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perlre ()
  • >> perlre (1) ( Solaris man: Команды и прикладные программы пользовательского уровня )
  • perlre (1) ( Разные man: Команды и прикладные программы пользовательского уровня )


         perlre - Perl regular expressions


         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 discussions of `m//', `s///', `qr//' and `??' in the
         Regexp Quote-Like Operators entry in the perlop manpage.
         Matching operations can have various modifiers.  Modifiers
         that relate to the interpretation of the regular expression
         inside are listed below.  Modifiers that alter the way a
         regular expression is used by Perl are detailed in the
         Regexp Quote-Like Operators entry in the perlop manpage and
         the Gory details of parsing quoted constructs entry 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
             "$" from matching the start or end of the string to
             matching the start or end of any line anywhere within
             the string.
         s   Treat string as single line.  That is, change "." to
             match any character whatsoever, even a newline, which
             normally it would not match.
             The `/s' and `/m' modifiers both override the `$*'
             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 really be a
         slash.  Any of these modifiers may also be embedded within
         the regular expression itself using the `(?...)' 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 character class.
         You can use this to break up your regular expression into
         (slightly) more readable parts.  The `#' character is also
         treated as a metacharacter introducing a comment, just as in
         ordinary Perl code.  This also means that if you want real
         whitespace or `#' characters in the pattern (outside a
         character class, where they are unaffected by `/x'), 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 Perl pattern matching derive from
         supplied in the Version 8 regex routines.  (The routines are
         derived (distantly) from Henry Spencer's freely
         redistributable reimplementation of the V8 routines.)  See
         the Version 8 Regular Expressions entry elsewhere in this
         document 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 "$" character only the end
         (or before the newline at the end), and Perl does certain
         optimizations with the assumption that the string contains
         only one line.  Embedded newlines will not be matched by "^"
         or "$".  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 simplify multi-line substitutions, the "." character
         never matches a newline unless you use the `/s' modifier,
         which in effect tells Perl to pretend the string is a single
         line--even if it isn't.  The `/s' modifier also overrides
         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
         (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
             \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
         the perllocale manpage.  For documentation of `\N{name}',
         see the charnames 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
             \pP Match P, named property.  Use \p{Prop} for longer names.
             \PP Match non-P
             \X  Match eXtended Unicode "combining character sequence",
                 equivalent to C<(?:\PM\pM*)>
             \C  Match a single C char (octet) even under utf8.
         A `\w' matches a single alphanumeric character, 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 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,
         but if you try to use them as endpoints of a range, that's
         not a range, the "-" is understood literally.  See the utf8
         manpage for details about `\pP', `\PP', and `\X'.
         The POSIX character class syntax
         is also available.  The available classes and their
         backslash equivalents (if available) are as follows:
             digit       \d
             space       \s
             word        \w
         For example use `[:upper:]' to match all the uppercase
         characters.  Note that the `[]' are part of the `[::]'
         construct, not part of the whole character class.  For
         matches one, zero, any alphabetic character, and the
         percentage sign.
         If the `utf8' pragma is used, the following equivalences to
         Unicode \p{} constructs hold:
             alpha       IsAlpha
             alnum       IsAlnum
             ascii       IsASCII
             cntrl       IsCntrl
             digit       IsDigit
             graph       IsGraph
             lower       IsLower
             print       IsPrint
             punct       IsPunct
             space       IsSpace
             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 isalpha(3) interface (except
         for `word', which is a Perl extension, mirroring `\w').
         The assumedly non-obviously named classes are:
             Any control character.  Usually characters that don't
             produce output as such but instead control the terminal
             somehow: for example newline and backspace are control
             characters.  All characters with ord() less than 32 are
             most often classified as control characters.
             Any alphanumeric or punctuation character.
             Any alphanumeric or punctuation character or space.
             Any punctuation character.
