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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 "Reg­
       exp Quote-Like Operators" in perlop.

       Matching operations can have various modifiers.  Modifiers
       that relate to the interpretation of the regular expres­
       sion 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.

       i   Do case-insensitive pattern matching.

           If "use locale" is in effect, the case map is taken
           from the current locale.  See perllocale.

       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.

           The "/s" and "/m" modifiers both override the $* set­
           ting.  That is, no matter what $* contains, "/s" with­
           out "/m" will force "^" to match only at the beginning
           of the string and "$" to match only at the end (or
           just before a newline at the end) of the string.
           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.

       x   Extend your pattern's legibility by permitting whites­
           pace and comments.

       These are usually written as "the "/x" modifier", even
       though the delimiter in question might not really be a
       slash.  Any of these modifiers may also be embedded within
       the regular expression itself using the "(?...)" con­
       struct.  See below.
       ful not to include the pattern delimiter in the com­
       ment--perl has no way of knowing you did not intend to
       close the pattern early.  See the C-comment deletion code
       in perlop.

       Regular Expressions

       The patterns used in Perl pattern matching derive from
       supplied in the Version 8 regex routines.  (The routines
       are derived (distantly) from Henry Spencer's freely redis­
       tributable reimplementation of the V8 routines.)  See
       "Version 8 Regular Expressions" for details.

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

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

       By default, the "^" character is guaranteed to match only
       the beginning of the string, the "$" character only the
       end (or before the newline at the end), and Perl does cer­
       tain optimizations with the assumption that the string
       contains only one line.  Embedded newlines will not be
       matched by "^" or "$".  You may, however, wish to treat a
       string as a multi-line buffer, such that the "^" will
       match after any newline within the string, 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 set­
       ting $*, but this practice is now deprecated.)

       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 sin­
       gle line--even if it isn't.  The "/s" modifier also over­
       rides the setting of $*, in case you have some (badly
       behaved) older code that sets it in another module.

       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

       it will match as many times as possible (given a particu­
       lar 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
           +?     Match 1 or more times
           ??     Match 0 or 1 time
           {n}?   Match exactly n times
           {n,}?  Match at least n times
           {n,m}? Match at least n but not more than m times

       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)
           \033        octal char (think of a PDP-11)
           \x1B        hex char
           \x{263a}    wide hex char         (Unicode SMILEY)
           \c[         control char
           \N{name}    named char
           \l          lowercase next char (think vi)
           \u          uppercase next char (think vi)
           \L          lowercase till \E (think vi)
           \U          uppercase till \E (think vi)
           \E          end case modification (think vi)
           \Q          quote (disable) pattern metacharacters till \E

       If "use locale" is in effect, the case map used by "\l",
       "\L", "\u" and "\U" is taken from the current locale.  See
       perllocale.  For documentation of "\N{name}", see char­
       names.

       You cannot include a literal "$" or "@" within a "\Q"
       sequence.  An unescaped "$" or "@" interpolates the corre­
       sponding variable, while escaping will cause the literal
       string "\$" to be matched.  You'll need to write something
       like "m/\Quser\E\@\Qhost/".

       In addition, Perl defines the following:

           \PP Match non-P
           \X  Match eXtended Unicode "combining character sequence",
               equivalent to (?:\PM\pM*)
           \C  Match a single C char (octet) even under Unicode.
               NOTE: breaks up characters into their UTF-8 bytes,
               so you may end up with malformed pieces of UTF-8.
               Unsupported in lookbehind.

       A "\w" matches a single alphanumeric character (an alpha­
       betic character, or a decimal digit) or "_", not a whole
       word.  Use "\w+" to match a string of Perl-identifier
       characters (which isn't the same as matching an English
       word).  If "use locale" is in effect, the list of alpha­
       betic characters generated by "\w" is taken from the cur­
       rent locale.  See perllocale.  You may use "\w", "\W",
       "\s", "\S", "\d", and "\D" within character classes, but
       if you try to use them as endpoints of a range, that's not
       a range, the "-" is understood literally.  If Unicode is
       in effect, "\s" matches also "\x{85}", "\x{2028}, and
       "\x{2029}", see perlunicode for more details about "\pP",
       "\PP", and "\X", and perluniintro about Unicode in gen­
       eral.  You can define your own "\p" and "\P" propreties,
       see perlunicode.

       The POSIX character class syntax

           [:class:]

       is also available.  The available classes and their back­
       slash equivalents (if available) are as follows:

           alpha
           alnum
           ascii
           blank               [1]
           cntrl
           digit       \d
           graph
           lower
           print
           punct
           space       \s      [2]
           upper
           word        \w      [3]
           xdigit

       [1] A GNU extension equivalent to "[ \t]", `all horizontal
           whitespace'.

