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


NAME
       perlre - Perl regular expressions



DESCRIPTION
       This page describes the syntax of regular expressions in
       Perl.  For a description of how to use regular expressions
       in matching operations, plus various examples of the same,
       see discussion of m//, s///, qr// and ?? in the section on
       Regexp Quote-Like Operators in the perlop manpage.

       The matching operations can have various modifiers.  The
       modifiers that relate to the interpretation of the regular
       expression inside are listed below.  For the modifiers
       that alter the way a regular expression is used by Perl,
       see the section on Regexp Quote-Like Operators in the
       perlop manpage and the section on Gory details of parsing
       quoted constructs in the perlop manpage.

       i   Do case-insensitive pattern matching.

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

       m   Treat string as multiple lines.  That is, change "^"
           and "$" from matching at only the very start or end of
           the string to 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
           it normally would not match.

           The /s and /m modifiers both override the $* setting.
           That is, no matter what $* contains, /s without /m
           will force "^" to match only at the beginning of the
           string and "$" 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 yet allowing "^" and "$" to match,
           respectively, just after and just before newlines
           within the string.

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

       These are usually written as "the /x modifier", even
       though the delimiter in question might not actually be a
       slash.  In fact, any of these modifiers may also be
       embedded within the regular expression itself using the
       new (?...) construct.  See below.

       The /x modifier itself needs a little more explanation.
       It tells the regular expression parser to ignore
       whitespace that is neither backslashed nor within a



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       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 of a character class,
       where they are unaffected by /x), that you'll either have
       to escape them or encode them using octal or hex 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 the perlop manpage.

       Regular Expressions

       The patterns used in pattern matching are regular
       expressions such as those supplied in the Version 8 regex
       routines.  (In fact, the routines are derived (distantly)
       from Henry Spencer's freely redistributable
       reimplementation of the V8 routines.)  See the section on
       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 at
       only the beginning of the string, the "$" character at
       only the end (or before the newline at the end) and Perl
       does certain optimizations with the assumption that the
       string contains only one line.  Embedded newlines will not
       be matched by "^" or "$".  You may, however, wish to treat
       a string as a multi-line buffer, such that the "^" will
       match after any newline within the string, 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 practice is now deprecated.)

       To facilitate 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.  The /s modifier also
       overrides the setting of $*, in case you have some (badly
       behaved) older code that sets it in another module.



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       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, it is
       treated as a regular character.)  The "*" modifier is
       equivalent to {0,}, the "+" modifier to {1,}, and the "?"
       modifier to {0,1}.  n and m are limited to integral values
       less than a preset limit defined when perl is built.  This
       is usually 32766 on the most common platforms.  The actual
       limit can be seen in the error message generated by code
       such as this:

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

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

           *?     Match 0 or more times
           +?     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
           \c[         control 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




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       If use locale is in effect, the case map used by \l, \L,
       \u and \U is taken from the current locale.  See the
       perllocale manpage.

       You cannot include a literal $ or @ within a \Q sequence.
       An unescaped $ or @ interpolates the corresponding
       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:

           \w  Match a "word" character (alphanumeric plus "_")
           \W  Match a non-word character
           \s  Match a whitespace character
           \S  Match a non-whitespace character
           \d  Match a digit character
           \D  Match a non-digit character

       A \w matches a single alphanumeric character, not a whole
       word.  To match a word you'd need to say \w+.  If use
       locale is in effect, the list of alphabetic characters
       generated by \w is taken from the current locale.  See the
       perllocale manpage. You may use \w, \W, \s, \S, \d, and \D
       within character classes (though not as either end of a
       range).

       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 where previous m//g left off (works only with /g)

       A word boundary (\b) is defined as 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.)  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, not ignoring newline, you
       can use \z.  The \G assertion can be used to chain global
       matches (using m//g), as described in the section on
       Regexp Quote-Like Operators in the perlop manpage.

       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



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       also be influenced by using pos() as an lvalue.  See the
       pos entry in the perlfunc manpage.