             Any hexadecimal digit.  Though this may feel silly
             (/0-9a-f/i would work just fine) it is included for
         You can negate the [::] character classes by prefixing the
         class name with a '^'. This is a Perl extension.  For
             POSIX       trad. Perl  utf8 Perl
             [:^digit:]      \D      \P{IsDigit}
             [:^space:]      \S      \P{IsSpace}
             [:^word:]       \W      \P{IsWord}
         The POSIX character classes [.cc.] and [=cc=] are recognized
         but not supported and trying to use them will cause an
         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 characters off
         the beginning and end of the string as matching a `\W'.
         (Within character classes `\b' represents backspace rather
         than a word boundary, just as it normally does in any
         double-quoted string.)  The `\A' and `\Z' are just like "^"
         and "$", except that they won't match multiple times when
         the `/m' modifier is used, while "^" and "$" will match at
         every internal line boundary.  To match the actual end of
         the string and not ignore an optional trailing newline, use
         The `\G' assertion can be used to chain global matches
         (using `m//g'), as described in the Regexp Quote-Like
         Operators entry 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 also be influenced
         by using `pos()' as an lvalue.  See the pos entry in the
         perlfunc manpage.
         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 circumstances outside the
         match.  See the warning below about \1 vs $1 for details.)
         Referring back to another part of the match is called a
         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 octal, so
         \011 is the 9'th ASCII character, a tab.)  Perl resolves
         this ambiguity by interpreting \10 as a backreference only
         if at least 10 left parentheses have opened before it.
         Likewise \11 is a backreference only if at least 11 left
         parentheses have opened before it.  And so on.  \1 through
         \9 are always interpreted as backreferences."
             s/^([^ ]*) *([^ ]*)/$2 $1/;     # swap first two words
              if (/(.)\1/) {                 # find first doubled char
                  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.  And `$'' returns everything after the matched
         The numbered variables ($1, $2, $3, etc.) and the related
         punctuation set (`<$+', `$&', `$`', and `$'') are all
         dynamically scoped until the end of the enclosing block or
         until the next successful match, whichever comes first.
         (See the Compound Statements entry in the perlsyn manpage.)
         WARNING: Once Perl sees that you need one of `$&', `$`', or
         `$'' anywhere in the program, it has to provide them for
         every pattern match.  This may substantially slow your
         program.  Perl uses the same mechanism to produce $1, $2,
         etc, so you also pay a price for each pattern that contains
         capturing parentheses.  (To avoid this cost while retaining
         the grouping behaviour, use the extended regular expression
         `(?: ... )' instead.)  But if you never use `$&', `$`' or
         `$'', then patterns without capturing parentheses will not
         be penalized.  So avoid `$&', `$'', and `$`' if you can, but
         if you can't (and some algorithms really appreciate them),
         once you've used them once, use them at will, because you've
         already paid the price.  As of 5.005, `$&' is not so costly
         as the other two.
         Backslashed metacharacters in Perl are alphanumeric, such as
         `\b', `\w', `\n'.  Unlike some other regular expression
         languages, there are no backslashed symbols that aren't
         alphanumeric.  So anything that looks like \\, \(, \), \<,
         \>, \{, or \} is always interpreted as a literal character,
         not a metacharacter.  This was once used in a common idiom
         to disable or quote the special meanings of regular
         expression metacharacters in a string that you want to use
         for a pattern. Simply quote all non-alphanumeric characters:
             $pattern =~ s/(\W)/\\$1/g;
         Today it is more common to use the quotemeta() function or
         the `\Q' metaquoting escape sequence to disable all
         metacharacters' special meanings like this:
         Beware that if you put literal backslashes (those not inside
         interpolated variables) between `\Q' and `\E', double-
         quotish backslash interpolation may lead to confusing
         results.  If you need to use literal backslashes within
         `\Q...\E', consult the Gory details of parsing quoted
         constructs entry in the perlop manpage.
         Extended Patterns
         Perl also defines a consistent extension syntax for features
         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.  Others are
         experimental and may change without warning or be completely
         removed.  Check the documentation on an individual feature
         to verify its current status.