       [2] Not exactly equivalent to "\s" since the "[[:space:]]"
           includes also the (very rare) `vertical tabulator',
           "\ck", chr(11).

       equivalent backslash character classes (if available),
       will hold:

           [:...:]     \p{...}         backslash

           alpha       IsAlpha
           alnum       IsAlnum
           ascii       IsASCII
           blank       IsSpace
           cntrl       IsCntrl
           digit       IsDigit        \d
           graph       IsGraph
           lower       IsLower
           print       IsPrint
           punct       IsPunct
           space       IsSpace
                       IsSpacePerl    \s
           upper       IsUpper
           word        IsWord
           xdigit      IsXDigit

       For example "[:lower:]" and "\p{IsLower}" are equivalent.

       If the "utf8" pragma is not used but the "locale" pragma
       is, the classes correlate with the usual isalpha(3) inter­
       face (except for `word' and `blank').

       The assumedly non-obviously named classes are:

       cntrl
           Any control character.  Usually characters that don't
           produce output as such but instead control the termi­
           nal somehow: for example newline and backspace are
           control characters.  All characters with ord() less
           than 32 are most often classified as control charac­
           ters (assuming ASCII, the ISO Latin character sets,
           and Unicode), as is the character with the ord() value
           of 127 ("DEL").

       graph
           Any alphanumeric or punctuation (special) character.

       print
           Any alphanumeric or punctuation (special) character or
           the space character.

       punct
           Any punctuation (special) character.

       xdigit
           Any hexadecimal digit.  Though this may feel silly
           ([0-9A-Fa-f] would work just fine) it is included for

       POSIX character classes [.cc.] and [=cc=] are recognized
       but not supported and trying to use them will cause an
       error.

       Perl defines the following zero-width assertions:

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

       A word boundary ("\b") is a spot between two characters
       that has a "\w" on one side of it and a "\W" on the other
       side of it (in either order), counting the imaginary char­
       acters 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 "$", except that they won't match multi­
       ple times when the "/m" modifier is used, while "^" and
       "$" 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 Opera­
       tors" 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. Currently
       "\G" is only fully supported when anchored to the start of
       the pattern; while it is permitted to use it elsewhere, as
       in "/(?<=\G..)./g", some such uses ("/.\G/g", for example)
       currently cause problems, and it is recommended that you
       avoid such usage for now.

       The bracketing construct "( ... )" creates capture
       buffers.  To refer to the digit'th buffer use \<digit>
       within the match.  Outside the match use "$" instead of
       "\".  (The \<digit> notation works in certain circum­
       stances outside the match.  See the warning below about \1
       vs $1 for details.)  Referring back to another part of the
       match is called a backreference.

       There is no limit to the number of captured substrings
       that you may use.  However Perl also uses \10, \11, etc.
       as aliases for \010, \011, etc.  (Recall that 0 means
                print "'$1' is the first doubled character\n";
            }

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

       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 pat­
       terns (see below), for example to assign a submatch to a
       variable.

       The numbered match variables ($1, $2, $3, etc.) and the
       related punctuation set ($+, $&, $`, $', and $^N) are all
       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 vari­
       ables, which makes 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 cap­
       turing parentheses.  (To avoid this cost while retaining
       the grouping behaviour, use the extended regular expres­
       sion "(?: ... )" 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.

       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 charac­
       ter, not a metacharacter.  This was once used in a common
       inside interpolated variables) between "\Q" and "\E", dou­
       ble-quotish backslash interpolation may lead to confusing
       results.  If you need to use literal backslashes within
       "\Q...\E", consult "Gory details of parsing quoted con­
       structs" in perlop.

       Extended Patterns

       Perl also defines a consistent extension syntax for fea­
       tures not found in standard tools like awk and lex.  The
       syntax 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.  Oth­
       ers are experimental and may change without warning or be
       completely removed.  Check the documentation on an indi­
       vidual 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.

       "(?imsx-imsx)"
                 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 remain­
                 der of the enclosing pattern group (if any).
                 This is particularly useful for dynamic pat­
                 terns, such as those read in from a configura­
                 tion file, read in as an argument, are specified
                 in a table somewhere, etc.  Consider the case
                 that some of which want to be case sensitive and
                 some do not.  The case insensitive ones need to
                 include merely "(?i)" at the front of the pat­
                 tern.  For example:

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

                     # more flexible:

                 This is for clustering, not capturing; it groups
                 subexpressions like "()", but doesn't make back­
                 references 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 "(?imsx-imsx)".  For example,

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

                 is equivalent to the more verbose

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

       "(?=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 how­
                 ever 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 pre­
                 ceded 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.  You would have to do something like
                 "/(?!foo)...bar/" for that.   We say "like"
                 because there's the case of your "bar" not hav­
                 ing three characters before it.  You could cover
                 that this way: "/(?:(?!foo)...|^.{0,2})bar/".
                 Sometimes it's still easier just to say:

                     if (/bar/ && $` !~ /foo$/)

                 For look-behind see below.

                 ture is considered highly experimental, and may
                 be changed or deleted without notice.

                 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 spe­
                 cial variable $^N to capture the results of sub­
                 matches 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";

                 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" opera­
                 tors are unwound.

