930 lines
36 KiB
Plaintext
930 lines
36 KiB
Plaintext
=head1 NAME
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perlre - Perl regular expressions
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=head1 DESCRIPTION
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This page describes the syntax of regular expressions in Perl. For a
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description of how to I<use> regular expressions in matching
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operations, plus various examples of the same, see discussion
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of C<m//>, C<s///>, C<qr//> and C<??> in L<perlop/"Regexp Quote-Like Operators">.
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The matching operations can have various modifiers. The modifiers
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that relate to the interpretation of the regular expression inside
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are listed below. For the modifiers that alter the way a regular expression
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is used by Perl, see L<perlop/"Regexp Quote-Like Operators"> and
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L<perlop/"Gory details of parsing quoted constructs">.
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=over 4
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=item i
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Do case-insensitive pattern matching.
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If C<use locale> is in effect, the case map is taken from the current
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locale. See L<perllocale>.
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=item m
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Treat string as multiple lines. That is, change "^" and "$" from matching
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at only the very start or end of the string to the start or end of any
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line anywhere within the string,
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=item s
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Treat string as single line. That is, change "." to match any character
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whatsoever, even a newline, which it normally would not match.
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The C</s> and C</m> modifiers both override the C<$*> setting. That is, no matter
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what C<$*> contains, C</s> without C</m> will force "^" to match only at the
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beginning of the string and "$" to match only at the end (or just before a
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newline at the end) of the string. Together, as /ms, they let the "." match
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any character whatsoever, while yet allowing "^" and "$" to match,
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respectively, just after and just before newlines within the string.
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=item x
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Extend your pattern's legibility by permitting whitespace and comments.
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=back
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These are usually written as "the C</x> modifier", even though the delimiter
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in question might not actually be a slash. In fact, any of these
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modifiers may also be embedded within the regular expression itself using
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the new C<(?...)> construct. See below.
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The C</x> modifier itself needs a little more explanation. It tells
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the regular expression parser to ignore whitespace that is neither
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backslashed nor within a character class. You can use this to break up
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your regular expression into (slightly) more readable parts. The C<#>
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character is also treated as a metacharacter introducing a comment,
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just as in ordinary Perl code. This also means that if you want real
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whitespace or C<#> characters in the pattern (outside of a character
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class, where they are unaffected by C</x>), that you'll either have to
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escape them or encode them using octal or hex escapes. Taken together,
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these features go a long way towards making Perl's regular expressions
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more readable. Note that you have to be careful not to include the
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pattern delimiter in the comment--perl has no way of knowing you did
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not intend to close the pattern early. See the C-comment deletion code
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in L<perlop>.
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=head2 Regular Expressions
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The patterns used in pattern matching are regular expressions such as
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those supplied in the Version 8 regex routines. (In fact, the
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routines are derived (distantly) from Henry Spencer's freely
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redistributable reimplementation of the V8 routines.)
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See L<Version 8 Regular Expressions> for details.
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In particular the following metacharacters have their standard I<egrep>-ish
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meanings:
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\ Quote the next metacharacter
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^ Match the beginning of the line
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. Match any character (except newline)
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$ Match the end of the line (or before newline at the end)
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| Alternation
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() Grouping
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[] Character class
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By default, the "^" character is guaranteed to match at only the
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beginning of the string, the "$" character at only the end (or before the
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newline at the end) and Perl does certain optimizations with the
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assumption that the string contains only one line. Embedded newlines
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will not be matched by "^" or "$". You may, however, wish to treat a
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string as a multi-line buffer, such that the "^" will match after any
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newline within the string, and "$" will match before any newline. At the
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cost of a little more overhead, you can do this by using the /m modifier
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on the pattern match operator. (Older programs did this by setting C<$*>,
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but this practice is now deprecated.)
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To facilitate multi-line substitutions, the "." character never matches a
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newline unless you use the C</s> modifier, which in effect tells Perl to pretend
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the string is a single line--even if it isn't. The C</s> modifier also
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overrides the setting of C<$*>, in case you have some (badly behaved) older
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code that sets it in another module.