         A question mark was chosen for this and for the minimal-
         matching construct because 1) question marks are rare in
         older regular expressions, and 2) whenever you see one, you
         should stop and "question" exactly what is going on.  That's
                   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.
                   One or more embedded pattern-match modifiers.
                   This is particularly useful for dynamic patterns,
                   such as those read in from a configuration 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 pattern.  For example:
                       $pattern = "foobar";
                       if ( /$pattern/i ) { }
                       # more flexible:
                       $pattern = "(?i)foobar";
                       if ( /$pattern/ ) { }
                   Letters after a `-' turn those modifiers off.
                   These modifiers are localized inside an enclosing
                   group (if any).  For example,
                       ( (?i) blah ) \s+ \1
                   will match a repeated (including the case!) word
                   `blah' in any case, assuming `x' modifier, and no
                   `i' modifier outside this group.
                   This is for clustering, not capturing; it groups
                   subexpressions like "()", but doesn't make
                   backreferences as "()" does.  So
                       @fields = split(/\b(?:a|b|c)\b/)
                   is like
                       @fields = split(/\b(a|b|c)\b/)
                   but doesn't spit out extra fields.  It's also
                   cheaper not to capture characters if you don't
                   need to.
                   Any letters between `?' and `:' act as flags
                   modifiers as with `(?imsx-imsx)'.  For example,
                   is equivalent to the more verbose
                   A zero-width positive look-ahead assertion.  For
                   example, `/\w+(?=\t)/' matches a word followed by
                   a tab, without including the tab in `$&'.
                   A zero-width negative look-ahead assertion.  For
                   example `/foo(?!bar)/' matches any occurrence of
                   "foo" that isn't followed by "bar".  Note however
                   that look-ahead and look-behind are NOT the same
                   thing.  You cannot use this for look-behind.
                   If you are looking for a "bar" that isn't preceded
                   by a "foo", `/(?!foo)bar/' will not do what you
                   want.  That's because the `(?!foo)' is just saying
                   that the next thing cannot be "foo"--and it's not,
                   it's a "bar", so "foobar" will match.  You would
                   have to do something like `/(?!foo)' 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 look-behind see below.
                   A zero-width positive look-behind assertion.  For
                   example, `/(?<=\t)\w+/' matches a word that
                   follows a tab, without including the tab in `$&'.
                   Works only for fixed-width look-behind.
                   A zero-width negative look-behind assertion.  For
                   example `/(?<!bar)foo/' matches any occurrence of
                   "foo" that does not follow "bar".  Works only for
                   fixed-width look-behind.
         `(?{ code })'
                   WARNING: This extended regular expression feature
                   is considered highly experimental, and may be
                   changed or deleted without notice.
                   This zero-width assertion evaluate any embedded
                   Perl code.  It always succeeds, and its `code' is
                   not interpolated.  Currently, the rules to
                   determine where the `code' ends are somewhat
                   The `code' is properly scoped in the following
                   sense: If the assertion is backtracked (compare
                   the section on "Backtracking"), all changes
                   introduced after `local'ization are undone, so
                     $_ = 'a' x 8;
                        (?{ $cnt = 0 })                    # Initialize $cnt.
                              local $cnt = $cnt + 1;       # Update $cnt, backtracking-safe.
                        (?{ $res = $cnt })                 # On success copy to non-localized
                                                           # location.
                   will set `$res = 4'.  Note that after the match,
                   $cnt returns to the globally introduced value,
                   because the scopes that restrict `local' operators
                   are unwound.
                   This assertion may be used as a
                   `(?(condition)yes-pattern|no-pattern)' switch.  If
                   not used in this way, the result of evaluation of
                   `code' is put into the special variable `$^R'.
                   This happens immediately, so `$^R' can be used
                   from other `(?{ code })' assertions inside the
                   same regular expression.
                   The assignment to `$^R' above is properly
                   localized, so the old value of `$^R' is restored
                   if the assertion is backtracked; compare the
                   section on "Backtracking".
