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 line only at the left and right ends
           of the string to matching them 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!

           ${^POSTMATCH} are available for use after matching.

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

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

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

       /x

       "/x" tells the regular expression parser to ignore most whitespace that
       is neither backslashed nor within a character class.  You can use this
       to break up your regular expression into (slightly) more readable
       parts.  The "#" character is also treated as a metacharacter
       introducing a comment, just as in ordinary Perl code.  This also means
       that if you want real whitespace or "#" characters in the pattern
       (outside a character class, where they are unaffected by "/x"), then
       you'll either have to escape them (using backslashes or "\Q...\E") or
       encode them using octal, hex, or "\N{}" escapes.  Taken together, these
       features go a long way towards making Perl's regular expressions more
       readable.  Note that you have to be careful not to include the pattern
       delimiter in the comment--perl has no way of knowing you did not intend
       to close the pattern early.  See the C-comment deletion code in perlop.
       Also note that anything inside a "\Q...\E" stays unaffected by "/x".
       And note that "/x" doesn't affect space interpretation within a single
       multi-character construct.  For example in "\x{...}", regardless of the
       "/x" modifier, there can be no spaces.  Same for a quantifier such as
       "{3}" or "{5,}".  Similarly, "(?:...)" can't have a space between the
       "(", "?", and ":".  Within any delimiters for such a construct, allowed
       spaces are not affected by "/x", and depend on the construct.  For
       example, "\x{...}" can't have spaces because hexadecimal numbers don't
       have spaces in them.  But, Unicode properties can have spaces, so in
       "\p{...}" there can be spaces that follow the Unicode rules, for which
       see "Properties accessible through \p{} and \P{}" in perluniprops.

       Character set modifiers

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

       The "/d", "/u", and "/l" modifiers are not likely to be of much use to
       effect at the time of the execution of the pattern match.

       "/u" sets the character set to Unicode.

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

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

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

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

        s/foo/\Ubar/il

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

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

        s/foo/\Lbar/iu

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

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

       /l

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

       /u

       means to use Unicode rules when pattern matching.  On ASCII platforms,
       this means that the code points between 128 and 255 take on their
       Latin-1 (ISO-8859-1) meanings (which are the same as Unicode's).
       (Otherwise Perl considers their meanings to be undefined.)  Thus, under
       this modifier, the ASCII platform effectively becomes a Unicode
       platform; and hence, for example, "\w" will match any of the more than
       100_000 word characters in Unicode.

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

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

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

       /d

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

       1.  the target string is encoded in UTF-8; or

       2.  the pattern is encoded in UTF-8; or

       3.  the pattern explicitly mentions a code point that is above 255 (say
           by "\x{100}"); or

       4.  the pattern uses a Unicode name ("\N{...}");  or

       5.  the pattern uses a Unicode property ("\p{...}"); or
       Here are some examples of how that works on an ASCII platform:

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

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

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

       /a (and /aa)

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

       When it appears singly, it causes the sequences "\d", "\s", "\w", and
       the Posix character classes to match only in the ASCII range.  They
       thus revert to their pre-5.6, pre-Unicode meanings.  Under "/a",  "\d"
       always means precisely the digits "0" to "9"; "\s" means the five
       characters "[ \f\n\r\t]", and starting in Perl v5.18, experimentally,
       the vertical tab; "\w" means the 63 characters "[A-Za-z0-9_]"; and
       likewise, all the Posix classes such as "[[:print:]]" match only the
       appropriate ASCII-range characters.

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

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

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

       Otherwise, "/a" behaves like the "/u" modifier, in that case-
       insensitive matching uses Unicode 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.

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

       Which character set modifier is in effect?

       Which of these modifiers is in effect at any given point in a regular
       expression depends on a fairly complex set of interactions.  These have
       been designed so that in general you don't have to worry about it, but
       this section gives the gory details.  As explained below in "Extended
       Patterns" it is possible to explicitly specify modifiers that apply
       only to portions of a regular expression.  The innermost always has
       priority over any outer ones, and one applying to the whole expression
       has priority over any of the default settings that are described in the
       remainder of this section.

