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 "$". You may, however, wish to treat a
-string as a multiline buffer, such that the "^" will match after any
+string as a multi-line buffer, such that the "^" will match after any
newline within the string, and "$" will match before any newline. At the
cost of a little more overhead, you can do this by using the /m modifier
on the pattern match operator. (Older programs did this by setting C<$*>,
but this practice is now deprecated.)
-To facilitate multiline substitutions, the "." character never matches a
+To facilitate multi-line substitutions, the "." character never matches a
newline unless you use the C</s> modifier, which in effect tells Perl to pretend
the string is a single line--even if it isn't. The C</s> modifier also
overrides the setting of C<$*>, in case you have some (badly behaved) older
\L lowercase till \E (think vi)
\U uppercase till \E (think vi)
\E end case modification (think vi)
- \Q quote regexp metacharacters till \E
+ \Q quote (disable) regexp 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>.
\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
+ \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
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 mix global
-matches (using C<m//g>) and non-global ones, as described in
+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 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 makes it
-simple to quote a string that you want to use for a pattern but that
-you are afraid might contain metacharacters. Quote simply all the
+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:
$pattern =~ s/(\W)/\\$1/g;
-You can also use the builtin quotemeta() function to do this.
-An even easier way to quote metacharacters right in the match operator
-is to say
+Now it is much more common to see either the quotemeta() function or
+the \Q escape sequence used to disable the metacharacters special
+meanings like this:
/$unquoted\Q$quoted\E$unquoted/
if (/foo/ && $` =~ /bar$/)
+For lookbehind see below.
+
+=item (?<=regexp)
+
+A zero-width positive lookbehind assertion. For example, C</(?=\t)\w+/>
+matches a word following a tab, without including the tab in C<$&>.
+Works only for fixed-width lookbehind.
+
+=item (?<!regexp)
+
+A zero-width negative lookbehind assertion. For example C</(?<!bar)foo/>
+matches any occurrence of "foo" that isn't following "bar".
+Works only for fixed-width lookbehind.
+
+=item (?{ code })
+
+Experimental "evaluate any Perl code" zero-width assertion. Always
+succeeds. Currently the quoting rules are somewhat convoluted, as is the
+determination where the C<code> ends.
+
+
+=item C<(?E<gt>regexp)>
+
+An "independend" subexpression. Matches the substring which a
+I<standalone> C<regexp> would match if anchored at the given position,
+B<and only this substring>.
+
+Say, C<^(?E<gt>a*)ab> will never match, since C<(?E<gt>a*)> (anchored
+at the beginning of string, as above) will match I<all> the 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
+less characters that a standalone C<a*>, since this makes the tail match.
+
+Note that a similar effect to C<(?E<gt>regexp)> may be achieved by
+
+ (?=(regexp))\1
+
+since the lookahead is in I<"logical"> context, thus matches the same
+substring as a standalone C<a+>. The following C<\1> eats the matched
+string, thus making a zero-length assertion into an analogue of
+C<(?>...)>. (The difference of these two constructions is that the
+second one uses a catching group, thus shifts ordinals of
+backreferences in the rest of a regular expression.)
+
+This construction is very useful for optimizations of "eternal"
+matches, since it will not backtrack (see L<"Backtracking">). Say,
+
+ / \( (
+ [^()]+
+ |
+ \( [^()]* \)
+ )+
+ \) /x
+
+will match a nonempty group with matching two-or-less-level-deep
+parentheses. It is very efficient in finding such groups. However,
+if there is no such group, it is going to take forever (on reasonably
+long string), since there are so many different ways to split a long
+string into several substrings (this is essentially what C<(.+)+> is
+doing, and this is a subpattern of the above pattern). Say, on
+C<((()aaaaaaaaaaaaaaaaaa> the above pattern detects no-match in 5sec
+(on kitchentop'96 processor), and each extra letter doubles this time.
+
+However, a tiny modification of this
+
+ / \( (
+ (?> [^()]+ )
+ |
+ \( [^()]* \)
+ )+
+ \) /x
+
+which uses (?>...) matches exactly when the above one does (it is a
+good excercise to check this), but finishes in a fourth of the above
+time on a similar string with 1000000 C<a>s.
+
+Note that on simple groups like the above C<(?> [^()]+ )> a similar
+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 (?(condition)yes-regexp|no-regexp)
+
+=item (?(condition)yes-regexp)
+
+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,
+
+ / ( \( )?
+ [^()]+
+ (?(1) \) )/x
+
+matches a chunk of non-parentheses, possibly included in parentheses
+themselves.
=item (?imsx)
$pattern = "(?i)foobar";
if ( /$pattern/ )
+Note that 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
The specific choice of question mark for this and the new minimal
=head2 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}?>.
+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
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>regexp)>>). 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>regexp)>>.
+
=head2 Version 8 Regular Expressions
In case you're not familiar with the "regular" Version 8 regexp
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.
+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,
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 SEE ALSO
+
+"Mastering Regular Expressions" (see L<perlbook>) by Jeffrey Friedl.
projects / p5sagit/p5-mst-13.2.git / blobdiff