                   For reasons of security, this construct is
                   forbidden if the regular expression involves run-
                   time interpolation of variables, unless the
                   perilous `use re 'eval'' pragma has been used (see
                   the re manpage), or the variables contain results
                   of `qr//' operator (see the qr/STRING/imosx entry
                   in the perlop manpage).
                   This restriction is because of the wide-spread and
                   remarkably convenient custom of using run-time
                   determined strings as patterns.  For example:
                       $re = <>;
                       chomp $re;
                       $string =~ /$re/;
                   Before Perl knew how to execute interpolated code
                   within a pattern, this operation was completely
                   safe from a security point of view, although it
                   could raise an exception from an illegal pattern.
                   If you turn on the `use re 'eval'', though, it is
                   no longer secure, so you should only do so if you
                   are also using taint checking.  Better yet, use
                   the carefully constrained evaluation within a Safe
                   module.  See the perlsec manpage for details about
                   both these mechanisms.
         `(??{ code })'
                   WARNING: This extended regular expression feature
                   is considered highly experimental, and may be
                   changed or deleted without notice.  A simplified
                   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 expression
                   and matched as if it were inserted instead of this
                   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
                     $re = qr{
                                   (?> [^()]+ )    # Non-parens without backtracking
                                   (??{ $re })     # Group with matching parens
                   WARNING: This extended regular expression feature
                   is considered highly experimental, and may be
                   changed or deleted without notice.
                   An "independent" subexpression, one which matches
                   the substring that a standalone `pattern' would
                   match if anchored at the given position, and it
                   matches nothing other than this substring.  This
                   construct is useful for optimizations of what
                   would otherwise be "eternal" matches, because it
                   will not backtrack (see the section on
                   "Backtracking").  It may also be useful in places
                   where the "grab all you can, and do not give
                   anything back" semantic is desirable.
                   For example: `^(?>a*)ab' will never match, since
                   `(?>a*)' (anchored at the beginning of string, as
                   above) will match all characters `a' at the
                   beginning of string, leaving no `a' for `ab' to
                   match.  In contrast, `a*ab' will match the same as
                   `a+b', since the match of the subgroup `a*' is
                   influenced by the following group `ab' (see 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 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 ordinals of
                   backreferences in the rest of a regular
                   Consider this pattern:
                       m{ \(
                               [^()]+              # 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+ )
                               \( [^()]* \)
                   which uses `(?>...)' matches exactly when the one
                   above does (verifying this yourself would be a
                   productive exercise), but finishes in a fourth the
                   time when used on a similar string with 1000000
                   `a's.  Be aware, however, that this pattern
                   currently triggers a warning message under the
                   `use warnings' pragma or -w switch saying it
                   `"matches the null string many times"'):
                   On simple groups, such as the pattern `(?> [^()]+
                   )', a comparable effect may be achieved by
                   negative look-ahead, as in `[^()]+ (?! [^()] )'.
                   This was only 4 times slower on a string with
                   1000000 `a's.
                   The "grab all you can, and do not give anything
                   back" semantic is desirable in many situations
                   where on the first sight a simple `()*' looks like
                   the correct solution.  Suppose we parse text with
                   comments being delimited by `#' followed by some
                   optional (horizontal) whitespace.  Contrary to its
                   appearence, `#[ \t]*' is not the correct
                   subexpression to match the comment delimiter,
                   because it may "give up" some whitespace if the
                   remainder of the pattern can be made to match that
                   way.  The correct answer is either one of these:
                       (?>#[ \t]*)
                       #[ \t]*(?![ \t])
                   For example, to grab non-empty comments into $1,
                   one should use either one of these:
                       / (?> \# [ \t]* ) (        .+ ) /x;
                       /     \# [ \t]*   ( [^ \t] .* ) /x;
                   Which one you pick depends on which of these
                   expressions better reflects the above
                   specification of comments.
                   WARNING: This extended regular expression feature
                   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
                   For example:
                       m{ ( \( )?