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The following standard quantifiers are recognized:
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* Match 0 or more times
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+ Match 1 or more times
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? Match 1 or 0 times
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{n} Match exactly n times
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{n,} Match at least n times
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{n,m} Match at least n but not more than m times
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(If a curly bracket occurs in any other context, it is treated
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as a regular character.) The "*" modifier is equivalent to C<{0,}>, the "+"
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modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. n and m are limited
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to integral values less than 65536.
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By default, a quantified subpattern is "greedy", that is, it will match as
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many times as possible (given a particular starting location) while still
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allowing the rest of the pattern to match. If you want it to match the
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minimum number of times possible, follow the quantifier with a "?". Note
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that the meanings don't change, just the "greediness":
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*? Match 0 or more times
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+? Match 1 or more times
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?? Match 0 or 1 time
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{n}? Match exactly n times
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{n,}? Match at least n times
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{n,m}? Match at least n but not more than m times
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Because patterns are processed as double quoted strings, the following
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also work:
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\t tab (HT, TAB)
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\n newline (LF, NL)
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\r return (CR)
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\f form feed (FF)
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\a alarm (bell) (BEL)
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\e escape (think troff) (ESC)
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\033 octal char (think of a PDP-11)
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\x1B hex char
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\c[ control char
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\l lowercase next char (think vi)
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\u uppercase next char (think vi)
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\L lowercase till \E (think vi)
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\U uppercase till \E (think vi)
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\E end case modification (think vi)
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\Q quote (disable) pattern metacharacters till \E
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If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
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and C<\U> is taken from the current locale. See L<perllocale>.
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You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
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An unescaped C<$> or C<@> interpolates the corresponding variable,
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while escaping will cause the literal string C<\$> to be matched.
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You'll need to write something like C<m/\Quser\E\@\Qhost/>.
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In addition, Perl defines the following:
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\w Match a "word" character (alphanumeric plus "_")
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\W Match a non-word character
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\s Match a whitespace character
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\S Match a non-whitespace character
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\d Match a digit character
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\D Match a non-digit character
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A C<\w> matches a single alphanumeric character, not a whole
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word. To match a word you'd need to say C<\w+>. If C<use locale> is in
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effect, the list of alphabetic characters generated by C<\w> is taken
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from the current locale. See L<perllocale>. You may use C<\w>, C<\W>,
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C<\s>, C<\S>, C<\d>, and C<\D> within character classes (though not as
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either end of a range).
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Perl defines the following zero-width assertions:
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\b Match a word boundary
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\B Match a non-(word boundary)
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\A Match only at beginning of string
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\Z Match only at end of string, or before newline at the end
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\z Match only at end of string
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\G Match only where previous m//g left off (works only with /g)
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A word boundary (C<\b>) is defined as a spot between two characters that
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has a C<\w> on one side of it and a C<\W> on the other side of it (in
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either order), counting the imaginary characters off the beginning and
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end of the string as matching a C<\W>. (Within character classes C<\b>
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represents backspace rather than a word boundary.) The C<\A> and C<\Z> are
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just like "^" and "$", except that they won't match multiple times when the
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C</m> modifier is used, while "^" and "$" will match at every internal line
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boundary. To match the actual end of the string, not ignoring newline,
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you can use C<\z>. The C<\G> assertion can be used to chain global
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matches (using C<m//g>), as described in
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L<perlop/"Regexp Quote-Like Operators">.
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It is also useful when writing C<lex>-like scanners, when you have several
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patterns that you want to match against consequent substrings of your
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string, see the previous reference.
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The actual location where C<\G> will match can also be influenced
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by using C<pos()> as an lvalue. See L<perlfunc/pos>.
