Yolinux.com

perlre manpage

Search topic Section


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



NAME
       perlre - Perl regular expressions

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

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

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

   Modifiers
       Matching operations can have various modifiers.	Modifiers that relate
       to the 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 or end of the string to matching the start or
	   end of any line anywhere within the string.

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

	   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 an inverted bracketed
	   character class, which otherwise could be highly confusing.	See
	   "Negation" in perlrecharclass.

	   Another bug involves character classes that match both a sequence
	   of multiple characters, and an initial sub-string of that sequence.
	   For example,

	    /[s\xDF]/i

	   should match both a single and a double "s", since "\xDF" (on ASCII
	   platforms) matches "ss".  However, this bug ([perl #89774]
	   <https://rt.perl.org/rt3/Ticket/Display.html?id=89774>) causes it
	   to only match a single "s", even if the final larger match fails,
	   and matching the double "ss" would have succeeded.

	   Also, Perl matching doesn't fully conform to the current Unicode
	   "/i" recommendations, which ask that the matching be made upon the
	   NFD (Normalization Form Decomposed) of the text.  However, Unicode
	   is in the process of reconsidering and revising their
	   recommendations.

       x   Extend your pattern's legibility by permitting whitespace and
	   comments.  Details in "/x"

       p   Preserve the string matched such that ${^PREMATCH}, ${^MATCH}, and
	   ${^POSTMATCH} are available for use after matching.

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

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

       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 "/imsxadlup" may also be embedded
       within the regular expression itself using the "(?...)" construct, see
       "Extended Patterns" below.

       /x

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

       Character set modifiers

       "/d", "/u", "/a", and "/l", available starting in 5.14, are called the
       character set modifiers; they affect the character set semantics 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.

       Perl only supports single-byte locales.	This means that code points
       above 255 are treated as Unicode no matter what locale is in effect.
       Under Unicode rules, there are a few case-insensitive matches that
       cross the 255/256 boundary.  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.

       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.

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

       This modifier may be specified to be the default by "use feature
       'unicode_strings", "use locale ':not_characters'", 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{...}")

       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".

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

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

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

       This modifier is automatically selected by default when none of 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]"; "\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 semantics; 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 explictly 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 line (or before newline at the end)
	   |	    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 5.9.)

       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.  In particular, the lower quantifier bound is not optional.
       However, in Perl v5.18, it is planned to issue a deprecation warning
       for all such occurrences, and in Perl v5.20 to require literal uses of
       a curly bracket to be escaped, say by preceding them with a backslash
       or enclosing them within square brackets, ("\{" or "[{]").  This change
       will allow for future syntax extensions (like making the lower bound of
       a quantifier optional), and better error checking of quantifiers.  Now,
       a typo in a quantifier silently causes it to be treated as the literal
       characters.  For example,

	   /o{4,3}/

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

       The "*" quantifier is equivalent to "{0,}", the "+" quantifier to
       "{1,}", and the "?" quantifier to "{0,1}".  n and m are 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

       By default, 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:

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

       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 till \E (think vi)
	\U	    uppercase till \E (think vi)
	\Q	    quote (disable) pattern metacharacters till \E
	\E	    end either case modification or quoted section, think vi

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

       Character Classes and other Special Escapes

       In addition, Perl defines the following:

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

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

       [2] See "POSIX Character Classes" in perlrecharclass for details.

       [3] See "Backslash sequences" in perlrecharclass for details.

       [4] See "Misc" in perlrebackslash for details.

       [5] See "Capture groups" below for details.

       [6] See "Extended Patterns" below for details.

       [7] Note that "\N" has two meanings.  When of the form "\N{NAME}", it
	   matches 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".

       Assertions

       Perl defines the following zero-width assertions:

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

       A word boundary ("\b") is a spot between two 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.

       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: 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.  As of 5.005, $& is not so costly as the
       other two.

       As a workaround for this problem, Perl 5.10.0 introduces
       "${^PREMATCH}", "${^MATCH}" and "${^POSTMATCH}", which are equivalent
       to "$`", $& and "$'", except that they are only guaranteed to be
       defined after 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.

   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.  If the "/x" modifier enables
	   whitespace formatting, a simple "#" will suffice.  Note that Perl
	   closes the comment as soon as it sees a ")", so there is no way to
	   put a literal ")" in the comment.