       When the bracketing construct ( ... ) is used, \<digit>
       matches the digit'th substring.  Outside of the pattern,
       always use "$" instead of "\" in front of the digit.
       (While the \<digit> notation can on rare occasion work
       outside the current pattern, this should not be relied
       upon.  See the WARNING below.) The scope of $<digit> (and
       $`, $&, and $') extends to the end of the enclosing BLOCK
       or eval string, or to the next successful pattern match,
       whichever comes first.  If you want to use parentheses to
       delimit a subpattern (e.g., a set of alternatives) without
       saving it as a subpattern, follow the ( with a ?:.

       You may have as many parentheses as you wish.  If you have
       more than 9 substrings, the variables $10, $11, ... refer
       to the corresponding substring.  Within the pattern, \10,
       \11, etc. refer back to substrings if there have been at
       least that many left parentheses before the backreference.
       Otherwise (for backward compatibility) \10 is the same as
       \010, a backspace, and \11 the same as \011, a tab.  And
       so on.  (\1 through \9 are always backreferences.)

       $+ returns whatever the last bracket match matched.  $&
       returns the entire matched string.  ($0 used to return the
       same thing, but not any more.)  $` returns everything
       before the matched string.  $' returns everything after
       the matched string.  Examples:

           s/^([^ ]*) *([^ ]*)/$2 $1/;     # swap first two words

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

       Once perl sees that you need one of $&, $` or $' anywhere
       in the program, it has to provide them on each and every
       pattern match.  This can slow your program down.  The same
       mechanism that handles these provides for the use of $1,
       $2, etc., so you pay the same price for each pattern that
       contains capturing parentheses. But if you never use $&,
       etc., in your script, then patterns without capturing
       parentheses won't 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



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       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-
       alphanumeric characters:

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

       Now it is much more common to see either the quotemeta()
       function or the \Q escape sequence used to disable all
       metacharacters' special meanings like this:

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

       Perl defines a consistent extension syntax for regular
       expressions.  The syntax is a pair of parentheses with a
       question mark as the first thing within the parentheses
       (this was a syntax error in older versions of Perl).  The
       character after the question mark gives the function of
       the extension.  Several extensions are already supported:

       (?#text)  A comment.  The text is ignored.  If the /x
                 switch is used to enable 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.

       (?:pattern)

       (?imsx-imsx: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.

                 The letters between ? and : act as flags
                 modifiers, see the (?imsx-imsx) manpage.  In
                 particular,

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

                 is equivalent to more verbose

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




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

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

                 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.  You would have to do something like
                 /(?!foo)...bar/ for that.   We say "like"
                 because there's the case of your "bar" not
                 having 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 lookbehind see below.

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

       (?<!pattern)
                 A zero-width negative lookbehind assertion.  For
                 example /(?<!bar)foo/ matches any occurrence of
                 "foo" that isn't following "bar".  Works only
                 for fixed-width lookbehind.

       (?{ code })
                 Experimental "evaluate any Perl code" zero-width
                 assertion.  Always succeeds.  code is not
                 interpolated.  Currently the rules to determine
                 where the code ends are somewhat convoluted.

                 The code is properly scoped in the following
                 sense: if the assertion is backtracked (compare
                 the section on Backtracking), all the changes
                 introduced after localisation are undone, so





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                   $_ = '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 0,
                 since the scopes which restrict local statements
                 are unwound.

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

                 The above assignment to $^R is properly
                 localized, thus the old value of $^R is restored
                 if the assertion is backtracked (compare the
                 section on Backtracking).

                 Due to security concerns, this construction is
                 not allowed if the regular expression involves
                 run-time interpolation of variables, unless use
                 re 'eval' pragma is used (see the re manpage),
                 or the variables contain results of qr()
                 operator (see the section on qr/STRING/imosx in
                 the perlop manpage).

                 This restriction is due to the wide-spread
                 (questionable) practice of using the construct

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

                 without tainting.  While this code is frowned
                 upon from security point of view, when (?{}) was
                 introduced, it was considered bad to add new
                 security holes to existing scripts.