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When the bracketing construct C<( ... )> is used, \E<lt>digitE<gt> matches the
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digit'th substring. Outside of the pattern, always use "$" instead of "\"
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in front of the digit. (While the \E<lt>digitE<gt> notation can on rare occasion work
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outside the current pattern, this should not be relied upon. See the
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WARNING below.) The scope of $E<lt>digitE<gt> (and C<$`>, C<$&>, and C<$'>)
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extends to the end of the enclosing BLOCK or eval string, or to the next
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successful pattern match, whichever comes first. If you want to use
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parentheses to delimit a subpattern (e.g., a set of alternatives) without
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saving it as a subpattern, follow the ( with a ?:.
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You may have as many parentheses as you wish. If you have more
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than 9 substrings, the variables $10, $11, ... refer to the
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corresponding substring. Within the pattern, \10, \11, etc. refer back
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to substrings if there have been at least that many left parentheses before
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the backreference. Otherwise (for backward compatibility) \10 is the
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same as \010, a backspace, and \11 the same as \011, a tab. And so
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on. (\1 through \9 are always backreferences.)
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C<$+> returns whatever the last bracket match matched. C<$&> returns the
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entire matched string. (C<$0> used to return the same thing, but not any
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more.) C<$`> returns everything before the matched string. C<$'> returns
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everything after the matched string. Examples:
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s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
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if (/Time: (..):(..):(..)/) {
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$hours = $1;
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$minutes = $2;
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$seconds = $3;
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}
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Once perl sees that you need one of C<$&>, C<$`> or C<$'> anywhere in
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the program, it has to provide them on each and every pattern match.
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This can slow your program down. The same mechanism that handles
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these provides for the use of $1, $2, etc., so you pay the same price
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for each pattern that contains capturing parentheses. But if you never
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use $&, etc., in your script, then patterns I<without> capturing
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parentheses won't be penalized. So avoid $&, $', and $` if you can,
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but if you can't (and some algorithms really appreciate them), once
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you've used them once, use them at will, because you've already paid
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the price. As of 5.005, $& is not so costly as the other two.
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Backslashed metacharacters in Perl are
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alphanumeric, such as C<\b>, C<\w>, C<\n>. Unlike some other regular
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expression languages, there are no backslashed symbols that aren't
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alphanumeric. So anything that looks like \\, \(, \), \E<lt>, \E<gt>,
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\{, or \} is always interpreted as a literal character, not a
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metacharacter. This was once used in a common idiom to disable or
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quote the special meanings of regular expression metacharacters in a
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string that you want to use for a pattern. Simply quote all
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non-alphanumeric characters:
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$pattern =~ s/(\W)/\\$1/g;
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Now it is much more common to see either the quotemeta() function or
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the C<\Q> escape sequence used to disable all metacharacters' special
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meanings like this:
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/$unquoted\Q$quoted\E$unquoted/
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Perl defines a consistent extension syntax for regular expressions.
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The syntax is a pair of parentheses with a question mark as the first
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thing within the parentheses (this was a syntax error in older
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versions of Perl). The character after the question mark gives the
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function of the extension. Several extensions are already supported:
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=over 10
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=item C<(?#text)>
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A comment. The text is ignored. If the C</x> switch is used to enable
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whitespace formatting, a simple C<#> will suffice. Note that perl closes
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the comment as soon as it sees a C<)>, so there is no way to put a literal
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C<)> in the comment.
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=item C<(?:pattern)>
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=item C<(?imsx-imsx:pattern)>
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This is for clustering, not capturing; it groups subexpressions like
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"()", but doesn't make backreferences as "()" does. So
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@fields = split(/\b(?:a|b|c)\b/)
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is like
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@fields = split(/\b(a|b|c)\b/)
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but doesn't spit out extra fields.
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The letters between C<?> and C<:> act as flags modifiers, see
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L<C<(?imsx-imsx)>>. In particular,
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/(?s-i:more.*than).*million/i
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is equivalent to more verbose
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/(?:(?s-i)more.*than).*million/i
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=item C<(?=pattern)>
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A zero-width positive lookahead assertion. For example, C</\w+(?=\t)/>
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matches a word followed by a tab, without including the tab in C<$&>.