                          (?(1) \) )
                   matches a chunk of non-parentheses, possibly
                   included in parentheses themselves.
         NOTE: This section presents an abstract approximation of
         regular expression behavior.  For a more rigorous (and
         complicated) view of the rules involved in selecting a match
         among possible alternatives, see the Combining pieces
         together entry elsewhere in this document.
         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}?'.
         Backtracking is often optimized internally, but the general
         principle outlined here is valid.
         For a regular expression to match, the entire regular
         expression must match, not just part of it.  So if the
         beginning of a pattern containing a quantifier succeeds in a
         way that causes later parts in the pattern to fail, the
         matching engine backs up and recalculates the beginning
         part--that's why it's called backtracking.
         Here is an example of backtracking:  Let's say you want to
         find the word following "foo" in the string "Food is on the
         foo table.":
             $_ = "Food is on the foo table.";
             if ( /\b(foo)\s+(\w+)/i ) {
                 print "$2 follows $1.\n";
         When the match runs, the first part of the regular
         expression (`\b(foo)') finds a possible match right at the
         beginning of the string, and loads up $1 with "Foo".
         However, as soon as the matching engine sees that there's no
         whitespace following the "Foo" that it had saved in $1, it
         realizes its mistake and starts over again one character
         after where it had the tentative match.  This time it goes
         all the way until the next occurrence of "foo". The complete
         regular expression matches this time, and you get the
         expected output of "table follows foo."
         Sometimes minimal matching can help a lot.  Imagine you'd
         like to match everything between "foo" and "bar".
         Initially, you write something like this:
             $_ =  "The food is under the bar in the barn.";
             if ( /foo(.*)bar/ ) {
                 print "got <$1>\n";
         Which perhaps unexpectedly yields:
           got <d is under the bar in the >
         That's because `.*' was greedy, so you get everything
         between the first "foo" and the last "bar".  Here it's more
         effective to use minimal matching to make sure you get the
         text between a "foo" and the first "bar" thereafter.
             if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
           got <d is under the >
         Here's another example: let's say you'd like to match a
         number at the end of a string, and you also want to keep the
         preceding part the match.  So you write this:
             $_ = "I have 2 numbers: 53147";
             if ( /(.*)(\d*)/ ) {                                # Wrong!
                 print "Beginning is <$1>, number is <$2>.\n";
         That won't work at all, because `.*' was greedy and gobbled
         up the whole string. As `\d*' can match on an empty string
         the complete regular expression matched successfully.
             Beginning is <I have 2 numbers: 53147>, number is <>.
         Here are some variants, most of which don't work:
             $_ = "I have 2 numbers: 53147";
             @pats = qw{
             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
         When using look-ahead 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 matches,
         contrary to popular expectations:
             $x = 'ABC123' ;
             $y = 'ABC445' ;
             print "1: got $1\n" if $x =~ /^(ABC)(?!123)/ ;
             print "2: got $1\n" if $y =~ /^(ABC)(?!123)/ ;
             print "3: got $1\n" if $x =~ /^(\D*)(?!123)/ ;
             print "4: got $1\n" if $y =~ /^(\D*)(?!123)/ ;
         This prints
             2: got ABC
             3: got AB
             4: got ABC
         You might have expected test 3 to fail because it seems to a
         more general purpose version of test 1.  The important
         difference between them is that test 3 contains a quantifier
         (`\D*') and so can use backtracking, whereas test 1 will
         not.  What's happening is that you've asked "Is it true that
         at the start of $x, following 0 or more non-digits, you have
         something that's not 123?"  If the pattern matcher had let
         `\D*' expand to "ABC", this would have caused the whole
         pattern to fail.
         The search engine will initially match `\D*' with "ABC".
         Then it will try to match `(?!123' with "123", which fails.
         But because a quantifier (`\D*') has been used in the
         regular expression, the search engine can backtrack and
         retry the match differently in the hope of matching the
         complete regular expression.