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

       Otherwise, "use locale" sets the default modifier to "/l"; and "use
       feature 'unicode_strings", or "use 5.012" (or higher) set the default
       to "/u" when not in the same scope as either "use locale" or "use
       bytes".  ("use locale ':not_characters'" also sets the default to "/u",
       overriding any plain "use locale".)  Unlike the mechanisms mentioned
       above, these affect operations besides regular expressions pattern
       matching, and so give more consistent results with other operators,
       including using "\U", "\l", etc. in substitution replacements.

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

       Character set modifier behavior prior to Perl 5.14

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

   Regular Expressions
       Metacharacters

       The patterns used in Perl pattern matching evolved from those supplied
           ()       Grouping
           []       Bracketed Character class

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

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

       Quantifiers

       The following standard quantifiers are recognized:

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

       (If a curly bracket occurs in any other context and does not form part
       of a backslashed sequence like "\x{...}", it is treated as a regular
       character.  In particular, the lower quantifier bound is not optional,
       and a typo in a quantifier silently causes it to be treated as the
       literal characters.  For example,

           /o{4,3}/

       looks like a quantifier that matches 0 times, since 4 is greater than
       3, but it really means to match the sequence of six characters
       "o { 4 , 3 }".  It is planned to eventually require literal uses of
       curly brackets to be escaped, say by preceding them with a backslash or
       enclosing them within square brackets, ("\{" or "[{]").  This change
       will allow for future syntax extensions (like making the lower bound of
       a quantifier optional), and better error checking.  In the meantime,
       you should get in the habit of escaping all instances where you mean a
       literal "{".)

       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:

           {n,}?     Match at least n times, not greedily
           {n,m}?    Match at least n but not more than m times, not greedily

       By default, when a quantified subpattern does not allow the rest of the
       overall pattern to match, Perl will backtrack. However, this behaviour
       is sometimes undesirable. Thus Perl provides the "possessive"
       quantifier form as well.

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

       For instance,

          'aaaa' =~ /a++a/

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

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

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

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

       Escape sequences

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

        \t          tab                   (HT, TAB)
        \n          newline               (LF, NL)
        \r          return                (CR)
        \f          form feed             (FF)
        \a          alarm (bell)          (BEL)
        \e          escape (think troff)  (ESC)
        \cK         control char          (example: VT)
        \x{}, \x00  character whose ordinal is the given hexadecimal number
        \N{name}    named Unicode character or character sequence
        \N{U+263D}  Unicode character     (example: FIRST QUARTER MOON)
        \o{}, \000  character whose ordinal is the given octal number
        \l          lowercase next char (think vi)
        \u          uppercase next char (think vi)
        \L          lowercase till \E (think vi)
                          bracketed character class defined by the "...".
                          Example: [a-z] matches "a" or "b" or "c" ... or "z"
         [[:...:]] [2]  Match a character according to the rules of the POSIX
                          character class "..." within the outer bracketed
                          character class.  Example: [[:upper:]] matches any
                          uppercase character.
         (?[...])  [8]  Extended bracketed character class
         \w        [3]  Match a "word" character (alphanumeric plus "_", plus
                          other connector punctuation chars plus Unicode
                          marks)
         \W        [3]  Match a non-"word" character
         \s        [3]  Match a whitespace character
         \S        [3]  Match a non-whitespace character
         \d        [3]  Match a decimal digit character
         \D        [3]  Match a non-digit character
         \pP       [3]  Match P, named property.  Use \p{Prop} for longer names
         \PP       [3]  Match non-P
         \X        [4]  Match Unicode "eXtended grapheme cluster"
         \C             Match a single C-language char (octet) even if that is
                          part of a larger UTF-8 character.  Thus it breaks up
                          characters into their UTF-8 bytes, so you may end up
                          with malformed pieces of UTF-8.  Unsupported in
                          lookbehind.
         \1        [5]  Backreference to a specific capture group or buffer.
                          '1' may actually be any positive integer.
         \g1       [5]  Backreference to a specific or previous group,
         \g{-1}    [5]  The number may be negative indicating a relative
                          previous group and may optionally be wrapped in
                          curly brackets for safer parsing.
         \g{name}  [5]  Named backreference
         \k<name>  [5]  Named backreference
         \K        [6]  Keep the stuff left of the \K, don't include it in $&
         \N        [7]  Any character but \n.  Not affected by /s modifier
         \v        [3]  Vertical whitespace
         \V        [3]  Not vertical whitespace
         \h        [3]  Horizontal whitespace
         \H        [3]  Not horizontal whitespace
         \R        [4]  Linebreak

