perlre


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!

           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, new in 5.14, affect which character-set semantics
           (Unicode, ASCII, etc.) are used, as described below in "Character
           set modifiers".

       These are usually written as "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.

       The "/x", "/l", "/u", "/a" and "/d" modifiers need a little more
       explanation.

       /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
       to any replacement done.  For example,

        s/foo/\Ubar/l

       will uppercase "bar", but the "/l" does not affect how the "\U"
       operates.  If "use locale" is in effect, the "\U" will use locale
       rules; if "use feature 'unicode_strings'" is in effect, it will use
       Unicode rules, etc.

       /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 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),
       whereas in strict ASCII their meanings are undefined.  Thus the
       platform effectively becomes a Unicode platform, 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 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.
       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", but see "Which character set modifier is in
       effect?".

       /a

       is the same as "/u", except that "\d", "\s", "\w", and the Posix
       character classes are restricted to matching in the ASCII range only.
       That is, with this modifier, "\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.
       With it, one can write "\d" with confidence that it will only match
       ASCII characters, and should the need arise to match beyond ASCII, you
       can use "\p{Digit}", or "\p{Word}" for "\w".  There are similar
       "\p{...}" constructs that can match white space and Posix classes
       beyond ASCII.  See "POSIX Character Classes" in perlrecharclass.

       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"

       To reiterate, 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'", 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
       results.  See "The "Unicode Bug"" in perlunicode.

       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.

       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.  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
       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".
       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
           .        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 bound is not optional.)  The "*"
       quantifier is equivalent to "{0,}", the "+" quantifier to "{1,}", and
       the "?" quantifier to "{0,1}".  n and m are limited to 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 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
         \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

       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:


       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";

           /((.)(.)(.)(.)(.)(.)(.)(.)(.))\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
       meanings like this:

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

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

   Extended Patterns
       Perl also defines a consistent extension syntax for features not found
       in standard tools like awk and lex.  The syntax 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/ ) { }

                 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


                     (?^: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:

                 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)"
                 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
                 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

       "(??{ 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 to refer to a capture group defined
                 outside.  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.
                 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) {

                 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

                 (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.

       "(?>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.

                 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
                     (?>#[ \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
               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).
                   '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)" "(*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).

               Its name comes from the observation that this operation
               combined with the alternation operator ("|") can be used to

               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.

           "(*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.

       Verbs without an argument
           "(*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

       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 >

       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";
           }


       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:
       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
       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:
           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
       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".
           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;
                     }
                     { $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.


perl v5.14.2                      2011-09-26                         PERLRE(1)
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