         The pattern really, really wants to succeed, so it uses the
         standard pattern back-off-and-retry and lets `\D*' expand to
         just "AB" this time.  Now there's indeed something following
         "AB" that is not "123".  It's "C123", which suffices.
         We can deal with this by using both an assertion and a
         negation.  We'll say that the first part in $1 must be
         followed both by a digit and by something that's not "123".
         Remember that the look-aheads are zero-width expressions--
         they only look, but don't consume any of the string in their
         match.  So rewriting this way produces what you'd expect;
         that is, case 5 will fail, but case 6 succeeds:
             print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ;
             print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/ ;
             6: got ABC
         In other words, the two zero-width assertions next to each
         other work as though they're ANDed together, just as you'd
         use any built-in assertions:  `/^$/' matches only if you're
         at the beginning of the line AND the end of the line
         simultaneously.  The deeper underlying truth is that
         juxtaposition in regular expressions always means AND,
         except when you write an explicit OR using the vertical bar.
         `/ab/' means match "a" AND (then) match "b", although the
         attempted matches are made at different positions because
         "a" is not a zero-width assertion, but a one-width
         WARNING: particularly complicated regular expressions can
         take exponential time to solve because of the immense number
         of possible ways they can use backtracking to try match.
         For example, without internal optimizations done by the
         regular expression engine, this will take a painfully long
         time to run:
             'aaaaaaaaaaaa' =~ /((a{0,5}){0,5}){0,5}[c]/
         And if you used `*''s instead of limiting it to 0 through 5
         matches, then it would take forever--or until you ran out of
         stack space.
         A powerful tool for optimizing such beasts is what is known
         as an "independent group", which does not backtrack (see the
         section on "`(?>pattern)'").  Note also that zero-length
         look-ahead/look-behind assertions will not backtrack to make
         the tail match, since they are in "logical" context: only
         whether they match is considered relevant.  For an example
         where side-effects of look-ahead might have influenced the
         following match, see the section on "`(?>pattern)'".
         Version 8 Regular Expressions
         In case you're not familiar with the "regular" Version 8
         regex routines, here are the pattern-matching rules not
         described above.
         Any single character matches itself, unless it is a
         metacharacter with a special meaning described here or
         above.  You can cause characters that normally function as
         metacharacters to be interpreted literally by prefixing them
         with a "\" (e.g., "\." matches a ".", not any character;
         "\\" matches a "\").  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 (possibly after a
         "^"), or escape it with a backslash.  "-" is also taken
         literally when it is at the end of the list, just before the
         closing "]".  (The following all specify the same class of
         three characters: `[-az]', `[az-]', and `[a\-z]'.  All are
         different from `[a-z]', which specifies a class containing
         twenty-six characters.)  Also, if you try to use the
         character classes `\w', `\W', `\s', `\S', `\d', or `\D' as
         endpoints of a range, that's not a range, the "-" is
         understood literally.
         Note also that the whole range idea is rather unportable
         between character sets--and even within character sets they
         may cause results you probably didn't expect.  A sound
         principle is to use only ranges that begin from and end at
         either 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.  That's why it's common practice to
         include alternatives in parentheses: to minimize confusion
         about where they start and end.
         Alternatives are tried from left to right, so the first
         alternative found for which the entire expression matches,
         is the one that is chosen. This means that alternatives are
         not necessarily greedy. For example: when matching
         `foo|foot' against "barefoot", only the "foo" part will
         match, as that is the first alternative tried, and it
         successfully matches the target string. (This might not seem
         important, but it is important when you are capturing
         matched text using parentheses.)
         Also remember that "|" is interpreted as a literal within
         square brackets, so if you write `[fee|fie|foe]' you're
         really only matching `[feio|]'.
