=head1 NAME
+X<regular expression> X<regex> X<regexp>
perlre - Perl regular expressions
=head1 DESCRIPTION
-This page describes the syntax of regular expressions in Perl. For a
-description of how to I<use> regular expressions in matching
-operations, plus various examples of the same, see C<m//> and C<s///> in
-L<perlop>.
+This page describes the syntax of regular expressions in Perl.
-The matching operations can have various modifiers. The modifiers
-which relate to the interpretation of the regular expression inside
-are listed below. For the modifiers that alter the behaviour of the
-operation, see L<perlop/"m//"> and L<perlop/"s//">.
+If you haven't used regular expressions before, a quick-start
+introduction is available in L<perlrequick>, and a longer tutorial
+introduction is available in L<perlretut>.
+
+For reference on how regular expressions are used in matching
+operations, plus various examples of the same, see discussions of
+C<m//>, C<s///>, C<qr//> and C<??> in L<perlop/"Regexp Quote-Like
+Operators">.
+
+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 L<perlop/"Regexp Quote-Like Operators"> and
+L<perlop/"Gory details of parsing quoted constructs">.
=over 4
=item i
+X</i> X<regex, case-insensitive> X<regexp, case-insensitive>
+X<regular expression, case-insensitive>
Do case-insensitive pattern matching.
locale. See L<perllocale>.
=item m
+X</m> X<regex, multiline> X<regexp, multiline> X<regular expression, multiline>
Treat string as multiple lines. That is, change "^" and "$" from matching
-at only the very start or end of the string to the start or end of any
-line anywhere within the string,
+the start or end of the string to matching the start or end of any
+line anywhere within the string.
=item s
+X</s> X<regex, single-line> X<regexp, single-line>
+X<regular expression, single-line>
Treat string as single line. That is, change "." to match any character
-whatsoever, even a newline, which it normally would not match.
+whatsoever, even a newline, which normally it would not match.
+
+Used together, as /ms, they let the "." match any character whatsoever,
+while still allowing "^" and "$" to match, respectively, just after
+and just before newlines within the string.
=item x
+X</x>
Extend your pattern's legibility by permitting whitespace and comments.
=back
These are usually written as "the C</x> modifier", even though the delimiter
-in question might not actually be a slash. In fact, any of these
+in question might not really be a slash. Any of these
modifiers may also be embedded within the regular expression itself using
-the new C<(?...)> construct. See below.
+the C<(?...)> construct. See below.
The C</x> modifier itself needs a little more explanation. It tells
the regular expression parser to ignore whitespace that is neither
your regular expression into (slightly) more readable parts. The C<#>
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 C<#> characters in the pattern that you'll have to either
-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. See the C comment deletion code in L<perlop>.
+whitespace or C<#> characters in the pattern (outside a character
+class, where they are unaffected by C</x>), then you'll either have to
+escape them (using backslashes or C<\Q...\E>) 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 L<perlop>. Also note that anything inside
+a C<\Q...\E> stays unaffected by C</x>.
+X</x>
=head2 Regular Expressions
-The patterns used in pattern matching are regular expressions such as
-those supplied in the Version 8 regexp routines. (In fact, the
-routines are derived (distantly) from Henry Spencer's freely
-redistributable reimplementation of the V8 routines.)
-See L<Version 8 Regular Expressions> for details.
+=head3 Metacharacters
+
+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 L<Version 8 Regular Expressions> for
+details.
In particular the following metacharacters have their standard I<egrep>-ish
meanings:
+X<metacharacter>
+X<\> X<^> X<.> X<$> X<|> X<(> X<()> X<[> X<[]>
+
\ Quote the next metacharacter
^ Match the beginning of the line
() Grouping
[] Character class
-By default, the "^" character is guaranteed to match at only the
-beginning of the string, the "$" character at only the end (or before the
-newline at the end) and Perl does certain optimizations with the
+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 "$" will match before any newline. At the
+newline within the string (except if the newline is the last character in
+the string), and "$" will match before any newline. At the
cost of a little more overhead, you can do this by using the /m modifier
on the pattern match operator. (Older programs did this by setting C<$*>,
-but this practice is now deprecated.)
+but this practice has been removed in perl 5.9.)
+X<^> X<$> X</m>
-To facilitate multi-line substitutions, the "." character never matches a
+To simplify multi-line substitutions, the "." character never matches a
newline unless you use the C</s> modifier, which in effect tells Perl to pretend
-the string is a single line--even if it isn't. The C</s> modifier also
-overrides the setting of C<$*>, in case you have some (badly behaved) older
-code that sets it in another module.
+the string is a single line--even if it isn't.
+X<.> X</s>
+
+=head3 Quantifiers
The following standard quantifiers are recognized:
+X<metacharacter> X<quantifier> X<*> X<+> X<?> X<{n}> X<{n,}> X<{n,m}>
* Match 0 or more times
+ Match 1 or more 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 C<{0,}>, the "+"
+as a regular character. In particular, the lower bound
+is not optional.) The "*" modifier is equivalent to C<{0,}>, the "+"
modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. n and m are limited
-to integral values less than 65536.
+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":
+X<metacharacter> X<greedy> X<greedyness>
+X<?> X<*?> X<+?> X<??> X<{n}?> X<{n,}?> X<{n,m}?>
*? Match 0 or more times
+? Match 1 or more times
{n,}? Match at least n times
{n,m}? Match at least n but not more than m times
+By default, when a quantified subpattern does not allow the rest of the
+overall pattern to match, Perl will backtrack. However, this behaviour is
+sometimes undesirable. Thus Perl provides the "possesive" quantifier form
+as well.
+
+ *+ Match 0 or more times and give nothing back
+ ++ Match 1 or more times and give nothing back
+ ?+ Match 0 or 1 time and give nothing back
+ {n}+ Match exactly n times and give nothing back (redundant)
+ {n,}+ Match at least n times and give nothing back
+ {n,m}+ Match at least n but not more than m times and give nothing back
+
+For instance,
+
+ 'aaaa' =~ /a++a/
+
+will never match, as the C<a++> will gobble up all the C<a>'s in the
+string and won't leave any for the remaining part of the pattern. This
+feature can be extremely useful to give perl hints about where it
+shouldn't backtrack. For instance, the typical "match a double-quoted
+string" problem can be most efficiently performed when written as:
+
+ /"(?:[^"\\]++|\\.)*+"/
+
+as we know that if the final quote does not match, bactracking will not
+help. See the independent subexpression C<< (?>...) >> for more details;
+possessive quantifiers are just syntactic sugar for that construct. For
+instance the above example could also be written as follows:
+
+ /"(?>(?:(?>[^"\\]+)|\\.)*)"/
+
+=head3 Escape sequences
+
Because patterns are processed as double quoted strings, the following
also work:
+X<\t> X<\n> X<\r> X<\f> X<\a> X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
+X<\0> X<\c> X<\N> X<\x>
\t tab (HT, TAB)
\n newline (LF, NL)
\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) regexp metacharacters till \E
+ \Q quote (disable) pattern metacharacters till \E
If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
-and <\U> is taken from the current locale. See L<perllocale>.
+and C<\U> is taken from the current locale. See L<perllocale>. For
+documentation of C<\N{name}>, see L<charnames>.
+
+You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
+An unescaped C<$> or C<@> interpolates the corresponding variable,
+while escaping will cause the literal string C<\$> to be matched.
+You'll need to write something like C<m/\Quser\E\@\Qhost/>.
+
+=head3 Character classes
In addition, Perl defines the following:
+X<metacharacter>
+X<\w> X<\W> X<\s> X<\S> X<\d> X<\D> X<\X> X<\p> X<\P> X<\C>
+X<word> X<whitespace>
+
+ \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 (?:\PM\pM*)
+ \C Match a single C char (octet) even under Unicode.
+ NOTE: breaks up characters into their UTF-8 bytes,
+ so you may end up with malformed pieces of UTF-8.
+ Unsupported in lookbehind.
+ \1 Backreference to a a specific group.
+ '1' may actually be any positive integer
+ \k<name> Named backreference
+ \N{name} Named unicode character, or unicode escape.