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=item C<(?!pattern)>
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A zero-width negative lookahead assertion. For example C</foo(?!bar)/>
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matches any occurrence of "foo" that isn't followed by "bar". Note
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however that lookahead and lookbehind are NOT the same thing. You cannot
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use this for lookbehind.
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If you are looking for a "bar" that isn't preceded by a "foo", C</(?!foo)bar/>
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will not do what you want. That's because the C<(?!foo)> is just saying that
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the next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will
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match. You would have to do something like C</(?!foo)...bar/> for that. We
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say "like" because there's the case of your "bar" not having three characters
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before it. You could cover that this way: C</(?:(?!foo)...|^.{0,2})bar/>.
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Sometimes it's still easier just to say:
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if (/bar/ && $` !~ /foo$/)
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For lookbehind see below.
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=item C<(?E<lt>=pattern)>
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A zero-width positive lookbehind assertion. For example, C</(?E<lt>=\t)\w+/>
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matches a word following a tab, without including the tab in C<$&>.
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Works only for fixed-width lookbehind.
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=item C<(?<!pattern)>
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A zero-width negative lookbehind assertion. For example C</(?<!bar)foo/>
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matches any occurrence of "foo" that isn't following "bar".
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Works only for fixed-width lookbehind.
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=item C<(?{ code })>
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Experimental "evaluate any Perl code" zero-width assertion. Always
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succeeds. C<code> is not interpolated. Currently the rules to
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determine where the C<code> ends are somewhat convoluted.
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The C<code> is properly scoped in the following sense: if the assertion
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is backtracked (compare L<"Backtracking">), all the changes introduced after
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C<local>isation are undone, so
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$_ = 'a' x 8;
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m<
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(?{ $cnt = 0 }) # Initialize $cnt.
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(
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a
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(?{
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local $cnt = $cnt + 1; # Update $cnt, backtracking-safe.
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})
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)*
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aaaa
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(?{ $res = $cnt }) # On success copy to non-localized
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# location.
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>x;
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will set C<$res = 4>. Note that after the match $cnt returns to the globally
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introduced value 0, since the scopes which restrict C<local> statements
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are unwound.
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This assertion may be used as L<C<(?(condition)yes-pattern|no-pattern)>>
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switch. If I<not> used in this way, the result of evaluation of C<code>
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is put into variable $^R. This happens immediately, so $^R can be used from
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other C<(?{ code })> assertions inside the same regular expression.
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The above assignment to $^R is properly localized, thus the old value of $^R
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is restored if the assertion is backtracked (compare L<"Backtracking">).
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Due to security concerns, this construction is not allowed if the regular
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expression involves run-time interpolation of variables, unless
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C<use re 'eval'> pragma is used (see L<re>), or the variables contain
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results of qr() operator (see L<perlop/"qr/STRING/imosx">).
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This restriction is due to the wide-spread (questionable) practice of
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using the construct
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$re = <>;
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chomp $re;
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$string =~ /$re/;
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without tainting. While this code is frowned upon from security point
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of view, when C<(?{})> was introduced, it was considered bad to add
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I<new> security holes to existing scripts.
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B<NOTE:> Use of the above insecure snippet without also enabling taint mode
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is to be severely frowned upon. C<use re 'eval'> does not disable tainting
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checks, thus to allow $re in the above snippet to contain C<(?{})>
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I<with tainting enabled>, one needs both C<use re 'eval'> and untaint
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the $re.
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=item C<(?E<gt>pattern)>
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An "independent" subexpression. Matches the substring that a
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I<standalone> C<pattern> would match if anchored at the given position,
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B<and only this substring>.
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Say, C<^(?E<gt>a*)ab> will never match, since C<(?E<gt>a*)> (anchored
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at the beginning of string, as above) will match I<all> characters
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C<a> at the beginning of string, leaving no C<a> for C<ab> to match.
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In contrast, C<a*ab> will match the same as C<a+b>, since the match of
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the subgroup C<a*> is influenced by the following group C<ab> (see
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L<"Backtracking">). In particular, C<a*> inside C<a*ab> will match
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fewer characters than a standalone C<a*>, since this makes the tail match.