                 This assertion may be used as a "(?(condi­
                 tion)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 local­
                 time determined strings as patterns.  For exam­
                 ple:

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

                 Before Perl knew how to execute interpolated
                 code within a pattern, this operation was com­
                 pletely 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 con­
                 strained evaluation within a Safe compartment.
                 See perlsec for details about both these mecha­
                 nisms.

       "(??{ code })"
                 WARNING: This extended regular expression fea­
                 ture is considered highly experimental, and may
                 be changed or deleted without notice.  A simpli­
                 fied version of the syntax may be introduced for
                 commonly used idioms.

                 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 as a regular expres­
                 sion and matched as if it were inserted instead
                 of this construct.

                 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;

       "(?>pattern)"
                 WARNING: This extended regular expression fea­
                 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 stan­
                 dalone "a*", since this makes the tail match.

                 An effect similar to "(?>pattern)" may be
                 achieved by writing "(?=(pattern))\1".  This
                 matches the same substring as a standalone "a+",
                 and the following "\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 ordi­
                 nals 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+ )
                           |

                 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 "#" fol­
                 lowed 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 whites­
                 pace 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 specifica­
                 tion of comments.

       "(?(condition)yes-pattern|no-pattern)"
       "(?(condition)yes-pattern)"
                 WARNING: This extended regular expression fea­
                 ture is considered highly experimental, and may
                 be changed or deleted without notice.

                 Conditional expression.  "(condition)" should be
                 either an integer in parentheses (which is valid
                 if the corresponding pair of parentheses
                 matched), or look-ahead/look-behind/evaluate
                 zero-width assertion.

                 For example:

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

                 matches a chunk of non-parentheses, possibly

       "{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 expres­
       sion ("\b(foo)") finds a possible match right at the
       beginning of the string, and loads up $1 with "Foo".  How­
       ever, 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 com­
       plete 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".  Ini­
       tially, 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" there­
       after.

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

       string the complete regular expression matched success­
       fully.

           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
           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 quanti­
       fier ("\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-dig­
       its, 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 some­
       thing 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
       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 influ­
       enced 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 charac­
       ter; "\\" matches a "\").  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 one 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 (possi­
       bly 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 coded character sets.)  Also, if you try to use the
       tab, "\r" a carriage return, "\f" a form feed, etc.  More
       generally, \nnn, where nnn is a string of octal digits,
       matches the character whose coded character set value is
       nnn.  Similarly, \xnn, where nn are hexadecimal digits,
       matches the character whose numeric value 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 ("(", "[", or
       the beginning of the pattern) up to the first "|", and the
       last alternative contains everything from the last "|" to
       the next pattern delimiter.  That's why it's common prac­
       tice 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 suc­
       cessfully 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.  Subpatterns are numbered
       based on the left to right order of their opening paren­
       thesis.  A backreference matches whatever actually matched
       the subpattern in the string being examined, not the rules
       for that subpattern.  Therefore, "(0|0x)\d*\s\1\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 num­
       ber.

       Warning on \1 vs $1

       Some people get too used to writing things like:

           $pattern =~ s/(\W)/\\\1/g;
           s/(\d+)/\1000/;

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

       Repeated patterns matching zero-length substring

       WARNING: Difficult material (and prose) ahead.  This sec­
       tion needs a rewrite.

       Regular expressions provide a terse and powerful program­
       ming 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 some­
       thing as innocuous as:

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

       The "o?" can match 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 "*" modifier.
       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 modifiers "*+{}",
       and for higher-level ones like the "/g" modifier or
       split() operator.

       The lower-level loops are interrupted (that is, the loop
       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 "<>second best match is what is
       matched by "\w".  Thus zero-length matches alternate with
       one-character-long matches.

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

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

       Combining pieces together

       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 pat­
       terns 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 "bet­
       ter"/"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,
           If "A" and "A'" coincide: "AB" is a better match than
           "AB'" if "B" is better match for "T" than "B'".

       "S|T"
           When "S" can match, it is a better match than when
           only "T" can match.

           Ordering of two matches for "S" is the same as for
           "S".  Similar for two matches for "T".

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

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

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

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

       "S??", "S*?", "S+?"
           Same as "S{0,1}?", "S{0,BIG_NUMBER}?", "S{1,BIG_NUM­
           BER}?" 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 })"
           The ordering is the same as for the regular expression
           which is the result of EXPR.

       "(?(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 subex­
           pression.

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

           package customre;
           use overload;

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

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

           my %rules = ( '\\' => '\\',
                         'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ );
           sub convert {
             my $re = shift;
             $re =~ s{
                       \\ ( \\ | Y . )
                     }
                     { $rules{$1} or invalid($re,$1) }sgex;
             return $re;
           }

       Now "use customre" enables the new escape in constant reg­
       ular expressions, i.e., those without any runtime variable
       interpolations.  As documented in overload, this conver­
       sion will work only over literal parts of regular expres­
       sions.  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|/;


BUGS

       This document varies from difficult to understand to com­
       pletely 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.


       perlebcdic.

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

perl v5.8.1                 2003-09-02                  PERLRE(1)
  
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