+ \x12 Hexadecimal escape sequence
+ \x{1234} Long hexadecimal escape sequence
+
+A C<\w> matches a single alphanumeric character (an alphabetic
+character, or a decimal digit) or C<_>, not a whole word. Use C<\w+>
+to match a string of Perl-identifier characters (which isn't the same
+as matching an English word). If C<use locale> is in effect, the list
+of alphabetic characters generated by C<\w> is taken from the current
+locale. See L<perllocale>. You may use C<\w>, C<\W>, C<\s>, C<\S>,
+C<\d>, and C<\D> within character classes, but if you try to use them
+as endpoints of a range, that's not a range, the "-" is understood
+literally. If Unicode is in effect, C<\s> matches also "\x{85}",
+"\x{2028}, and "\x{2029}", see L<perlunicode> for more details about
+C<\pP>, C<\PP>, and C<\X>, and L<perluniintro> about Unicode in general.
+You can define your own C<\p> and C<\P> properties, see L<perlunicode>.
+X<\w> X<\W> X<word>
+
+The POSIX character class syntax
+X<character class>
+
+ [:class:]
+
+is also available. Note that the C<[> and C<]> braces are I<literal>;
+they must always be used within a character class expression.
+
+ # this is correct:
+ $string =~ /[[:alpha:]]/;
+
+ # this is not, and will generate a warning:
+ $string =~ /[:alpha:]/;
+
+The available classes and their backslash equivalents (if available) are
+as follows:
+X<character class>
+X<alpha> X<alnum> X<ascii> X<blank> X<cntrl> X<digit> X<graph>
+X<lower> X<print> X<punct> X<space> X<upper> X<word> X<xdigit>
+
+ alpha
+ alnum
+ ascii
+ blank [1]
+ cntrl
+ digit \d
+ graph
+ lower
+ print
+ punct
+ space \s [2]
+ upper
+ word \w [3]
+ xdigit
+
+=over
+
+=item [1]
+
+A GNU extension equivalent to C<[ \t]>, "all horizontal whitespace".
+
+=item [2]
+
+Not exactly equivalent to C<\s> since the C<[[:space:]]> includes
+also the (very rare) "vertical tabulator", "\ck", chr(11).
+
+=item [3]
+
+A Perl extension, see above.
+
+=back
+
+For example use C<[:upper:]> to match all the uppercase characters.
+Note that the C<[]> are part of the C<[::]> construct, not part of the
+whole character class. For example:
+
+ [01[:alpha:]%]
+
+matches zero, one, any alphabetic character, and the percentage sign.
+
+The following equivalences to Unicode \p{} constructs and equivalent
+backslash character classes (if available), will hold:
+X<character class> X<\p> X<\p{}>
+
+ [[:...:]] \p{...} backslash
+
+ alpha IsAlpha
+ alnum IsAlnum
+ ascii IsASCII
+ blank IsSpace
+ cntrl IsCntrl
+ digit IsDigit \d
+ graph IsGraph
+ lower IsLower
+ print IsPrint
+ punct IsPunct
+ space IsSpace
+ IsSpacePerl \s
+ upper IsUpper
+ word IsWord
+ xdigit IsXDigit
+
+For example C<[[:lower:]]> and C<\p{IsLower}> are equivalent.
+
+If the C<utf8> pragma is not used but the C<locale> pragma is, the
+classes correlate with the usual isalpha(3) interface (except for
+"word" and "blank").
+
+The assumedly non-obviously named classes are:
+
+=over 4
+
+=item cntrl
+X<cntrl>
- \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
+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 (assuming ASCII,
+the ISO Latin character sets, and Unicode), as is the character with
+the ord() value of 127 (C<DEL>).
-Note that C<\w> matches a single alphanumeric character, not a whole
-word. To match a word you'd need to say C<\w+>. If C<use locale> is in
-effect, the list of alphabetic characters generated by C<\w> is taken
-from the current locale. See L<perllocale>. You may use C<\w>, C<\W>,
-C<\s>, C<\S>, C<\d>, and C<\D> within character classes (though not as
-either end of a range).
+=item graph
+X<graph>
+
+Any alphanumeric or punctuation (special) character.
+
+=item print
+X<print>
+
+Any alphanumeric or punctuation (special) character or the space character.
+
+=item punct
+X<punct>
+
+Any punctuation (special) character.
+
+=item xdigit
+X<xdigit>
+
+Any hexadecimal digit. Though this may feel silly ([0-9A-Fa-f] would
+work just fine) it is included for completeness.
+
+=back
+
+You can negate the [::] character classes by prefixing the class name
+with a '^'. This is a Perl extension. For example:
+X<character class, negation>
+
+ POSIX traditional Unicode
+
+ [[:^digit:]] \D \P{IsDigit}
+ [[:^space:]] \S \P{IsSpace}
+ [[:^word:]] \W \P{IsWord}
+
+Perl respects the POSIX standard in that POSIX character classes are
+only supported within a character class. The POSIX character classes
+[.cc.] and [=cc=] are recognized but B<not> supported and trying to
+use them will cause an error.
+
+=head3 Assertions
Perl defines the following zero-width assertions:
+X<zero-width assertion> X<assertion> X<regex, zero-width assertion>
+X<regexp, zero-width assertion>
+X<regular expression, zero-width assertion>
+X<\b> X<\B> X<\A> X<\Z> X<\z> X<\G>
\b Match a word boundary
\B Match a non-(word boundary)
- \A Match at only beginning of string
- \Z Match at only end of string (or before newline at the end)
- \G Match only where previous m//g left off (works only with /g)
-
-A word boundary (C<\b>) is defined as a spot between two characters that
-has a C<\w> on one side of it and a C<\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 C<\W>. (Within character classes C<\b>
-represents backspace rather than a word boundary.) The C<\A> and C<\Z> are
-just like "^" and "$" except that they won't match multiple times when the
-C</m> modifier is used, while "^" and "$" will match at every internal line
-boundary. To match the actual end of the string, not ignoring newline,
-you can use C<\Z(?!\n)>. The C<\G> assertion can be used to chain global
-matches (using C<m//g>), as described in
-L<perlop/"Regexp Quote-Like Operators">.
-
-It is also useful when writing C<lex>-like scanners, when you have several
-regexps which you want to match against consequent substrings of your
-string, see the previous reference.
-The actual location where C<\G> will match can also be influenced
-by using C<pos()> as an lvalue. See L<perlfunc/pos>.
-
-When the bracketing construct C<( ... )> is used, \E<lt>digitE<gt> matches the
-digit'th substring. Outside of the pattern, always use "$" instead of "\"
-in front of the digit. (While the \E<lt>digitE<gt> notation can on rare occasion work
-outside the current pattern, this should not be relied upon. See the
-WARNING below.) The scope of $E<lt>digitE<gt> (and C<$`>, C<$&>, and C<$'>)
-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, $11, ... refer to the
-corresponding substring. Within the pattern, \10, \11, etc. refer back
-to substrings if there have been at least that many left parentheses before
-the backreference. Otherwise (for backward compatibility) \10 is the
-same as \010, a backspace, and \11 the same as \011, a tab. And so
-on. (\1 through \9 are always backreferences.)
-
-C<$+> returns whatever the last bracket match matched. C<$&> returns the
-entire matched string. (C<$0> used to return the same thing, but not any
-more.) C<$`> returns everything before the matched string. C<$'> returns
-everything after the matched string. Examples:
+ \A Match only at beginning of string
+ \Z Match only at end of string, or before newline at the end
+ \z Match only at end of string
+ \G Match only at pos() (e.g. at the end-of-match position
+ of prior m//g)
+
+A word boundary (C<\b>) is a spot between two characters
+that has a C<\w> on one side of it and a C<\W> on the other side
+of it (in either order), counting the imaginary characters off the
+beginning and end of the string as matching a C<\W>. (Within
+character classes C<\b> represents backspace rather than a word
+boundary, just as it normally does in any double-quoted string.)
+The C<\A> and C<\Z> are just like "^" and "$", except that they
+won't match multiple times when the C</m> modifier is used, while
+"^" and "$" will match at every internal line boundary. To match
+the actual end of the string and not ignore an optional trailing
+newline, use C<\z>.