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An effect similar to C<(?E<gt>pattern)> may be achieved by
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(?=(pattern))\1
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since the lookahead is in I<"logical"> context, thus matches the same
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substring as a standalone C<a+>. The following C<\1> eats the matched
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string, thus making a zero-length assertion into an analogue of
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C<(?E<gt>...)>. (The difference between these two constructs is that the
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second one uses a catching group, thus shifting ordinals of
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backreferences in the rest of a regular expression.)
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This construct is useful for optimizations of "eternal"
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matches, because it will not backtrack (see L<"Backtracking">).
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m{ \(
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(
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[^()]+
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\( [^()]* \)
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)+
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\)
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}x
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That will efficiently match a nonempty group with matching
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two-or-less-level-deep parentheses. However, if there is no such group,
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it will take virtually forever on a long string. That's because there are
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so many different ways to split a long string into several substrings.
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This is what C<(.+)+> is doing, and C<(.+)+> is similar to a subpattern
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of the above pattern. Consider that the above pattern detects no-match
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on C<((()aaaaaaaaaaaaaaaaaa> in several seconds, but that each extra
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letter doubles this time. This exponential performance will make it
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appear that your program has hung.
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However, a tiny modification of this pattern
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m{ \(
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(
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(?> [^()]+ )
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\( [^()]* \)
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)+
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\)
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}x
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which uses C<(?E<gt>...)> matches exactly when the one above does (verifying
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this yourself would be a productive exercise), but finishes in a fourth
|
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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
|
|
B<-w> saying it C<"matches the null string many times">):
|
|
|
|
On simple groups, such as the pattern C<(?> [^()]+ )>, a comparable
|
|
effect may be achieved by negative lookahead, as in C<[^()]+ (?! [^()] )>.
|
|
This was only 4 times slower on a string with 1000000 C<a>s.
|
|
|
|
=item C<(?(condition)yes-pattern|no-pattern)>
|
|
|
|
=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), or lookahead/lookbehind/evaluate zero-width assertion.
|
|
|
|
Say,
|
|
|
|
m{ ( \( )?
|
|
[^()]+
|
|
(?(1) \) )
|
|
}x
|
|
|
|
matches a chunk of non-parentheses, possibly included in parentheses
|
|
themselves.
|
|
|
|
=item C<(?imsx-imsx)>
|
|
|
|
One or more embedded pattern-match modifiers. This is particularly
|
|
useful for patterns that are specified in a table somewhere, some of
|
|
which want to be case sensitive, and some of which don't. The case
|
|
insensitive ones need to include merely C<(?i)> at the front of the
|
|
pattern. For example:
|
|
|
|
$pattern = "foobar";
|
|
if ( /$pattern/i ) { }
|
|
|
|
# more flexible:
|
|
|
|
$pattern = "(?i)foobar";
|
|
if ( /$pattern/ ) { }
|
|
|
|
Letters after C<-> switch modifiers off.
|
|
|
|
These modifiers are localized inside an enclosing group (if any). Say,
|
|
|
|
( (?i) blah ) \s+ \1
|
|
|
|
(assuming C<x> modifier, and no C<i> modifier outside of this group)
|
|
will match a repeated (I<including the case>!) word C<blah> in any
|
|
case.
|
|
|
|
=back
|
|
|
|
A question mark was chosen for this and for the new minimal-matching
|
|
construct because 1) question mark is pretty rare in older regular
|
|
expressions, and 2) whenever you see one, you should stop and "question"
|
|
exactly what is going on. That's psychology...
|
|
|
|
=head2 Backtracking
|
|
|
|
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}?>.
|
|
|
|
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
|
|
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 (C<\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 C<.*> was greedy, so you get everything between the
|
|
I<first> "foo" and the I<last> "bar". In this case, it's more effective
|
|
to use minimal matching to make sure you get the text between a "foo"
|
|
and the first "bar" thereafter.