       "(?adlupimsx-imsx)"
       "(?^alupimsx)"
	   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-imsx".
	   Flags (except "d") may follow the caret to override it.  But a
	   minus sign is not legal with it.

	   Note that the "a", "d", "l", "p", and "u" modifiers are special 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)"
       "(?adluimsx-imsx:pattern)"
       "(?^aluimsx:pattern)"
	   This is for clustering, not capturing; it groups subexpressions
	   like "()", but doesn't make backreferences as "()" does.  So

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

	   is like

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

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

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

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

	   is equivalent to the more verbose

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

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

	       (?^x:foo)

	   is equivalent to

	       (?x-ims:foo)

	   The caret tells Perl that this cluster doesn't inherit the flags of
	   any surrounding pattern, but uses the system defaults ("d-imsx"),
	   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", 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: This extended regular expression feature is considered
	   experimental, and may be changed without notice. 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.

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

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

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

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

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

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

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

	   This assertion may be used as a
	   "(?(condition)yes-pattern|no-pattern)" switch.  If not used in this
	   way, the result of evaluation of "code" is put into the 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".

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

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

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

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

	   WARNING: Use of lexical ("my") variables in these blocks is broken.
	   The result is unpredictable and will make perl unstable. The
	   workaround is to use global ("our") variables.

	   WARNING: In perl 5.12.x and earlier, the regex engine was not re-
	   entrant, so interpolated code could not safely invoke the regex
	   engine either directly with "m//" or "s///"), or indirectly with
	   functions such as "split". Invoking the regex engine in these
	   blocks would make perl unstable.

       "(??{ code })"
	   WARNING: This extended regular expression feature is considered
	   experimental, and may be changed without notice. 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.

	   This is a "postponed" regular subexpression.	 The "code" is
	   evaluated at run time, at the moment this subexpression may match.
	   The result of evaluation is considered a regular expression and
	   matched as if it were inserted instead of this construct.  Note
	   that this means that the contents of capture groups defined inside
	   an eval'ed pattern are not available outside of the pattern, and
	   vice versa, there is no way for the inner pattern returned from the
	   code block 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,

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

	   will match, it will not set $1.

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

	   The following pattern matches a parenthesized group:

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

	   See also "(?PARNO)" for a different, more efficient way to
	   accomplish the same task.

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

	   In perl 5.12.x and earlier, because the regex engine was not re-
	   entrant, delayed code could not safely invoke the regex engine
	   either directly with "m//" or "s///"), or indirectly with functions
	   such as "split".

	   Recursing deeper than 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)"
	   Similar to "(??{ code })" except it does not involve compiling any
	   code, instead it treats the contents of a capture group as an
	   independent pattern that must match at the current position.
	   Capture groups contained by the pattern will have the value as
	   determined by the outermost recursion.

	   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 either an integer in parentheses (which is
	   valid if the corresponding pair of parentheses matched), a
	   look-ahead/look-behind/evaluate zero-width assertion, a name in
	   angle brackets or single quotes (which is valid if a group with the
	   given name matched), or 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 optimiser 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>....)
		(?<ADRESS_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})

   Special Backtracking Control Verbs
       WARNING: These patterns are experimental and subject to change or
       removal in a future version of Perl. Their usage in production code
       should be noted to avoid problems during upgrades.

       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 (*THEN) D ) /

	      is not the same as

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

	      as after matching the A but failing on the B the "(*THEN)" verb
	      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)"
	      WARNING: This feature is highly experimental. It is not
	      recommended for production code.

	      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--and even within character sets they may cause results
       you probably didn't expect.  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.  If in doubt,
       spell out the character sets in full.

       Characters may be specified using a metacharacter syntax much like that
       used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return,
       "\f" a form feed, etc.  More 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|/;

   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)".

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

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

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

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

SEE ALSO
       perlrequick.

       perlretut.

       "Regexp Quote-Like Operators" in perlop.

       "Gory details of parsing quoted constructs" in perlop.

       perlfaq6.

       "pos" in perlfunc.

       perllocale.

       perlebcdic.

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



perl v5.16.3			  2013-03-04			     PERLRE(1)