+X<\b> X<\A> X<\Z> X<\z> X</m>
+
+The C<\G> assertion can be used to chain global matches (using
+C<m//g>), as described in L<perlop/"Regexp Quote-Like Operators">.
+It is also useful when writing C<lex>-like scanners, when you have
+several patterns that you want to match against consequent substrings
+of your string, see the previous reference. The actual location
+where C<\G> will match can also be influenced by using C<pos()> as
+an lvalue: see L<perlfunc/pos>. Currently C<\G> is only fully
+supported when anchored to the start of the pattern; while it
+is permitted to use it elsewhere, as in C</(?<=\G..)./g>, some
+such uses (C</.\G/g>, for example) currently cause problems, and
+it is recommended that you avoid such usage for now.
+X<\G>
+
+=head3 Capture buffers
+
+The bracketing construct C<( ... )> creates capture buffers. To
+refer to the digit'th buffer use \<digit> within the
+match. Outside the match use "$" instead of "\". (The
+\<digit> notation works in certain circumstances outside
+the match. See the warning below about \1 vs $1 for details.)
+Referring back to another part of the match is called a
+I<backreference>.
+X<regex, capture buffer> X<regexp, capture buffer>
+X<regular expression, capture buffer> X<backreference>
+
+There is no limit to the number of captured substrings that you may
+use. However Perl also uses \10, \11, etc. as aliases for \010,
+\011, etc. (Recall that 0 means octal, so \011 is the character at
+number 9 in your coded character set; which would be the 10th character,
+a horizontal tab under ASCII.) Perl resolves this
+ambiguity by interpreting \10 as a backreference only if at least 10
+left parentheses have opened before it. Likewise \11 is a
+backreference only if at least 11 left parentheses have opened
+before it. And so on. \1 through \9 are always interpreted as
+backreferences.
+
+Additionally, as of Perl 5.10 you may use named capture buffers and named
+backreferences. The notation is C<< (?<name>...) >> and C<< \k<name> >>
+(you may also use single quotes instead of angle brackets to quote the
+name). The only difference with named capture buffers and unnamed ones is
+that multiple buffers may have the same name and that the contents of
+named capture buffers is available via the C<%+> hash. When multiple
+groups share the same name C<$+{name}> and C<< \k<name> >> refer to the
+leftmost defined group, thus it's possible to do things with named capture
+buffers that would otherwise require C<(??{})> code to accomplish. Named
+capture buffers are numbered just as normal capture buffers are and may be
+referenced via the magic numeric variables or via numeric backreferences
+as well as by name.
+
+Examples:
s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
- if (/Time: (..):(..):(..)/) {
+ /(.)\1/ # find first doubled char
+ and print "'$1' is the first doubled character\n";
+
+ /(?<char>.)\k<char>/ # ... a different way
+ and print "'$+{char}' is the first doubled character\n";
+
+ /(?<char>.)\1/ # ... mix and match
+ and print "'$1' is the first doubled character\n";
+
+ if (/Time: (..):(..):(..)/) { # parse out values
$hours = $1;
$minutes = $2;
$seconds = $3;
}
-Once perl sees that you need one of C<$&>, C<$`> or C<$'> 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 regexp that contains capturing parentheses. But if you never
-use $&, etc., in your script, then regexps I<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.
-
-You will note that all backslashed metacharacters in Perl are
-alphanumeric, such as C<\b>, C<\w>, C<\n>. Unlike some other regular
-expression languages, there are no backslashed symbols that aren't
-alphanumeric. So anything that looks like \\, \(, \), \E<lt>, \E<gt>,
-\{, 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 the
-non-alphanumeric characters:
+Several special variables also refer back to portions of the previous
+match. C<$+> returns whatever the last bracket match matched.
+C<$&> returns the entire matched string. (At one point C<$0> did
+also, but now it returns the name of the program.) C<$`> returns
+everything before the matched string. C<$'> returns everything
+after the matched string. And C<$^N> contains whatever was matched by
+the most-recently closed group (submatch). C<$^N> can be used in
+extended patterns (see below), for example to assign a submatch to a
+variable.
+X<$+> X<$^N> X<$&> X<$`> X<$'>
+
+The numbered match variables ($1, $2, $3, etc.) and the related punctuation
+set (C<$+>, C<$&>, C<$`>, C<$'>, and C<$^N>) are all dynamically scoped
+until the end of the enclosing block or until the next successful
+match, whichever comes first. (See L<perlsyn/"Compound Statements">.)
+X<$+> X<$^N> X<$&> X<$`> X<$'>
+X<$1> X<$2> X<$3> X<$4> X<$5> X<$6> X<$7> X<$8> X<$9>
+
+
+B<NOTE>: failed matches in Perl do not reset the match variables,
+which makes it easier to write code that tests for a series of more
+specific cases and remembers the best match.
+
+B<WARNING>: Once Perl sees that you need one of C<$&>, C<$`>, or
+C<$'> 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, $2, etc, so you also pay a
+price for each pattern that contains capturing parentheses. (To
+avoid this cost while retaining the grouping behaviour, use the
+extended regular expression C<(?: ... )> instead.) But if you never
+use C<$&>, C<$`> or C<$'>, then patterns I<without> capturing
+parentheses will not be penalized. So avoid C<$&>, C<$'>, and C<$`>
+if you can, but if you can't (and some algorithms really appreciate
+them), once you've used them once, use them at will, because you've
+already paid the price. As of 5.005, C<$&> is not so costly as the
+other two.
+X<$&> X<$`> X<$'>
+
+Backslashed metacharacters in Perl are alphanumeric, such as C<\b>,
+C<\w>, C<\n>. Unlike some other regular expression languages, there
+are no backslashed symbols that aren't alphanumeric. So anything
+that looks like \\, \(, \), \<, \>, \{, or \} is always
+interpreted as a literal character, not a metacharacter. This was
+once used in a common idiom to disable or quote the special meanings
+of regular expression metacharacters in a string that you want to
+use for a pattern. Simply quote all non-"word" characters:
$pattern =~ s/(\W)/\\$1/g;
-Now it is much more common to see either the quotemeta() function or
-the \Q escape sequence used to disable the metacharacters special
+(If C<use locale> is set, then this depends on the current locale.)
+Today it is more common to use the quotemeta() function or the C<\Q>
+metaquoting escape sequence 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:
+Beware that if you put literal backslashes (those not inside
+interpolated variables) between C<\Q> and C<\E>, double-quotish
+backslash interpolation may lead to confusing results. If you
+I<need> to use literal backslashes within C<\Q...\E>,
+consult L<perlop/"Gory details of parsing quoted constructs">.
+
+=head2 Extended Patterns
+
+Perl also defines a consistent extension syntax for features not
+found in standard tools like B<awk> and B<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 psychology...
=over 10
-=item (?#text)
+=item C<(?#text)>
+X<(?#)>
+
+A comment. The text is ignored. If the C</x> modifier enables
+whitespace formatting, a simple C<#> will suffice. Note that Perl closes
+the comment as soon as it sees a C<)>, so there is no way to put a literal
+C<)> in the comment.
+
+=item C<(?imsx-imsx)>
+X<(?)>
+
+One or more embedded pattern-match modifiers, to be turned on (or
+turned off, if preceded by C<->) for the remainder of the pattern or
+the remainder of the enclosing pattern group (if any). This is
+particularly useful for dynamic patterns, such as those read in from a
+configuration file, 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 C<(?i)> at the front of the pattern. For example:
+
+ $pattern = "foobar";
+ if ( /$pattern/i ) { }
+
+ # more flexible:
+
+ $pattern = "(?i)foobar";
+ if ( /$pattern/ ) { }
+
+These modifiers are restored at the end of the enclosing group. For example,
+
+ ( (?i) blah ) \s+ \1
+
+will match a repeated (I<including the case>!) word C<blah> in any
+case, assuming C<x> modifier, and no C<i> modifier outside this
+group.
-A comment. The text is ignored. If the C</x> switch is used to enable
-whitespace formatting, a simple C<#> will suffice.