|
|
|
|
if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
|
|
got <d is under the >
|
|
|
|
Here's another example: let's say you'd like to match a number at the end
|
|
of a string, and you also want to keep the preceding part the match.
|
|
So you write this:
|
|
|
|
$_ = "I have 2 numbers: 53147";
|
|
if ( /(.*)(\d*)/ ) { # Wrong!
|
|
print "Beginning is <$1>, number is <$2>.\n";
|
|
}
|
|
|
|
That won't work at all, because C<.*> was greedy and gobbled up the
|
|
whole string. As C<\d*> can match on an empty string the complete
|
|
regular expression matched successfully.
|
|
|
|
Beginning is <I have 2 numbers: 53147>, number is <>.
|
|
|
|
Here are some variants, most of which don't work:
|
|
|
|
$_ = "I have 2 numbers: 53147";
|
|
@pats = qw{
|
|
(.*)(\d*)
|
|
(.*)(\d+)
|
|
(.*?)(\d*)
|
|
(.*?)(\d+)
|
|
(.*)(\d+)$
|
|
(.*?)(\d+)$
|
|
(.*)\b(\d+)$
|
|
(.*\D)(\d+)$
|
|
};
|
|
|
|
for $pat (@pats) {
|
|
printf "%-12s ", $pat;
|
|
if ( /$pat/ ) {
|
|
print "<$1> <$2>\n";
|
|
} else {
|
|
print "FAIL\n";
|
|
}
|
|
}
|
|
|
|
That will print out:
|
|
|
|
(.*)(\d*) <I have 2 numbers: 53147> <>
|
|
(.*)(\d+) <I have 2 numbers: 5314> <7>
|
|
(.*?)(\d*) <> <>
|
|
(.*?)(\d+) <I have > <2>
|
|
(.*)(\d+)$ <I have 2 numbers: 5314> <7>
|
|
(.*?)(\d+)$ <I have 2 numbers: > <53147>
|
|
(.*)\b(\d+)$ <I have 2 numbers: > <53147>
|
|
(.*\D)(\d+)$ <I have 2 numbers: > <53147>
|
|
|
|
As you see, this can be a bit tricky. It's important to realize that a
|
|
regular expression is merely a set of assertions that gives a definition
|
|
of success. There may be 0, 1, or several different ways that the
|
|
definition might succeed against a particular string. And if there are
|
|
multiple ways it might succeed, you need to understand backtracking to
|
|
know which variety of success you will achieve.
|
|
|
|
When using lookahead assertions and negations, this can all get even
|
|
tricker. Imagine you'd like to find a sequence of non-digits not
|
|
followed by "123". You might try to write that as
|
|
|
|
$_ = "ABC123";
|
|
if ( /^\D*(?!123)/ ) { # Wrong!
|
|
print "Yup, no 123 in $_\n";
|
|
}
|
|
|
|
But that isn't going to match; at least, not the way you're hoping. It
|
|
claims that there is no 123 in the string. Here's a clearer picture of
|
|
why it that pattern 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 (C<\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 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
|
|
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.
|
|
|
|
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.
|
|
|
|
We can deal with this by using both an assertion and a negation. We'll
|
|
say that the first part in $1 must be followed by a digit, and in fact, it
|
|
must also be followed by something that's not "123". Remember that the
|
|
lookaheads are zero-width expressions--they only look, but don't consume
|
|
any of the string in their match. So rewriting this way produces what
|
|
you'd expect; that is, case 5 will fail, but case 6 succeeds:
|
|
|
|
print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ;
|
|
print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/ ;
|
|
|
|
6: got ABC
|
|
|
|
In other words, the two zero-width assertions next to each other work as though
|
|
they're ANDed together, just as you'd use any builtin assertions: C</^$/>
|
|
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
|
|
using the vertical bar. C</ab/> means match "a" AND (then) match "b",
|
|
although the attempted matches are made at different positions because "a"
|
|
is not a zero-width assertion, but a one-width assertion.
