idea is rather unportable between character sets, except
for four situations that Perl handles specially. Any subset of the ranges "[A-Z]",
"[a-z]", and "[0-9]" are guaranteed to match the expected subset of ASCII characters, no
matter what character set the platform is running. The fourth portable way to specify
ranges is to use the "\N{...}" syntax to specify either end point of the range. For
example, "[\N{U+04}-\N{U+07}]" means to match the Unicode code points "\N{U+04}",
"\N{U+05}", "\N{U+06}", and "\N{U+07}", whatever their native values may be on the
platform. Under use re 'strict' or within a ""(?[ ])"", a warning is raised, if enabled,
and the other end point of a range which has a "\N{...}" endpoint is not portably
specified. For example,

[\N{U+00}-\x06] # Warning under "use re 'strict'".

It is hard to understand without digging what exactly matches ranges other than subsets of
"[A-Z]", "[a-z]", and "[0-9]". A sound principle is to use only ranges that begin from
and end at either alphabetics of equal case ([a-e], [A-E]), or digits ([0-9]). Anything
else is unsafe or unclear. If in doubt, spell out the range in full.

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

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

Alternatives are tried from left to right, so the first alternative found for which the
entire expression matches, is the one that is chosen. This means that alternatives are not
necessarily greedy. For example: when matching "foo|foot" against "barefoot", only the
"foo" part will match, as that is the first alternative tried, and it successfully matches
the target string. (This might not seem important, but it is important when you are
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 reference by enclosing them in
parentheses, and you may refer back to the nth subpattern later in the pattern using the
metacharacter \n or \gn. Subpatterns are numbered based on the left to right order of
their opening parenthesis. A backreference matches whatever actually matched the
subpattern in the string being examined, not the rules for that subpattern. Therefore,
"(0|0x)\d*\s\g1\d*" will match "0x1234 0x4321", but not "0x1234 01234", because subpattern
1 matched "0x", even though the rule "0|0x" could potentially match the leading 0 in the
second number.

Warning on \1 Instead of $1
Some people get too used to writing things like:

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

This is grandfathered (for \1 to \9) for the RHS of a substitute to avoid shocking the sed
addicts, but it's a dirty habit to get into. That's because in PerlThink, the righthand
side of an "s///" is a double-quoted string. "\1" in the usual double-quoted string means
a control-A. The customary Unix meaning of "\1" is kludged in for "s///". However, if
you get into the habit of doing that, you get yourself into trouble if you then add an
"/e" modifier.

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

Or if you try to do

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

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

Repeated Patterns Matching a Zero-length Substring
WARNING: Difficult material (and prose) ahead. This section needs a rewrite.

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

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

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

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

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

or

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

or the loop implied by split().

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

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

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

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

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

is made equivalent to

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

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 sixth iteration of
the outermost "(?:)*" matches a zero-length string, it stops the "*".

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

For example:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

"(??{ EXPR })", "(?PARNO)"
The ordering is the same as for the regular expression which is the 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 is already
determined. The ordering of the matches is the same as for the chosen subexpression.

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

Creating Custom RE Engines
As of Perl 5.10.0, one can create custom regular expression engines. This is not for the
faint of heart, as they have to plug in at the C level. See perlreapi for more details.

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

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

package customre;
use overload;

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

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

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

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

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

Embedded Code Execution Frequency
The exact rules for how often (??{}) and (?{}) are executed in a pattern are unspecified.
In the case of a successful match you can assume that they DWIM and will be executed in
left to right order the appropriate number of times in the accepting path of the pattern
as would any other meta-pattern. How non-accepting pathways and match failures affect the
number of times a pattern is executed is specifically unspecified and may vary depending
on what optimizations can be applied to the pattern and is likely to change from version
to version.

For instance in

"aaabcdeeeee"=~/a(?{print "a"})b(?{print "b"})cde/;

the exact number of times "a" or "b" are printed out is unspecified for failure, but you
may assume they will be printed at least once during a successful match, additionally you
may assume that if "b" is printed, it will be preceded by at least one "a".

In the case of branching constructs like the following:

/a(b|(?{ print "a" }))c(?{ print "c" })/;

you can assume that the input "ac" will output "ac", and that "abc" will output only "c".

When embedded code is quantified, successful matches will call the code once for each
matched iteration of the quantifier. For example:

"good" =~ /g(?:o(?{print "o"}))*d/;

will output "o" twice.

PCRE/Python Support
As of Perl 5.10.0, Perl supports several Python/PCRE-specific extensions to the regex
syntax. While Perl programmers are encouraged to use the Perl-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)".

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