+=item C<(?:pattern)>
+X<(?:)>
-=item (?:regexp)
+=item C<(?imsx-imsx:pattern)>
-This groups things like "()" but doesn't make backreferences like "()" does. So
+This is for clustering, not capturing; it groups subexpressions like
+"()", but doesn't make backreferences as "()" does. So
- split(/\b(?:a|b|c)\b/)
+ @fields = split(/\b(?:a|b|c)\b/)
is like
- split(/\b(a|b|c)\b/)
+ @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 C<?> and C<:> act as flags modifiers as with
+C<(?imsx-imsx)>. For example,
+
+ /(?s-i:more.*than).*million/i
+
+is equivalent to the more verbose
-but doesn't spit out extra fields.
+ /(?:(?s-i)more.*than).*million/i
-=item (?=regexp)
+=item C<(?=pattern)>
+X<(?=)> X<look-ahead, positive> X<lookahead, positive>
-A zero-width positive lookahead assertion. For example, C</\w+(?=\t)/>
+A zero-width positive look-ahead assertion. For example, C</\w+(?=\t)/>
matches a word followed by a tab, without including the tab in C<$&>.
-=item (?!regexp)
+=item C<(?!pattern)>
+X<(?!)> X<look-ahead, negative> X<lookahead, negative>
-A zero-width negative lookahead assertion. For example C</foo(?!bar)/>
+A zero-width negative look-ahead assertion. For example C</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: C</(?!foo)bar/> will not find an occurrence of
-"bar" that is preceded by something which is not "foo". That's because
-the C<(?!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 C</(?!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: C</(?:(?!foo)...|^..?)bar/>. Sometimes it's still
-easier just to say:
+however that look-ahead and look-behind are NOT the same thing. You cannot
+use this for look-behind.
- if (/foo/ && $` =~ /bar$/)
+If you are looking for a "bar" that isn't preceded by a "foo", C</(?!foo)bar/>
+will not do what you want. That's because the C<(?!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 C</(?!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: C</(?:(?!foo)...|^.{0,2})bar/>.
+Sometimes it's still easier just to say:
+ if (/bar/ && $` !~ /foo$/)
-=item (?imsx)
+For look-behind see below.
-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 C<(?i)> at the front of the
-pattern. For example:
+=item C<(?<=pattern)>
+X<(?<=)> X<look-behind, positive> X<lookbehind, positive>
- $pattern = "foobar";
- if ( /$pattern/i )
+A zero-width positive look-behind assertion. For example, C</(?<=\t)\w+/>
+matches a word that follows a tab, without including the tab in C<$&>.
+Works only for fixed-width look-behind.
- # more flexible:
+=item C<(?<!pattern)>
+X<(?<!)> X<look-behind, negative> X<lookbehind, negative>
- $pattern = "(?i)foobar";
- if ( /$pattern/ )
+A zero-width negative look-behind assertion. For example C</(?<!bar)foo/>
+matches any occurrence of "foo" that does not follow "bar". Works
+only for fixed-width look-behind.
+
+=item C<(?'NAME'pattern)>
+
+=item C<< (?<NAME>pattern) >>
+X<< (?<NAME>) >> X<(?'NAME')> X<named capture> X<capture>
+
+A named capture buffer. Identical in every respect to normal capturing
+parens C<()> but for the additional fact that C<%+> may be used after
+a succesful match to refer to a named buffer. See C<perlvar> for more
+details on the C<%+> hash.
+
+If multiple distinct capture buffers have the same name then the
+$+{NAME} will refer to the leftmost defined buffer in the match.
+
+The forms C<(?'NAME'pattern)> and C<(?<NAME>pattern)> are equivalent.
+
+B<NOTE:> While the notation of this construct is the same as the similar
+function in .NET regexes, the behavior is not, in Perl the buffers are
+numbered sequentially regardless of being named or not. Thus in the
+pattern
+
+ /(x)(?<foo>y)(z)/
+
+$+{foo} will be the same as $2, and $3 will contain 'z' instead of
+the opposite which is what a .NET regex hacker might expect.
+
+Currently NAME is restricted to word chars only. In other words, it
+must match C</^\w+$/>.
+
+=item C<< \k<name> >>
+
+=item C<< \k'name' >>
+
+Named backreference. Similar to numeric backreferences, except that
+the group is designated by name and not number. If multiple groups
+have the same name then it refers to the leftmost defined group in
+the current match.
+
+It is an error to refer to a name not defined by a C<(?<NAME>)>
+earlier in the pattern.
+
+Both forms are equivalent.
+
+=item C<(?{ code })>
+X<(?{})> X<regex, code in> X<regexp, code in> X<regular expression, code in>
+
+B<WARNING>: This extended regular expression feature is considered
+experimental, and may be changed without notice. Code executed that
+has side effects may not perform identically from version to version
+due to the effect of future optimisations in the regex engine.
+
+This zero-width assertion evaluates any embedded Perl code. It
+always succeeds, and its C<code> is not interpolated. Currently,
+the rules to determine where the C<code> ends are somewhat convoluted.
+
+This feature can be used together with the special variable C<$^N> to
+capture the results of submatches in variables without having to keep
+track of the number of nested parentheses. For example:
+
+ $_ = "The brown fox jumps over the lazy dog";
+ /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i;
+ print "color = $color, animal = $animal\n";
+
+Inside the C<(?{...})> block, C<$_> refers to the string the regular
+expression is matching against. You can also use C<pos()> to know what is
+the current position of matching within this string.
+
+The C<code> is properly scoped in the following sense: If the assertion
+is backtracked (compare L<"Backtracking">), all changes introduced after
+C<local>ization are undone, so that
+
+ $_ = 'a' x 8;
+ m<
+ (?{ $cnt = 0 }) # Initialize $cnt.
+ (
+ a
+ (?{
+ local $cnt = $cnt + 1; # Update $cnt, backtracking-safe.
+ })
+ )*
+ aaaa
+ (?{ $res = $cnt }) # On success copy to non-localized
+ # location.
+ >x;
+
+will set C<$res = 4>. Note that after the match, $cnt returns to the globally
+introduced value, because the scopes that restrict C<local> operators
+are unwound.
+
+This assertion may be used as a C<(?(condition)yes-pattern|no-pattern)>
+switch. If I<not> used in this way, the result of evaluation of
+C<code> is put into the special variable C<$^R>. This happens
+immediately, so C<$^R> can be used from other C<(?{ code })> assertions
+inside the same regular expression.
+
+The assignment to C<$^R> above is properly localized, so the old
+value of C<$^R> is restored if the assertion is backtracked; compare
+L<"Backtracking">.
+
+Due to an unfortunate implementation issue, the Perl code contained in these
+blocks is treated as a compile time closure that can have seemingly bizarre
+consequences when used with lexically scoped variables inside of subroutines
+or loops. There are various workarounds for this, including simply using
+global variables instead. If you are using this construct and strange results
+occur then check for the use of lexically scoped variables.
+
+For reasons of security, this construct is forbidden if the regular
+expression involves run-time interpolation of variables, unless the
+perilous C<use re 'eval'> pragma has been used (see L<re>), or the
+variables contain results of C<qr//> operator (see
+L<perlop/"qr/STRING/imosx">).
+
+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 C<use re 'eval'>, though, it is no longer secure,
+so you should only do so if you are also using taint checking.
+Better yet, use the carefully constrained evaluation within a Safe
+compartment. See L<perlsec> for details about both these mechanisms.
+
+Because perl's regex engine is not currently re-entrant, interpolated
+code may not invoke the regex engine either directly with C<m//> or C<s///>),
+or indirectly with functions such as C<split>.
+
+=item C<(??{ code })>
+X<(??{})>
+X<regex, postponed> X<regexp, postponed> X<regular expression, postponed>
+
+B<WARNING>: This extended regular expression feature is considered
+experimental, and may be changed without notice. Code executed that
+has side effects may not perform identically from version to version
+due to the effect of future optimisations in the regex engine.
+
+This is a "postponed" regular subexpression. The C<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 construct. Note that this means
+that the contents of capture buffers defined inside an eval'ed pattern
+are not available outside of the pattern, and vice versa, there is no
+way for the inner pattern to refer to a capture buffer defined outside.
+Thus,
+
+ ('a' x 100)=~/(??{'(.)' x 100})/
+
+B<will> match, it will B<not> set $1.