|
|
|
|
One warning: particularly complicated regular expressions can take
|
|
exponential time to solve due to the immense number of possible ways they
|
|
can use backtracking to try match. For example this will take a very long
|
|
time to run
|
|
|
|
/((a{0,5}){0,5}){0,5}/
|
|
|
|
And if you used 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.
|
|
|
|
A powerful tool for optimizing such beasts is "independent" groups,
|
|
which do not backtrace (see L<C<(?E<gt>pattern)>>). Note also that
|
|
zero-length lookahead/lookbehind assertions will not backtrace to make
|
|
the tail match, since they are in "logical" context: only the fact
|
|
whether they match or not is considered relevant. For an example
|
|
where side-effects of a lookahead I<might> have influenced the
|
|
following match, see L<C<(?E<gt>pattern)>>.
|
|
|
|
=head2 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 I<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 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 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 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
|
|
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.)
|
|
|
|
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>).
|
|
|
|
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>). The
|
|
first alternative includes everything from the last pattern delimiter
|
|
("(", "[", or the beginning of the pattern) up to the first "|", and
|
|
the last alternative contains everything from the last "|" to the next
|
|
pattern delimiter. For this reason, it's common practice to include
|
|
alternatives in parentheses, to minimize confusion about where they
|
|
start and end.
|
|
|
|
Alternatives are tried from left to right, so the first
|
|
alternative found for which the entire expression matches, is the one that
|
|
is chosen. This means that alternatives are not necessarily greedy. For
|
|
example: when mathing 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
|
|
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:
|
|
|
|
$pattern =~ s/(\W)/\\\1/g;
|
|
|
|
This is grandfathered for the RHS of a substitute to avoid shocking the
|
|
B<sed> addicts, but it's a dirty habit to get into. That's because in
|
|
PerlThink, the righthand side of a C<s///> is a double-quoted string. C<\1> in
|
|
the usual double-quoted string means a control-A. The customary Unix
|
|
meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit
|
|
of doing that, you get yourself into trouble if you then add an C</e>
|
|
modifier.
|
|
|
|
s/(\d+)/ \1 + 1 /eg; # causes warning under -w
|
|
|
|
Or if you try to do
|
|
|
|
s/(\d+)/\1000/;
|
|
|
|
You can't disambiguate that by saying C<\{1}000>, whereas you can fix it with
|
|
C<${1}000>. Basically, the operation of interpolation should not be confused
|
|
with the operation of matching a backreference. Certainly they mean two
|
|
different things on the I<left> side of the C<s///>.
|
|
|
|
=head2 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 innocous 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
|
|
due to 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 which
|
|
may match zero-length substrings, with a simple example being:
|
|
|
|
@chars = split //, $string; # // is not magic in split
|
|
($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// /
|
|
|
|
Thus Perl allows the C</()/> construct, which I<forcefully breaks
|
|
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> when it is detected that a
|
|
repeated expression did match a zero-length substring, thus
|
|
|
|
m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;
|
|
|
|
is made equivalent to
|
|
|
|
m{ (?: NON_ZERO_LENGTH )*
|
|
|
|
|
(?: ZERO_LENGTH )?
|
|
}x;
|
|
|
|
The higher level-loops preserve an additional state between iterations:
|
|
whether the last match was zero-length. To break the loop, the following
|
|
match after a zero-length match is prohibited to have a length of zero.
|
|
This prohibition interacts with backtracking (see L<"Backtracking">),
|
|
and so the I<second best> match is chosen if the I<best> match is of
|
|
zero length.
|
|
|
|
Say,
|
|
|
|
$_ = 'bar';
|
|
s/\w??/<$&>/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 to
|
|
the matched string, and is reset by each assignment to pos().
|
|
|
|
=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 white-space 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]'"}
|
|
|
|
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|/;
|
|
|
|
=head2 SEE ALSO
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L<perlop/"Regexp Quote-Like Operators">.
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L<perlop/"Gory details of parsing quoted constructs">.
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L<perlfunc/pos>.
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L<perllocale>.
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I<Mastering Regular Expressions> (see L<perlbook>) by Jeffrey Friedl.
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