                 NOTE:  Use of the above insecure snippet without
                 also enabling taint mode is to be severely
                 frowned upon.  use re 'eval' does not disable



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                 tainting checks, thus to allow $re in the above
                 snippet to contain (?{}) with tainting enabled,
                 one needs both use re 'eval' and untaint the
                 $re.

       (?>pattern)
                 An "independent" subexpression.  Matches the
                 substring that a standalone pattern would match
                 if anchored at the given position, and only this
                 substring.

                 Say, ^(?>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 the section on
                 Backtracking).  In particular, a* inside a*ab
                 will match fewer characters than a standalone
                 a*, since this makes the tail match.

                 An effect similar to (?>pattern) may be achieved
                 by

                    (?=(pattern))\1

                 since the lookahead is in "logical" context,
                 thus matches the same substring as a standalone
                 a+.  The following \1 eats the matched string,
                 thus making a zero-length assertion into an
                 analogue of (?>...).  (The difference between
                 these two constructs is that the second one uses
                 a catching group, thus shifting ordinals of
                 backreferences in the rest of a regular
                 expression.)

                 This construct is useful for optimizations of
                 "eternal" matches, because it will not backtrack
                 (see the section on Backtracking).

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

                 That will efficiently match a nonempty group
                 with matching two-or-less-level-deep
                 parentheses.  However, if there is no such
                 group, it will take virtually forever on a long



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                 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 that the above
                 pattern 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 modification of this pattern

                     m{ \(
                           (
                             (?> [^()]+ )
                           |
                             \( [^()]* \)
                           )+
                        \)
                      }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 as.  Be aware, however, that this
                 pattern currently triggers a warning message
                 under -w saying it "matches the null string many
                 times"):

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

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

       (?(condition)yes-pattern)
                 Conditional expression.  (condition) should be
                 either an integer in parentheses (which is valid
                 if the corresponding pair of parentheses
                 matched), or lookahead/lookbehind/evaluate zero-
                 width assertion.

                 Say,

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

                 matches a chunk of non-parentheses, possibly



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                 included in parentheses themselves.

       (?imsx-imsx)
                 One or more embedded pattern-match modifiers.
                 This is particularly useful for patterns that
                 are specified in a table somewhere, some of
                 which want to be case sensitive, and some of
                 which don't.  The case insensitive ones need to
                 include merely (?i) at the front of the pattern.
                 For example:

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

                     # more flexible:

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

                 Letters after - switch modifiers off.

                 These modifiers are localized inside an
                 enclosing group (if any).  Say,

                     ( (?i) blah ) \s+ \1

                 (assuming x modifier, and no i modifier outside
                 of this group) will match a repeated (including
                 the case!) word blah in any case.

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

       Backtracking

       A fundamental feature of regular expression matching
       involves the notion called backtracking, which is
       currently used (when needed) by all regular expression
       quantifiers, namely *, *?, +, +?, {n,m}, and {n,m}?.

       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.":




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

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

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

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

       Which perhaps unexpectedly yields:

         got <d is under the bar in the >

       That's because .* was greedy, so you get everything
       between the first "foo" and the last "bar".  In this case,
       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 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:



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           $_ = "I have 2 numbers: 53147";
           @pats = qw{
               (.*)(\d*)
               (.*)(\d+)
               (.*?)(\d*)
               (.*?)(\d+)
               (.*)(\d+)$
               (.*?)(\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 lookahead assertions and negations, this can
       all get even tricker.  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 it that pattern



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       matches, contrary to popular expectations:

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

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

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

       This prints

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

       You might have expected test 3 to fail because it seems to
       a more general purpose version of test 1.  The important
       difference between them is that test 3 contains a
       quantifier (\D*) and so can use backtracking, whereas test
       1 will not.  What's happening is that you've asked "Is it
       true that at the start of $x, following 0 or more non-
       digits, you have something that's not 123?"  If the
       pattern matcher had let \D* expand to "ABC", this would
       have caused the whole pattern to fail.  The search engine
       will initially match \D* with "ABC".  Then it will try to
       match (?!123 with "123", which of course fails.  But
       because a quantifier (\D*) has been used in the regular
       expression, the search engine can backtrack and retry the
       match differently in the hope of matching the complete
       regular expression.