+
+The C<code> is not interpolated. As before, the rules to determine
+where the C<code> ends are currently somewhat convoluted.
+
+The following pattern matches a parenthesized group:
+
+ $re = qr{
+ \(
+ (?:
+ (?> [^()]+ ) # Non-parens without backtracking
+ |
+ (??{ $re }) # Group with matching parens
+ )*
+ \)
+ }x;
+
+See also C<(?PARNO)> for a different, more efficient way to accomplish
+the same task.
+
+Because perl's regex engine is not currently re-entrant, delayed
+code may not invoke the regex engine either directly with C<m//> or C<s///>),
+or indirectly with functions such as C<split>.
+
+Recursing deeper than 50 times without consuming any input string will
+result in a fatal error. The maximum depth is compiled into perl, so
+changing it requires a custom build.
+
+=item C<(?PARNO)> C<(?R)> C<(?0)>
+X<(?PARNO)> X<(?1)> X<(?R)> X<(?0)>
+X<regex, recursive> X<regexp, recursive> X<regular expression, recursive>
+
+Similar to C<(??{ code })> except it does not involve compiling any code,
+instead it treats the contents of a capture buffer as an independent
+pattern that must match at the current position. Capture buffers
+contained by the pattern will have the value as determined by the
+outermost recursion.
+
+PARNO is a sequence of digits (not starting with 0) whose value reflects
+the paren-number of the capture buffer to recurse to. C<(?R)> recurses to
+the beginning of the whole pattern. C<(?0)> is an alternate syntax for
+C<(?R)>.
+
+The following pattern matches a function foo() which may contain
+balanced parentheses as the argument.
+
+ $re = qr{ ( # paren group 1 (full function)
+ foo
+ ( # paren group 2 (parens)
+ \(
+ ( # paren group 3 (contents of parens)
+ (?:
+ (?> [^()]+ ) # Non-parens without backtracking
+ |
+ (?2) # Recurse to start of paren group 2
+ )*
+ )
+ \)
+ )
+ )
+ }x;
+
+If the pattern was used as follows
+
+ 'foo(bar(baz)+baz(bop))'=~/$re/
+ and print "\$1 = $1\n",
+ "\$2 = $2\n",
+ "\$3 = $3\n";
+
+the output produced should be the following:
+
+ $1 = foo(bar(baz)+baz(bop))
+ $2 = (bar(baz)+baz(bop))
+ $3 = bar(baz)+baz(bop)
+
+If there is no corresponding capture buffer defined, then it is a
+fatal error. Recursing deeper than 50 times without consuming any input
+string will also result in a fatal error. The maximum depth is compiled
+into perl, so changing it requires a custom build.
+
+B<Note> that this pattern does not behave the same way as the equivalent
+PCRE or Python construct of the same form. In perl you can backtrack into
+a recursed group, in PCRE and Python the recursed into group is treated
+as atomic. Also, constructs like (?i:(?1)) or (?:(?i)(?1)) do not affect
+the pattern being recursed into.
+
+=item C<(?&NAME)>
+X<(?&NAME)>
+
+Recurse to a named subpattern. Identical to (?PARNO) except that the
+parenthesis to recurse to is determined by name. If multiple parens have
+the same name, then it recurses to the leftmost.
+
+It is an error to refer to a name that is not declared somewhere in the
+pattern.
+
+=item C<(?(condition)yes-pattern|no-pattern)>
+X<(?()>
+
+=item C<(?(condition)yes-pattern)>
+
+Conditional expression. C<(condition)> should be either an integer in
+parentheses (which is valid if the corresponding pair of parentheses
+matched), a look-ahead/look-behind/evaluate zero-width assertion, a
+name in angle brackets or single quotes (which is valid if a buffer
+with the given name matched), or the special symbol (R) (true when
+evaluated inside of recursion or eval). Additionally the R may be
+followed by a number, (which will be true when evaluated when recursing
+inside of the appropriate group), or by C<&NAME>, in which case it will
+be true only when evaluated during recursion in the named group.
+
+Here's a summary of the possible predicates:
+
+=over 4
+
+=item (1) (2) ...
+
+Checks if the numbered capturing buffer has matched something.
+
+=item (<NAME>) ('NAME')
+
+Checks if a buffer with the given name has matched something.
+
+=item (?{ CODE })
+
+Treats the code block as the condition.
+
+=item (R)
+
+Checks if the expression has been evaluated inside of recursion.
+
+=item (R1) (R2) ...
+
+Checks if the expression has been evaluated while executing directly
+inside of the n-th capture group. This check is the regex equivalent of
+
+ if ((caller(0))[3] eq 'subname') { ... }
+
+In other words, it does not check the full recursion stack.
+
+=item (R&NAME)
+
+Similar to C<(R1)>, this predicate checks to see if we're executing
+directly inside of the leftmost group with a given name (this is the same
+logic used by C<(?&NAME)> to disambiguate). It does not check the full
+stack, but only the name of the innermost active recursion.
+
+=item (DEFINE)
+
+In this case, the yes-pattern is never directly executed, and no
+no-pattern is allowed. Similar in spirit to C<(?{0})> but more efficient.
+See below for details.
+
+=back
+
+For example:
+
+ m{ ( \( )?
+ [^()]+
+ (?(1) \) )
+ }x
+
+matches a chunk of non-parentheses, possibly included in parentheses
+themselves.
+
+A special form is the C<(DEFINE)> predicate, which never executes directly
+its yes-pattern, and does not allow a no-pattern. This allows to define
+subpatterns which will be executed only by using the recursion mechanism.
+This way, you can define a set of regular expression rules that can be
+bundled into any pattern you choose.
+
+It is recommended that for this usage you put the DEFINE block at the
+end of the pattern, and that you name any subpatterns defined within it.
+
+Also, it's worth noting that patterns defined this way probably will
+not be as efficient, as the optimiser is not very clever about
+handling them.
+
+An example of how this might be used is as follows:
+
+ /(?<NAME>(&NAME_PAT))(?<ADDR>(&ADDRESS_PAT))
+ (?(DEFINE)
+ (<NAME_PAT>....)
+ (<ADRESS_PAT>....)
+ )/x
+
+Note that capture buffers matched inside of recursion are not accessible
+after the recursion returns, so the extra layer of capturing buffers are
+necessary. Thus C<$+{NAME_PAT}> would not be defined even though
+C<$+{NAME}> would be.
+
+=item C<< (?>pattern) >>
+X<backtrack> X<backtracking> X<atomic> X<possessive>
+
+An "independent" subexpression, one which matches the substring
+that a I<standalone> C<pattern> would match if anchored at the given
+position, and it matches I<nothing other than this substring>. This
+construct is useful for optimizations of what would otherwise be
+"eternal" matches, because it will not backtrack (see L<"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: C<< ^(?>a*)ab >> will never match, since C<< (?>a*) >>
+(anchored at the beginning of string, as above) will match I<all>
+characters C<a> at the beginning of string, leaving no C<a> for
+C<ab> to match. In contrast, C<a*ab> will match the same as C<a+b>,
+since the match of the subgroup C<a*> is influenced by the following
+group C<ab> (see L<"Backtracking">). In particular, C<a*> inside
+C<a*ab> will match fewer characters than a standalone C<a*>, since
+this makes the tail match.
+
+An effect similar to C<< (?>pattern) >> may be achieved by writing
+C<(?=(pattern))\1>. This matches the same substring as a standalone
+C<a+>, and the following C<\1> eats the matched string; it therefore
+makes a zero-length assertion into an analogue of C<< (?>...) >>.
+(The difference between these two constructs is that the second one
+uses a capturing group, thus shifting ordinals of backreferences
+in the rest of a regular expression.)