         Within a pattern, you may designate subpatterns for later
         reference by enclosing them in parentheses, and you may
         refer back to the nth subpattern later in the pattern using
         the metacharacter \n.  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 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:
             $pattern =~ s/(\W)/\\\1/g;
         This is grandfathered for the RHS of a substitute to avoid
         shocking the sed addicts, but it's a dirty habit to get
         into.  That's because in PerlThink, the righthand side of a
         `s///' is a double-quoted string.  `\1' in the usual
         double-quoted string means a control-A.  The customary Unix
         meaning of `\1' is kludged in for `s///'.  However, if you
         get into the habit of doing that, you get yourself into
         trouble if you then add an `/e' modifier.
             s/(\d+)/ \1 + 1 /eg;        # causes warning under -w
         Or if you try to do
         You can't disambiguate that by saying `\{1}000', whereas you
         can fix it with `${1}000'.  The operation of interpolation
         should not be confused with the operation of matching a
         backreference.  Certainly they mean two different things on
         the left side of the `s///'.
         Repeated patterns matching 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 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 );
             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()
         The lower-level loops are interrupted (that is, the loop is
         broken) when Perl detects that a repeated expression matched
         a zero-length substring.   Thus
            m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;
         is made equivalent to
            m{   (?: NON_ZERO_LENGTH )*
                 (?: ZERO_LENGTH )?
         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.
         For example:
             $_ = 'bar';
         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
         patterns using combining operators `ST', `S|T', `S*' etc (in
         these examples `S' and `T' are regular subexpressions).
         Such combinations can include alternatives, leading to a
         problem of choice:  if we match a regular expression `a|ab'
         against `"abc"', will it match substring `"a"' or `"ab"'?
         One way to describe which substring is actually matched is
         the concept of backtracking (see the section on
         "Backtracking").  However, this description is too low-level
         and makes you think in terms of a particular implementation.
         Another description starts with notions of "better"/"worse".
         All the substrings which may be matched by the given regular
         expression can be sorted from the "best" match to the
         "worst" match, and it is the "best" match which is chosen.
         This substitutes the question of "what is chosen?"  by the
         question of "which matches are better, and which are
         Again, for elementary pieces there is no such question,
         since at most one match at a given position is possible.
         This section describes the notion of better/worse for
         combining operators.  In the description below `S' and `T'
         are regular subexpressions.
             Consider two possible matches, `AB' and `A'B'', `A' and
             `A'' are substrings which can be matched by `S', `B' and
             `B'' are substrings which can be matched by `T'.
             If `A' is better match for `S' than `A'', `AB' is a
             better match than `A'B''.
             If `A' and `A'' coincide: `AB' is a better match than
             `AB'' if `B' is better match for `T' than `B''.
             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'.
             Matches as `SSS...S' (repeated as many times as
             Matches as `S{max}|S{max-1}|...|S{min+1}|S{min}'.
             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}'
         `S??', `S*?', `S+?'
             Same as `S{0,1}?', `S{0,BIG_NUMBER}?',
             `S{1,BIG_NUMBER}?' respectively.
             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
         `(??{ EXPR })'
             The ordering is the same as for the regular expression
             which is the result of EXPR.
             Recall that which of `yes-pattern' or `no-pattern'
             actually matches is already determined.  The ordering of
             the matches is the same as for the chosen subexpression.
         The above recipes describe the ordering of matches at a
         given position.  One more rule is needed to understand how a
         match is determined for the whole regular expression: a
         match at an earlier position is always better than a match
         at a later position.
         Creating custom RE engines
         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 {
               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):
             use customre;
             $re = <>;
             chomp $re;
             $re = customre::convert $re;


         This document varies from difficult to understand to
         completely and utterly opaque.  The wandering prose riddled
         with jargon is hard to fathom in several places.
         This document needs a rewrite that separates the tutorial
         content from the reference content.


         the Regexp Quote-Like Operators entry in the perlop manpage.
         the Gory details of parsing quoted constructs entry in the
         perlop manpage.
         the perlfaq6 manpage.
         the pos entry in the perlfunc manpage.
         the perllocale manpage.
         Mastering Regular Expressions by Jeffrey Friedl, published
         by O'Reilly and Associates.

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