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

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

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

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

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

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

       [7] Note that "\N" has two meanings.  When of the form "\N{NAME}", it
           matches the character or character sequence whose name is "NAME";
           and similarly when of the form "\N{U+hex}", it matches the
           \A  Match only at beginning of string
           \Z  Match only at end of string, or before newline at the end
           \z  Match only at end of string
           \G  Match only at pos() (e.g. at the end-of-match position
               of prior m//g)

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

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

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

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

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

       Capture groups

       The bracketing construct "( ... )" creates capture groups (also
       referred to as capture buffers). To refer to the current contents of a
       group later on, within the same pattern, use "\g1" (or "\g{1}") for the
       first, "\g2" (or "\g{2}") for the second, and so on.  This is called a
       backreference.





       preceding capture group, and "\g-2" and "\g{-2}" both refer to the
       group before it.  For example:

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

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

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

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

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

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

           /(?<char>.)\k<char>/            # ... a different way
                and print "'$+{char}' is the first doubled character\n";

           /(?'char'.)\g1/                 # ... mix and match
                and print "'$1' is the first doubled character\n";

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

           /(.)(.)(.)(.)(.)(.)(.)(.)(.)\g10/   # \g10 is a backreference
           /(.)(.)(.)(.)(.)(.)(.)(.)(.)\10/    # \10 is octal
           /((.)(.)(.)(.)(.)(.)(.)(.)(.))\10/  # \10 is a backreference
           /((.)(.)(.)(.)(.)(.)(.)(.)(.))\010/ # \010 is octal

           $a = '(.)\1';        # Creates problems when concatenated.
           $b = '(.)\g{1}';     # Avoids the problems.
           "aa" =~ /${a}/;      # True
           "aa" =~ /${b}/;      # True
           "aa0" =~ /${a}0/;    # False!
           "aa0" =~ /${b}0/;    # True
           "aa\x08" =~ /${a}0/;  # True!
           "aa\x08" =~ /${b}0/;  # False

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

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

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

       WARNING: Once Perl sees that you need one of $&, "$`", or "$'" anywhere
       in the program, it has to provide them for every pattern match.  This
       may substantially slow your program.  Perl uses the same mechanism to
       produce $1, $2, etc, so you also pay a price for each pattern that
       contains capturing parentheses.  (To avoid this cost while retaining
       the grouping behaviour, use the extended regular expression "(?: ... )"
       instead.)  But if you never use $&, "$`" or "$'", then patterns without
       capturing parentheses will not be penalized.  So avoid $&, "$'", and
       "$`" if you can, but if you can't (and some algorithms really
       at the trade-off that you have to tell perl when you want to use them.

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

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

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

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

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

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

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

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

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

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

       "(?adlupimsx-imsx)"
       "(?^alupimsx)"
               if ( /$pattern/i ) { }

               # more flexible:

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

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

               ( (?i) blah ) \s+ \g1

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

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

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

           Starting in Perl 5.14, a "^" (caret or circumflex accent)
           immediately after the "?" is a shorthand equivalent to "d-imsx".
           Flags (except "d") may follow the caret to override it.  But a
           minus sign is not legal with it.

           Note that the "a", "d", "l", "p", and "u" modifiers are special in
           that they can only be enabled, not disabled, and the "a", "d", "l",
           and "u" modifiers are mutually exclusive: specifying one de-
           specifies the others, and a maximum of one (or two "a"'s) may
           appear in the construct.  Thus, for example, "(?-p)" will warn when
           compiled under "use warnings"; "(?-d:...)" and "(?dl:...)" are
           fatal errors.

           Note also that the "p" modifier is special in that its presence
           anywhere in a pattern has a global effect.

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

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

           is like

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

           but doesn't spit out extra fields.  It's also cheaper not to
           immediately after the "?" is a shorthand equivalent to "d-imsx".
           Any positive flags (except "d") may follow the caret, so

               (?^x:foo)

           is equivalent to

               (?x-ims:foo)

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

           The caret allows for simpler stringification of compiled regular
           expressions.  These look like

               (?^:pattern)

           with any non-default flags appearing between the caret and the
           colon.  A test that looks at such stringification thus doesn't need
           to have the system default flags hard-coded in it, just the caret.
           If new flags are added to Perl, the meaning of the caret's
           expansion will change to include the default for those flags, so
           the test will still work, unchanged.