+
+Consider this pattern:
+
+ m{ \(
+ (
+ [^()]+ # x+
+ |
+ \( [^()]* \)
+ )+
+ \)
+ }x
+
+That will efficiently match a nonempty group with matching parentheses
+two levels deep or less. However, if there is no such group, it
+will take virtually forever on a long string. That's because there
+are so many different ways to split a long string into several
+substrings. This is what C<(.+)+> is doing, and C<(.+)+> is similar
+to a subpattern of the above pattern. Consider how the pattern
+above detects no-match on C<((()aaaaaaaaaaaaaaaaaa> in several
+seconds, but that each extra letter doubles this time. This
+exponential performance will make it appear that your program has
+hung. However, a tiny change to this pattern
+
+ m{ \(
+ (
+ (?> [^()]+ ) # change x+ above to (?> x+ )
+ |
+ \( [^()]* \)
+ )+
+ \)
+ }x
+
+which uses C<< (?>...) >> 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 C<a>s. Be aware,
+however, that this pattern currently triggers a warning message under
+the C<use warnings> pragma or B<-w> switch saying it
+C<"matches null string many times in regex">.
+
+On simple groups, such as the pattern C<< (?> [^()]+ ) >>, a comparable
+effect may be achieved by negative look-ahead, as in C<[^()]+ (?! [^()] )>.
+This was only 4 times slower on a string with 1000000 C<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 C<()*> looks like
+the correct solution. Suppose we parse text with comments being delimited
+by C<#> followed by some optional (horizontal) whitespace. Contrary to
+its appearance, C<#[ \t]*> I<is not> the correct subexpression to match
+the comment delimiter, because it may "give up" some whitespace if
+the remainder of the pattern can be made to match that way. The correct
+answer is either one of these:
+
+ (?>#[ \t]*)
+ #[ \t]*(?![ \t])
+
+For example, to grab non-empty comments into $1, one should use either
+one of these:
+
+ / (?> \# [ \t]* ) ( .+ ) /x;
+ / \# [ \t]* ( [^ \t] .* ) /x;
+
+Which one you pick depends on which of these expressions better reflects
+the above specification of comments.
+
+In some literature this construct is called "atomic matching" or
+"possessive matching".
+
+Possessive quantifiers are equivalent to putting the item they are applied
+to inside of one of these constructs. The following equivalences apply:
+
+ Quantifier Form Bracketing Form
+ --------------- ---------------
+ PAT*+ (?>PAT*)
+ PAT++ (?>PAT+)
+ PAT?+ (?>PAT?)
+ PAT{min,max}+ (?>PAT{min,max})
+
+=back
+
+=head2 Special Backtracking Control Verbs
+
+B<WARNING:> These patterns are experimental and subject to change or
+removal in a future version of perl. Their usage in production code should
+be noted to avoid problems during upgrades.
+
+These special patterns are generally of the form C<(*VERB:ARG)>. Unless
+otherwise stated the ARG argument is optional; in some cases, it is
+forbidden.
+
+Any pattern containing a special backtracking verb that allows an argument
+has the special behaviour that when executed it sets the current packages'
+C<$REGERROR> variable. In this case, the following rules apply:
+
+On failure, this variable will be set to the ARG value of the verb
+pattern, if the verb was involved in the failure of the match. If the ARG
+part of the pattern was omitted, then C<$REGERROR> will be set to TRUE.
+
+On a successful match this variable will be set to FALSE.
+
+B<NOTE:> C<$REGERROR> is not a magic variable in the same sense than
+C<$1> and most other regex related variables. It is not local to a
+scope, nor readonly but instead a volatile package variable similar to
+C<$AUTOLOAD>. Use C<local> to localize changes to it to a specific scope
+if necessary.
+
+If a pattern does not contain a special backtracking verb that allows an
+argument, then C<$REGERROR> is not touched at all.
+
+=over 4
+
+=item Verbs that take an argument
+
+=over 4
+
+=item C<(*NOMATCH)> C<(*NOMATCH:NAME)>
+X<(*NOMATCH)> X<(*NOMATCH:NAME)>
+
+This zero-width pattern commits the match at the current point, preventing
+the engine from backtracking on failure to the left of the this point.
+Consider the pattern C<A (*NOMATCH) B>, where A and B are complex patterns.
+Until the C<(*NOMATCH)> is reached, A may backtrack as necessary to match.
+Once it is reached, matching continues in B, which may also backtrack as
+necessary; however, should B not match, then no further backtracking will
+take place, and the pattern will fail outright at that starting position.
+
+The following example counts all the possible matching strings in a
+pattern (without actually matching any of them).
+
+ 'aaab' =~ /a+b?(?{print "$&\n"; $count++})(*FAIL)/;
+ print "Count=$count\n";
+
+which produces:
+
+ aaab
+ aaa
+ aa
+ a
+ aab
+ aa
+ a
+ ab
+ a
+ Count=9
+
+If we add a C<(*NOMATCH)> before the count like the following
+
+ 'aaab' =~ /a+b?(*NOMATCH)(?{print "$&\n"; $count++})(*FAIL)/;
+ print "Count=$count\n";
+
+we prevent backtracking and find the count of the longest matching
+at each matching startpoint like so:
+
+ aaab
+ aab
+ ab
+ Count=3
+
+Any number of C<(*NOMATCH)> assertions may be used in a pattern.
+
+See also C<< (?>pattern) >> and possessive quantifiers for other
+ways to control backtracking.
+
+=item C<(*MARK)> C<(*MARK:NAME)>
+X<(*MARK)>
+
+This zero-width pattern can be used to mark the point in a string
+reached when a certain part of the pattern has been successfully
+matched. This mark may be given a name. A later C<(*CUT)> pattern
+will then cut at that point if backtracked into on failure. Any
+number of (*MARK) patterns are allowed, and the NAME portion is
+optional and may be duplicated.
+
+See C<*CUT> for more detail.
+
+=item C<(*CUT)> C<(*CUT:NAME)>
+X<(*CUT)>
+
+This zero-width pattern is similar to C<(*NOMATCH)>, except that on
+failure it also signifies that whatever text that was matched leading up
+to the C<(*CUT)> pattern being executed cannot be part of a match, I<even
+if started from a later point>. This effectively means that the regex
+engine moves forward to this position on failure and tries to match
+again, (assuming that there is sufficient room to match).
+
+The name of the C<(*CUT:NAME)> pattern has special significance. If a
+C<(*MARK:NAME)> was encountered while matching, then it is the position
+where that pattern was executed that is used for the "cut point" in the
+string. If no mark of that name was encountered, then the cut is done at
+the point where the C<(*CUT)> was. Similarly if no NAME is specified in
+the C<(*CUT)>, and if a C<(*MARK)> with any name (or none) is encountered,
+then that C<(*MARK)>'s cursor point will be used. If the C<(*CUT)> is not
+preceded by a C<(*MARK)>, then the cut point is where the string was when
+the C<(*CUT)> was encountered.
+
+Compare the following to the examples in C<(*NOMATCH)>, note the string
+is twice as long:
+
+ 'aaabaaab' =~ /a+b?(*CUT)(?{print "$&\n"; $count++})(*FAIL)/;
+ print "Count=$count\n";
+
+outputs
+
+ aaab
+ aaab
+ Count=2
+
+Once the 'aaab' at the start of the string has matched, and the C<(*CUT)>
+executed, the next startpoint will be where the cursor was when the
+C<(*CUT)> was executed.
+
+=item C<(*COMMIT)>
+X<(*COMMIT)>
+
+This zero-width pattern is similar to C<(*CUT)> except that it causes
+the match to fail outright. No attempts to match will occur again.
+
+ 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/;
+ print "Count=$count\n";
+
+outputs
+
+ aaab
+ Count=1
+
+In other words, once the C<(*COMMIT)> has been entered, and if the pattern
+does not match, the regex engine will not try any further matching on the
+rest of the string.
=back
-The specific choice of question mark for this and the new minimal
-matching construct was 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...
+=item Verbs without an argument
+
+=over 4
+
+=item C<(*FAIL)> C<(*F)>
+X<(*FAIL)> X<(*F)>
+
+This pattern matches nothing and always fails. It can be used to force the
+engine to backtrack. It is equivalent to C<(?!)>, but easier to read. In
+fact, C<(?!)> gets optimised into C<(*FAIL)> internally.
+
+It is probably useful only when combined with C<(?{})> or C<(??{})>.
+
+=item C<(*ACCEPT)>
+X<(*ACCEPT)>
+
+B<WARNING:> This feature is highly experimental. It is not recommended
+for production code.
