operations, plus various examples of the same, see C<m//> and C<s///> in
L<perlop>.
-The matching operations can
-have various modifiers, some of which relate to the interpretation of
-the regular expression inside. These are:
+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//">.
=over 4
If C<use locale> is in effect, the case map is taken from the current
locale. See L<perllocale>.
-=item m
+=item m
Treat string as multiple lines. That is, change "^" and "$" from matching
at only the very start or end of the string to the start or end of any
line anywhere within the string,
-=item s
+=item s
Treat string as single line. That is, change "." to match any character
whatsoever, even a newline, which it normally would not match.
-=item x
+=item x
Extend your pattern's legibility by permitting whitespace and comments.
the regular expression parser to ignore whitespace that is neither
backslashed nor within a character class. You can use this to break up
your regular expression into (slightly) more readable parts. The C<#>
-character is also treated as a meta-character introducing a comment,
+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,
In particular the following metacharacters have their standard I<egrep>-ish
meanings:
- \ Quote the next meta-character
+ \ Quote the next metacharacter
^ Match the beginning of the line
. Match any character (except newline)
$ Match the end of the line (or before newline at the end)
modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. n and m are limited
to integral values less than 65536.
-By default, a quantified sub-pattern is "greedy", that is, it will match as
-many times as possible without causing the rest of the pattern not to match.
-The standard quantifiers are all "greedy", in that they match as many
-occurrences as possible (given a particular starting location) without
-causing the pattern to fail. If you want it to match the minimum number
-of times possible, follow the quantifier with a "?" after any of them.
-Note that the meanings don't change, just the "gravity":
+By default, a quantified subpattern is "greedy", that is, it will match as
+many times as possible (given a particular starting location) while still
+allowing the rest of the pattern to match. If you want it to match the
+minimum number of times possible, follow the quantifier with a "?". Note
+that the meanings don't change, just the "greediness":
*? Match 0 or more times
+? Match 1 or more times
\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 and a C<\W> on the other side of it (in
+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 mix global
-matches (using C<m//g>) and non-global ones, as described in L<perlop>.
+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>.
$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 meta-character. 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 built-in 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$unquoted/
=over 10
-=item (?#text)
+=item C<(?#text)>
A comment. The text is ignored. If the C</x> switch is used to enable
whitespace formatting, a simple C<#> will suffice.
-=item (?:regexp)
+=item C<(?:regexp)>
This groups things like "()" but doesn't make backreferences like "()" does. So
but doesn't spit out extra fields.
-=item (?=regexp)
+=item C<(?=regexp)>
A zero-width positive lookahead assertion. For example, C</\w+(?=\t)/>
matches a word followed by a tab, without including the tab in C<$&>.
-=item (?!regexp)
+=item C<(?!regexp)>
A zero-width negative lookahead 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
+use this for lookbehind. If you are looking for a "bar" which isn't preceeded
+"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)...|^..?)bar/>. Sometimes it's still
+cover that this way: C</(?:(?!foo)...|^.{0,2})bar/>. Sometimes it's still
easier just to say:
- if (/foo/ && $` =~ /bar$/)
+ if (/bar/ && $` !~ /foo$/)
+
+For lookbehind see below.
+
+=item C<(?<=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 C<(?<!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 C<(?{ code })>
+
+Experimental "evaluate any Perl code" zero-width assertion. Always
+succeeds. C<code> is not interpolated. Currently the rules to
+determine where the C<code> ends are somewhat convoluted.
+
+=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
-=item (?imsx)
+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 C<(?(condition)yes-regexp|no-regexp)>
+
+=item C<(?(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 C<(?imsx)>
One or more embedded pattern-match modifiers. This is particularly
useful for patterns that are specified in a table somewhere, some of
$pattern = "(?i)foobar";
if ( /$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
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 had the
+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."
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
+tricker. Imagine you'd like to find a sequence of non-digits not
followed by "123". You might try to write that as
$_ = "ABC123";
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.
+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.
+in the hope of matching the complete regular expression.
-Well now,
+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
time. Now there's indeed something following "AB" that is not
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
routines, here are the pattern-matching rules not described above.
-Any single character matches itself, unless it is a I<meta-character>
+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
literally by prefixing them with a "\" (e.g., "\." matches a ".", not any
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 meta-character syntax much like that
+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 "." meta-character matches any
+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
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|]>.
+start and end.
+
+Note that 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.)
-Within a pattern, you may designate sub-patterns for later reference by
+Also note 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
-sub-pattern later in the pattern using the meta-character \I<n>.
-Sub-patterns are numbered based on the left to right order of their
+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 sub-pattern in the string being examined, not the
-rules for that sub-pattern. Therefore, C<(0|0x)\d*\s\1\d*> will
-match "0x1234 0x4321",but not "0x1234 01234", because sub-pattern 1
+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.
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
+
+L<perlop/"Regexp Quote-Like Operators">.
+
+L<perlfunc/pos>.
+
+L<perllocale>.
+
+"Mastering Regular Expressions" (see L<perlbook>) by Jeffrey Friedl.
projects / p5sagit/p5-mst-13.2.git / blobdiff