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

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

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

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

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

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

           Consider the following pattern.  The numbers underneath show in
           which group the captured content will be stored.

               # before  ---------------branch-reset----------- after
               / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
               # 1            2         2  3        2     3     4

             "12" =~ /(?| (?<a> \d+ ) | (?<b> \D+))/x;
             say $+ {a};   # Prints '12'
             say $+ {b};   # *Also* prints '12'.

           The problem here is that both the group named "a" and the group
           named "b" are aliases for the group belonging to $1.

       Look-Around Assertions
           Look-around assertions are zero-width patterns which match a
           specific pattern without including it in $&. Positive assertions
           match when their subpattern matches, negative assertions match when
           their subpattern fails. Look-behind matches text up to the current
           match position, look-ahead matches text following the current match
           position.

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

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

               If you are looking for a "bar" that isn't preceded by a "foo",
               "/(?!foo)bar/" will not do what you want.  That's because the
               "(?!foo)" is just saying that the next thing cannot be
               "foo"--and it's not, it's a "bar", so "foobar" will match.  Use
               look-behind instead (see below).

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

               There is a special form of this construct, called "\K", which
               causes the regex engine to "keep" everything it had matched
               prior to the "\K" and not include it in $&. This effectively
               provides variable-length look-behind. The use of "\K" inside of
               another look-around assertion is allowed, but the behaviour is
               currently not well defined.

               For various reasons "\K" may be significantly more efficient
               than the equivalent "(?<=...)" construct, and it is especially
               useful in situations where you want to efficiently remove
               something following something else in a string. For instance

                 s/(foo)bar/$1/g;

           capturing parentheses "()" but for the additional fact that the
           group can be referred to by name in various regular expression
           constructs (like "\g{NAME}") and can be accessed by name after a
           successful match via "%+" or "%-". See perlvar for more details on
           the "%+" and "%-" hashes.

           If multiple distinct capture groups have the same name then the
           $+{NAME} will refer to the leftmost defined group in the match.

           The forms "(?'NAME'pattern)" and "(?<NAME>pattern)" are equivalent.

           NOTE: While the notation of this construct is the same as the
           similar function in .NET regexes, the behavior is not. In Perl the
           groups are numbered sequentially regardless of being named or not.
           Thus in the pattern

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

           $+{foo} will be the same as $2, and $3 will contain 'z' instead of
           the opposite which is what a .NET regex hacker might expect.

           Currently NAME is restricted to simple identifiers only.  In other
           words, it must match "/^[_A-Za-z][_A-Za-z0-9]*\z/" or its Unicode
           extension (see utf8), though it isn't extended by the locale (see
           perllocale).

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

       "\k<NAME>"
       "\k'NAME'"
           Named backreference. Similar to numeric backreferences, except that
           the group is designated by name and not number. If multiple groups
           have the same name then it refers to the leftmost defined group in
           the current match.

           It is an error to refer to a name not defined by a "(?<NAME>)"
           earlier in the pattern.

           Both forms are equivalent.

           NOTE: In order to make things easier for programmers with
           experience with the Python or PCRE regex engines, the pattern
           "(?P=NAME)" may be used instead of "\k<NAME>".

       "(?{ code })"
           WARNING: This extended regular expression feature is considered
           experimental, and may be changed without notice. Code executed that
           has side effects may not perform identically from version to
           version due to the effect of future optimisations in the regex
           engine. The implementation of this feature was radically overhauled

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

           In particular, braces do not need to be balanced:

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

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

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

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

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

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

               /AAA(?{ BBB })CCC/

           behaves approximately like

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

           Similarly,

               qr/AAA(?{ BBB })CCC/

           behaves approximately like

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

           In particular:

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

           Inside a "(?{...})" block, $_ refers to the string the regular
           expression is matching against. You can also use "pos()" to know
           what is the current position of matching within this string.
                (
                  a
                  (?{
                      local $cnt = $cnt + 1;  # Update $cnt,
                                              # backtracking-safe.
                  })
                )*
                aaaa
                (?{ $res = $cnt })            # On success copy to
                                              # non-localized location.
              >x;

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

           This assertion may be used as the condition in a

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

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

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

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

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

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

           During the matching of this sub-pattern, it has its own set of
           groups.)  Thus, although

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

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

           The following pattern matches a parenthesized group:

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

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

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

       "(?PARNO)" "(?-PARNO)" "(?+PARNO)" "(?R)" "(?0)"
           Similar to "(??{ code })" except that it does not involve executing
           any code or potentially compiling a returned pattern string;
           instead it treats the part of the current pattern contained within
           a specified capture group as an independent pattern that must match
           at the current position.  Capture groups contained by the pattern
           will have the value as determined by the outermost recursion.