+
+This pattern matches nothing and causes the end of successful matching at
+the point at which the C<(*ACCEPT)> pattern was encountered, regardless of
+whether there is actually more to match in the string. When inside of a
+nested pattern, such as recursion or a dynamically generated subbpattern
+via C<(??{})>, only the innermost pattern is ended immediately.
+
+If the C<(*ACCEPT)> is inside of capturing buffers then the buffers are
+marked as ended at the point at which the C<(*ACCEPT)> was encountered.
+For instance:
+
+ 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x;
+
+will match, and C<$1> will be C<AB> and C<$2> will be C<B>, C<$3> will not
+be set. If another branch in the inner parens were matched, such as in the
+string 'ACDE', then the C<D> and C<E> would have to be matched as well.
+
+=back
+
+=back
=head2 Backtracking
+X<backtrack> X<backtracking>
-A fundamental feature of regular expression matching involves the notion
-called I<backtracking>. which is used (when needed) by all regular
-expression quantifiers, namely C<*>, C<*?>, C<+>, C<+?>, C<{n,m}>, and
-C<{n,m}?>.
+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 L<Combining pieces together>.
+
+A fundamental feature of regular expression matching involves the
+notion called I<backtracking>, which is currently used (when needed)
+by all regular expression quantifiers, namely C<*>, C<*?>, C<+>,
+C<+?>, C<{n,m}>, and C<{n,m}?>. Backtracking is often optimized
+internally, but the general principle outlined here is valid.
For a regular expression to match, the I<entire> regular expression must
match, not just part of it. So if the beginning of a pattern containing a
got <d is under the bar in the >
That's because C<.*> was greedy, so you get everything between the
-I<first> "foo" and the I<last> "bar". In this case, it's more effective
+I<first> "foo" and the I<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.
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.
+of a string, and you also want to keep the preceding part of the match.
So you write this:
$_ = "I have 2 numbers: 53147";
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.
+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
+When using look-ahead assertions and negations, this can all get even
+trickier. Imagine you'd like to find a sequence of non-digits not
followed by "123". You might try to write that as
- $_ = "ABC123";
- if ( /^\D*(?!123)/ ) { # Wrong!
- print "Yup, no 123 in $_\n";
- }
+ $_ = "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:
+why that pattern matches, contrary to popular expectations:
- $x = 'ABC123' ;
- $y = 'ABC445' ;
+ $x = 'ABC123';
+ $y = 'ABC445';
- print "1: got $1\n" if $x =~ /^(ABC)(?!123)/ ;
- print "2: got $1\n" if $y =~ /^(ABC)(?!123)/ ;
+ 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)/ ;
+ print "3: got $1\n" if $x =~ /^(\D*)(?!123)/;
+ print "4: got $1\n" if $y =~ /^(\D*)(?!123)/;
This prints
non-digits, you have something that's not 123?" If the pattern matcher had
let C<\D*> expand to "ABC", this would have caused the whole pattern to
fail.
+
The search engine will initially match C<\D*> with "ABC". Then it will
-try to match C<(?!123> with "123" which, of course, fails. But because
+try to match C<(?!123> with "123", which fails. But because
a quantifier (C<\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.
-Well now,
-the pattern really, I<really> wants to succeed, so it uses the
-standard regexp back-off-and-retry and lets C<\D*> expand to just "AB" this
+The pattern really, I<really> wants to succeed, so it uses the
+standard pattern back-off-and-retry and lets C<\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.
+"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 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
+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)/ ;
+ print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/;
+ print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/;
6: got ABC
-In other words, the two zero-width assertions next to each other work like
-they're ANDed together, just as you'd use any builtin assertions: C</^$/>
+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: C</^$/>
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
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 C<*>'s instead of limiting it to 0 through 5 matches, then
-it would take literally forever--or until you ran out of stack space.
+B<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})*[c]/
+
+And if you used C<*>'s in the internal groups instead of limiting them
+to 0 through 5 matches, then it would take forever--or until you ran
+out of stack space. Moreover, these internal optimizations are not
+always applicable. For example, if you put C<{0,5}> instead of C<*>
+on the external group, no current optimization is applicable, and the
+match takes a long time to finish.
+
+A powerful tool for optimizing such beasts is what is known as an
+"independent group",
+which does not backtrack (see L<C<< (?>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 I<might> have influenced the
+following match, see L<C<< (?>pattern) >>>.
=head2 Version 8 Regular Expressions
+X<regular expression, version 8> X<regex, version 8> X<regexp, version 8>
-In case you're not familiar with the "regular" Version 8 regexp
+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 I<metacharacter>
with a special meaning described here or above. You can cause
-characters which normally function as metacharacters to be interpreted
+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 C<blurfl>
would match "blurfl" in the target string.
You can specify a character class, by enclosing a list of characters
-in C<[]>, which will match any one of the characters in the list. If the
+in C<[]>, 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 C<a-z> represents all the characters between "a" and "z",
-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
+in the list. Within a list, the "-" character specifies a
+range, so that C<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: C<[-az]>,
C<[az-]>, and C<[a\-z]>. All are different from C<[a-z]>, which
-specifies a class containing twenty-six characters.)
+specifies a class containing twenty-six characters, even on EBCDIC
+based coded character sets.) Also, if you try to use the character
+classes C<\w>, C<\W>, C<\s>, C<\S>, C<\d>, or C<\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, \I<nnn>, where I<nnn> is a string
-of octal digits, matches the character whose ASCII value is I<nnn>.
-Similarly, \xI<nn>, where I<nn> are hexadecimal digits, matches the
-character whose ASCII value is I<nn>. The expression \cI<x> matches the
-ASCII character control-I<x>. Finally, the "." metacharacter matches any
-character except "\n" (unless you use C</s>).
+of octal digits, matches the character whose coded character set value
+is I<nnn>. Similarly, \xI<nn>, where I<nn> are hexadecimal digits,
+matches the character whose numeric value is I<nn>. The expression \cI<x>
+matches the character control-I<x>. Finally, the "." metacharacter
+matches any character except "\n" (unless you use C</s>).
You can specify a series of alternatives for a pattern using "|" to
separate them, so that C<fee|fie|foe> will match any of "fee", "fie",
-or "foe" in the target string (as would C<f(e|i|o)e>). Note that the
+or "foe" in the target string (as would C<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. Note however that "|" is interpreted as a literal with
-square brackets, so if you write C<[fee|fie|foe]> you're really only
-matching C<[feio|]>.
-
-Within a pattern, you may designate subpatterns for later reference by
-enclosing them in parentheses, and you may refer back to the I<n>th
-subpattern later in the pattern using the metacharacter \I<n>.
-Subpatterns are numbered based on the left to right order of their
-opening parenthesis. Note that a backreference matches whatever
-actually matched the subpattern in the string being examined, not the
-rules for that subpattern. Therefore, C<(0|0x)\d*\s\1\d*> will
-match "0x1234 0x4321",but not "0x1234 01234", because subpattern 1
-actually matched "0x", even though the rule C<0|0x> could
-potentially match the leading 0 in the second number.
-
-=head2 WARNING on \1 vs $1
-
-Some people get too used to writing things like
+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 C<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 C<[fee|fie|foe]> you're really only matching C<[feio|]>.
+
+Within a pattern, you may designate subpatterns for later reference
+by enclosing them in parentheses, and you may refer back to the
+I<n>th subpattern later in the pattern using the metacharacter
+\I<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, C<(0|0x)\d*\s\1\d*> will
+match "0x1234 0x4321", but not "0x1234 01234", because subpattern
+1 matched "0x", even though the rule C<0|0x> could potentially match
+the leading 0 in the second number.
+
+=head2 Warning on \1 vs $1
+
+Some people get too used to writing things like:
$pattern =~ s/(\W)/\\\1/g;
This is grandfathered for the RHS of a substitute to avoid shocking the
B<sed> addicts, but it's a dirty habit to get into. That's because in
-PerlThink, the righthand side of a C<s///> is a double-quoted string. C<\1> in
+PerlThink, the righthand side of an C<s///> is a double-quoted string. C<\1> in
the usual double-quoted string means a control-A. The customary Unix
meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit
of doing that, you get yourself into trouble if you then add an C</e>
modifier.