       The pattern really, really wants to succeed, so it uses
       the standard pattern back-off-and-retry and lets \D*
       expand to just "AB" this time.  Now there's indeed
       something following "AB" that is not "123".  It's in fact
       "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 by a digit, and in fact, it must also be followed
       by something that's not "123".  Remember that the
       lookaheads 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



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       other work as though they're ANDed together, just as you'd
       use any builtin 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.

       One warning: particularly complicated regular expressions
       can take exponential time to solve due to the immense
       number of possible ways they can use backtracking to try
       match.  For example this will take a very long time to run

           /((a{0,5}){0,5}){0,5}/

       And if you used *'s instead of limiting it to 0 through 5
       matches, then it would take literally forever--or until
       you ran out of stack space.

       A powerful tool for optimizing such beasts is
       "independent" groups, which do not backtrace (see the
       (?>pattern) manpage).  Note also that zero-length
       lookahead/lookbehind assertions will not backtrace to make
       the tail match, since they are in "logical" context: only
       the fact whether they match or not is considered relevant.
       For an example where side-effects of a lookahead might
       have influenced the following match, see the (?>pattern)
       manpage.

       Version 8 Regular Expressions

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

       Any single character matches itself, unless it is a
       metacharacter with a special meaning described here or
       above.  You can cause characters that normally function as
       metacharacters to be interpreted literally by prefixing
       them with a "\" (e.g., "\." matches a ".", not any
       character; "\\" matches a "\").  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 is used to specify a range, so
       that a-z represents all characters between "a" and "z",



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       inclusive.  If you want "-" itself to be a member of a
       class, put it at the start or end of the list, or escape
       it with a backslash.  (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.)

       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 alphabets 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 octal digits,
       matches the character whose ASCII value is nnn.
       Similarly, \xnn, where nn are hexadecimal digits, matches
       the character whose ASCII value is nn. The expression \cx
       matches the ASCII 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.  For this reason, 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



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       using the metacharacter \n.  Subpatterns are numbered
       based on the left to right order of their opening
       parenthesis.  A backreference matches whatever actually
       matched the subpattern in the string being examined, not
       the rules for that subpattern.  Therefore,
       (0|0x)\d*\s\1\d* will match "0x1234 0x4321", but not
       "0x1234 01234", because subpattern 1 actually matched
       "0x", even though the rule 0|0x could potentially match
       the leading 0 in the second number.

       WARNING on \1 vs $1

       Some people get too used to writing things like:

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

       This is grandfathered for the RHS of a substitute to avoid
       shocking the sed addicts, but it's a dirty habit to get
       into.  That's because in PerlThink, the righthand side of
       a 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.  Basically, 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 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? can match at the beginning of 'foo', and since the
       position in the string is not moved by the match, o? would



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       match again and again due to 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 which may match zero-length substrings,
       with a simple example being:

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

       Thus Perl allows the /()/ construct, which forcefully
       breaks 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 when it is detected
       that a repeated expression did match a zero-length
       substring, thus

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

       is made equivalent to

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

       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 the
       section on Backtracking), and so the second best match is
       chosen if the best match is of zero length.

       Say,

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

       results in "<<b><><a><><r><>">.  At each position of the
       string the best match given by non-greedy ?? is the zero-



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       length match, and the second best match is what is matched
       by \w.  Thus zero-length matches alternate with one-
       character-long matches.

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

       The additional state of being matched with zero-length is
       associated to the matched string, and is reset by each
       assignment to pos().

       Creating custom RE engines

       Overloaded constants (see the overload manpage) provide a
       simple way to extend the functionality of the RE engine.

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

           package customre;
           use overload;

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

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

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

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



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


       SEE ALSO

       the section on Regexp Quote-Like Operators in the perlop
       manpage.

       the section on Gory details of parsing quoted constructs
       in the perlop manpage.

       the pos entry in the perlfunc manpage.

       the perllocale manpage.

       Mastering Regular Expressions (see the perlbook manpage)
       by Jeffrey Friedl.




































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