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

           The following pattern matches a function foo() which may contain
           balanced parentheses as the argument.

             $re = qr{ (                   # paren group 1 (full function)
                         foo
                         (                 # paren group 2 (parens)
                           \(
                             (             # paren group 3 (contents of parens)
                             (?:

               'foo(bar(baz)+baz(bop))'=~/$re/
                   and print "\$1 = $1\n",
                             "\$2 = $2\n",
                             "\$3 = $3\n";

           the output produced should be the following:

               $1 = foo(bar(baz)+baz(bop))
               $2 = (bar(baz)+baz(bop))
               $3 = bar(baz)+baz(bop)

           If there is no corresponding capture group defined, then it is a
           fatal error.  Recursing deeper than 50 times without consuming any
           input string will also result in a fatal error.  The maximum depth
           is compiled into perl, so changing it requires a custom build.

           The following shows how using negative indexing can make it easier
           to embed recursive patterns inside of a "qr//" construct for later
           use:

               my $parens = qr/(\((?:[^()]++|(?-1))*+\))/;
               if (/foo $parens \s+ \+ \s+ bar $parens/x) {
                  # do something here...
               }

           Note that this pattern does not behave the same way as the
           equivalent PCRE or Python construct of the same form. In Perl you
           can backtrack into a recursed group, in PCRE and Python the
           recursed into group is treated as atomic. Also, modifiers are
           resolved at compile time, so constructs like (?i:(?1)) or
           (?:(?i)(?1)) do not affect how the sub-pattern will be processed.

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

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

           NOTE: In order to make things easier for programmers with
           experience with the Python or PCRE regex engines the pattern
           "(?P>NAME)" may be used instead of "(?&NAME)".

       "(?(condition)yes-pattern|no-pattern)"
       "(?(condition)yes-pattern)"
           Conditional expression. Matches "yes-pattern" if "condition" yields
           a true value, matches "no-pattern" otherwise. A missing pattern
           always matches.

           "(condition)" should be one of: 1) an integer in parentheses (which
           is valid if the corresponding pair of parentheses matched); 2) a
           look-ahead/look-behind/evaluate zero-width assertion; 3) a name in
           angle brackets or single quotes (which is valid if a group with the
           (<NAME>) ('NAME')
               Checks if a group with the given name has matched something.

           (?=...) (?!...) (?<=...) (?<!...)
               Checks whether the pattern matches (or does not match, for the
               '!'  variants).

           (?{ CODE })
               Treats the return value of the code block as the condition.

           (R) Checks if the expression has been evaluated inside of
               recursion.

           (R1) (R2) ...
               Checks if the expression has been evaluated while executing
               directly inside of the n-th capture group. This check is the
               regex equivalent of

                 if ((caller(0))[3] eq 'subname') { ... }

               In other words, it does not check the full recursion stack.

           (R&NAME)
               Similar to "(R1)", this predicate checks to see if we're
               executing directly inside of the leftmost group with a given
               name (this is the same logic used by "(?&NAME)" to
               disambiguate). It does not check the full stack, but only the
               name of the innermost active recursion.

           (DEFINE)
               In this case, the yes-pattern is never directly executed, and
               no no-pattern is allowed. Similar in spirit to "(?{0})" but
               more efficient.  See below for details.

           For example:

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

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

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

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

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

           Finally, keep in mind that subpatterns created inside a DEFINE
           block count towards the absolute and relative number of captures,
           so this:

               my @captures = "a" =~ /(.)                  # First capture
                                      (?(DEFINE)
                                          (?<EXAMPLE> 1 )  # Second capture
                                      )/x;
               say scalar @captures;

           Will output 2, not 1. This is particularly important if you intend
           to compile the definitions with the "qr//" operator, and later
           interpolate them in another pattern.