- s/(\d+)/ \1 + 1 /eg;
+ s/(\d+)/ \1 + 1 /eg; # causes warning under -w
Or if you try to do
s/(\d+)/\1000/;
You can't disambiguate that by saying C<\{1}000>, whereas you can fix it with
-C<${1}000>. Basically, the operation of interpolation should not be confused
+C<${1}000>. The operation of interpolation should not be confused
with the operation of matching a backreference. Certainly they mean two
different things on the I<left> side of the C<s///>.
-=head2 SEE ALSO
+=head2 Repeated patterns matching zero-length substring
+
+B<WARNING>: Difficult material (and prose) ahead. This section needs a rewrite.
+
+Regular expressions provide a terse and powerful programming language. As
+with most other power tools, power comes together with the ability
+to wreak havoc.
+
+A common abuse of this power stems from the ability to make infinite
+loops using regular expressions, with something as innocuous as:
+
+ 'foo' =~ m{ ( o? )* }x;
+
+The C<o?> can match at the beginning of C<'foo'>, and since the position
+in the string is not moved by the match, C<o?> would match again and again
+because of the C<*> modifier. Another common way to create a similar cycle
+is with the looping modifier C<//g>:
+
+ @matches = ( 'foo' =~ m{ o? }xg );
+
+or
+
+ print "match: <$&>\n" while 'foo' =~ m{ o? }xg;
+
+or the loop implied by split().
+
+However, long experience has shown that many programming tasks may
+be significantly simplified by using repeated subexpressions that
+may match zero-length substrings. Here's a simple example being:
+
+ @chars = split //, $string; # // is not magic in split
+ ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// /
+
+Thus Perl allows such constructs, by I<forcefully breaking
+the infinite loop>. The rules for this are different for lower-level
+loops given by the greedy modifiers C<*+{}>, and for higher-level
+ones like the C</g> modifier or split() operator.
+
+The lower-level loops are I<interrupted> (that is, the loop is
+broken) when Perl detects that a repeated expression matched a
+zero-length substring. Thus
+
+ m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;
+
+is made equivalent to
+
+ m{ (?: NON_ZERO_LENGTH )*
+ |
+ (?: ZERO_LENGTH )?
+ }x;
+
+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 L<"Backtracking">),
+and so the I<second best> match is chosen if the I<best> match is of
+zero length.
+
+For example:
+
+ $_ = 'bar';
+ s/\w??/<$&>/g;
+
+results in C<< <><b><><a><><r><> >>. At each position of the string the best
+match given by non-greedy C<??> is the zero-length match, and the I<second
+best> match is what is matched by C<\w>. Thus zero-length matches
+alternate with one-character-long matches.
+
+Similarly, for repeated C<m/()/g> the second-best match is the match at the
+position one notch further in the string.
+
+The additional state of being I<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 C<split>.
+
+=head2 Combining pieces together
+
+Each of the elementary pieces of regular expressions which were described
+before (such as C<ab> or C<\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 C<ST>, C<S|T>, C<S*> etc
+(in these examples C<S> and C<T> are regular subexpressions).
+
+Such combinations can include alternatives, leading to a problem of choice:
+if we match a regular expression C<a|ab> against C<"abc">, will it match
+substring C<"a"> or C<"ab">? One way to describe which substring is
+actually matched is the concept of backtracking (see L<"Backtracking">).
+However, this description is too low-level and makes you think
+in terms of a particular implementation.
+
+Another description starts with notions of "better"/"worse". All the
+substrings which may be matched by the given regular expression can be
+sorted from the "best" match to the "worst" match, and it is the "best"
+match which is chosen. This substitutes the question of "what is chosen?"
+by the question of "which matches are better, and which are worse?".
+
+Again, for elementary pieces there is no such question, since at most
+one match at a given position is possible. This section describes the
+notion of better/worse for combining operators. In the description
+below C<S> and C<T> are regular subexpressions.
+
+=over 4
+
+=item C<ST>
+
+Consider two possible matches, C<AB> and C<A'B'>, C<A> and C<A'> are
+substrings which can be matched by C<S>, C<B> and C<B'> are substrings
+which can be matched by C<T>.
+
+If C<A> is better match for C<S> than C<A'>, C<AB> is a better
+match than C<A'B'>.
+
+If C<A> and C<A'> coincide: C<AB> is a better match than C<AB'> if
+C<B> is better match for C<T> than C<B'>.
+
+=item C<S|T>
+
+When C<S> can match, it is a better match than when only C<T> can match.
+
+Ordering of two matches for C<S> is the same as for C<S>. Similar for
+two matches for C<T>.
+
+=item C<S{REPEAT_COUNT}>
+
+Matches as C<SSS...S> (repeated as many times as necessary).
+
+=item C<S{min,max}>
+
+Matches as C<S{max}|S{max-1}|...|S{min+1}|S{min}>.
+
+=item C<S{min,max}?>
+
+Matches as C<S{min}|S{min+1}|...|S{max-1}|S{max}>.
+
+=item C<S?>, C<S*>, C<S+>
+
+Same as C<S{0,1}>, C<S{0,BIG_NUMBER}>, C<S{1,BIG_NUMBER}> respectively.
+
+=item C<S??>, C<S*?>, C<S+?>
+
+Same as C<S{0,1}?>, C<S{0,BIG_NUMBER}?>, C<S{1,BIG_NUMBER}?> respectively.
+
+=item C<< (?>S) >>
+
+Matches the best match for C<S> and only that.
+
+=item C<(?=S)>, C<(?<=S)>
+
+Only the best match for C<S> is considered. (This is important only if
+C<S> has capturing parentheses, and backreferences are used somewhere
+else in the whole regular expression.)
+
+=item C<(?!S)>, C<(?<!S)>
+
+For this grouping operator there is no need to describe the ordering, since
+only whether or not C<S> can match is important.
+
+=item C<(??{ EXPR })>, C<(?PARNO)>
+
+The ordering is the same as for the regular expression which is
+the result of EXPR, or the pattern contained by capture buffer PARNO.
+
+=item C<(?(condition)yes-pattern|no-pattern)>
+
+Recall that which of C<yes-pattern> or C<no-pattern> actually matches is
+already determined. The ordering of the matches is the same as for the
+chosen subexpression.
+
+=back
+
+The above recipes describe the ordering of matches I<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.
+
+=head2 Creating custom RE engines
+
+Overloaded constants (see L<overload>) provide a simple way to extend
+the functionality of the RE engine.
+
+Suppose that we want to enable a new RE escape-sequence C<\Y|> which
+matches at boundary between whitespace characters and non-whitespace
+characters. Note that C<(?=\S)(?<!\S)|(?!\S)(?<=\S)> matches exactly
+at these positions, so we want to have each C<\Y|> in the place of the
+more complicated version. We can create a module C<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]'"}
+
+ # We must also take care of not escaping the legitimate \\Y|
+ # sequence, hence the presence of '\\' in the conversion rules.
+ my %rules = ( '\\' => '\\\\',
+ 'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ );
+ sub convert {
+ my $re = shift;
+ $re =~ s{
+ \\ ( \\ | Y . )
+ }
+ { $rules{$1} or invalid($re,$1) }sgex;
+ return $re;
+ }
+
+Now C<use customre> enables the new escape in constant regular
+expressions, i.e., those without any runtime variable interpolations.
+As documented in L<overload>, this conversion will work only over
+literal parts of regular expressions. For C<\Y|$re\Y|> the variable
+part of this regular expression needs to be converted explicitly
+(but only if the special meaning of C<\Y|> should be enabled inside $re):
+
+ use customre;
+ $re = <>;
+ chomp $re;
+ $re = customre::convert $re;
+ /\Y|$re\Y|/;
+
+=head1 BUGS
+
+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.
+
+=head1 SEE ALSO
+
+L<perlrequick>.
+
+L<perlretut>.
+
+L<perlop/"Regexp Quote-Like Operators">.
+
+L<perlop/"Gory details of parsing quoted constructs">.
+
+L<perlfaq6>.
+
+L<perlfunc/pos>.
+
+L<perllocale>.
+
+L<perlebcdic>.
-"Mastering Regular Expressions" (see L<perlbook>) by Jeffrey Friedl.
+I<Mastering Regular Expressions> by Jeffrey Friedl, published
+by O'Reilly and Associates.
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