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

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

           "(?>pattern)" does not disable backtracking altogether once it has
           matched. It is still possible to backtrack past the construct, but
           not into it. So "((?>a*)|(?>b*))ar" will still match "bar".

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

           Consider this pattern:

               m{ \(
                     (
                       [^()]+           # x+

           how the pattern above detects no-match on "((()aaaaaaaaaaaaaaaaaa"
           in several seconds, but that each extra letter doubles this time.
           This exponential performance will make it appear that your program
           has hung.  However, a tiny change to this pattern

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

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

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

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

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

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

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

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

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

           Possessive quantifiers are equivalent to putting the item they are
           applied to inside of one of these constructs. The following

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

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

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

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

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

       NOTE: $REGERROR and $REGMARK are not magic variables like $1 and most
       other regex-related variables. They are not local to a scope, nor
       readonly, but instead are volatile package variables similar to
       $AUTOLOAD.  Use "local" to localize changes to them to a specific scope
       if necessary.

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

       Verbs that take an argument
          "(*PRUNE)" "(*PRUNE:NAME)"
              This zero-width pattern prunes the backtracking tree at the
              current point when backtracked into on failure. Consider the
              pattern "A (*PRUNE) B", where A and B are complex patterns.
              Until the "(*PRUNE)" verb is reached, A may backtrack as
              necessary to match. Once it is reached, matching continues in B,
              which may also backtrack as necessary; however, should B not
              match, then no further backtracking will take place, and the
              pattern will fail outright at the current starting position.

              The following example counts all the possible matching strings
              in a pattern (without actually matching any of them).

                  'aaab' =~ /a+b?(?{print "$&\n"; $count++})(*FAIL)/;
                  print "Count=$count\n";

              which produces:

                  aaab
                  'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/;
                  print "Count=$count\n";

              we prevent backtracking and find the count of the longest
              matching string at each matching starting point like so:

                  aaab
                  aab
                  ab
                  Count=3

              Any number of "(*PRUNE)" assertions may be used in a pattern.

              See also "(?>pattern)" and possessive quantifiers for other ways
              to control backtracking. In some cases, the use of "(*PRUNE)"
              can be replaced with a "(?>pattern)" with no functional
              difference; however, "(*PRUNE)" can be used to handle cases that
              cannot be expressed using a "(?>pattern)" alone.

          "(*SKIP)" "(*SKIP:NAME)"
              This zero-width pattern is similar to "(*PRUNE)", except that on
              failure it also signifies that whatever text that was matched
              leading up to the "(*SKIP)" pattern being executed cannot be
              part of any match of this pattern. This effectively means that
              the regex engine "skips" forward to this position on failure and
              tries to match again, (assuming that there is sufficient room to
              match).

              The name of the "(*SKIP:NAME)" pattern has special significance.
              If a "(*MARK:NAME)" was encountered while matching, then it is
              that position which is used as the "skip point". If no "(*MARK)"
              of that name was encountered, then the "(*SKIP)" operator has no
              effect. When used without a name the "skip point" is where the
              match point was when executing the (*SKIP) pattern.

              Compare the following to the examples in "(*PRUNE)"; note the
              string is twice as long:

               'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/;
               print "Count=$count\n";

              outputs

                  aaab
                  aaab
                  Count=2

              Once the 'aaab' at the start of the string has matched, and the
              "(*SKIP)" executed, the next starting point will be where the
              cursor was when the "(*SKIP)" was executed.

          "(*MARK:NAME)" "(*:NAME)"
              This zero-width pattern can be used to mark the point reached in
              a string when a certain part of the pattern has been
              involved in the match.

              This can be used to determine which branch of a pattern was
              matched without using a separate capture group for each branch,
              which in turn can result in a performance improvement, as perl
              cannot optimize "/(?:(x)|(y)|(z))/" as efficiently as something
              like "/(?:x(*MARK:x)|y(*MARK:y)|z(*MARK:z))/".

              When a match has failed, and unless another verb has been
              involved in failing the match and has provided its own name to
              use, the $REGERROR variable will be set to the name of the most
              recently executed "(*MARK:NAME)".

              See "(*SKIP)" for more details.

              As a shortcut "(*MARK:NAME)" can be written "(*:NAME)".

          "(*THEN)" "(*THEN:NAME)"
              This is similar to the "cut group" operator "::" from Perl 6.
              Like "(*PRUNE)", this verb always matches, and when backtracked
              into on failure, it causes the regex engine to try the next
              alternation in the innermost enclosing group (capturing or
              otherwise) that has alternations.  The two branches of a
              "(?(condition)yes-pattern|no-pattern)" do not count as an
              alternation, as far as "(*THEN)" is concerned.

              Its name comes from the observation that this operation combined
              with the alternation operator ("|") can be used to create what
              is essentially a pattern-based if/then/else block:

                ( COND (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ )

              Note that if this operator is used and NOT inside of an
              alternation then it acts exactly like the "(*PRUNE)" operator.

                / A (*PRUNE) B /

              is the same as

                / A (*THEN) B /

              but

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

              is not the same as

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

              as after matching the A but failing on the B the "(*THEN)" verb
              will backtrack and try C; but the "(*PRUNE)" verb will simply
              fail.

       Verbs without an argument
                  aaab
                  Count=1

              In other words, once the "(*COMMIT)" has been entered, and if
              the pattern does not match, the regex engine will not try any
              further matching on the rest of the string.

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

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

          "(*ACCEPT)"
              WARNING: This feature is highly experimental. It is not
              recommended for production code.

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

              If the "(*ACCEPT)" is inside of capturing groups then the groups
              are marked as ended at the point at which the "(*ACCEPT)" was
              encountered.  For instance:

                'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x;

              will match, and $1 will be "AB" and $2 will be "B", $3 will not
              be set. If another branch in the inner parentheses was matched,
              such as in the string 'ACDE', then the "D" and "E" would have to
              be matched as well.

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

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

       For a regular expression to match, the entire regular expression must
       match, not just part of it.  So if the beginning of a pattern
       containing a quantifier succeeds in a way that causes later parts in
       the pattern to fail, the matching engine backs up and recalculates the
       string, and loads up $1 with "Foo".  However, as soon as the matching
       engine sees that there's no whitespace following the "Foo" that it had
       saved in $1, it realizes its mistake and starts over again one
       character after where it had the tentative match.  This time it goes
       all the way until the next occurrence of "foo". The complete regular
       expression matches this time, and you get the expected output of "table
       follows foo."

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

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

       Which perhaps unexpectedly yields:

         got <d is under the bar in the >

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

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

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

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

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

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

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

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

       That will print out:

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

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

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

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

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

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

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

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

       This prints

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

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

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

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

           6: got ABC

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

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

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

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

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

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

       Any single character matches itself, unless it is a metacharacter with
       a special meaning described here or above.  You can cause characters
       that normally function as metacharacters to be interpreted literally by
       prefixing them with a "\" (e.g., "\." matches a ".", not any character;
       want either "-" or "]" itself to be a member of a class, put it at the
       start of the list (possibly after a "^"), or escape it with a
       backslash.  "-" is also taken literally when it is at the end of the
       list, just before the closing "]".  (The following all specify the same
       class of three characters: "[-az]", "[az-]", and "[a\-z]".  All are
       different from "[a-z]", which specifies a class containing twenty-six
       characters, even on EBCDIC-based character sets.)  Also, if you try to
       use the character classes "\w", "\W", "\s", "\S", "\d", or "\D" as
       endpoints of a range, the "-" is understood literally.

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

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

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

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

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

       Within a pattern, you may designate subpatterns for later reference by
       enclosing them in parentheses, and you may refer back to the nth
       subpattern later in the pattern using the metacharacter \n or \gn.
       Subpatterns are numbered based on the left to right order of their
       opening parenthesis.  A backreference matches whatever actually matched
       avoid shocking the sed addicts, but it's a dirty habit to get into.
       That's because in PerlThink, the righthand side of an "s///" is a
       double-quoted string.  "\1" in the usual double-quoted string means a
       control-A.  The customary Unix meaning of "\1" is kludged in for
       "s///".  However, if you get into the habit of doing that, you get
       yourself into trouble if you then add an "/e" modifier.

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

       Or if you try to do

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

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

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

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

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

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

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

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

       or

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

       or the loop implied by split().

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

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

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

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

       prints

          hello
          aaaaa
          b

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

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

       For example:

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

       results in "<>PERLRE(1)
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