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;
+ $_ **= $_ , / {$_} / 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
In addition, Perl defines the following:
\w Match a "word" character (alphanumeric plus "_")
- \W Match a non-word character
+ \W Match a non-"word" character
\s Match a whitespace character
\S Match a non-whitespace character
\d Match a digit character
equivalent to C<(?:\PM\pM*)>
\C Match a single C char (octet) even under utf8.
-A C<\w> matches a single alphanumeric character, not a whole word.
+A C<\w> matches a single alphanumeric character 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 (though not as either end of
-a range). See L<utf8> for details about C<\pP>, C<\PP>, and C<\X>.
+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.
+See L<utf8> for details about C<\pP>, C<\PP>, and C<\X>.
The POSIX character class syntax
- [:class:]
+ [:class:]
is also available. The available classes and their backslash
equivalents (if available) are as follows:
Note that the C<[]> are part of the C<[::]> construct, not part of the whole
character class. For example:
- [01[:alpha:]%]
+ [01[:alpha:]%]
matches one, zero, any alphabetic character, and the percentage sign.
=item cntrl
- 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 control
- classified as characters.
+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.
=item graph
- Any alphanumeric or punctuation character.
+Any alphanumeric or punctuation character.
=item print
- Any alphanumeric or punctuation character or space.
+Any alphanumeric or punctuation character or space.
=item punct
- Any punctuation character.
+Any punctuation character.
=item xdigit
- Any hexadecimal digit. Though this may feel silly
- (/0-9a-f/i would work just fine) it is included
- for completeness.
-
-=item
+Any hexadecimal digit. Though this may feel silly (/0-9a-f/i would
+work just fine) it is included for completeness.
=back
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>.
-
+
The bracketing construct C<( ... )> creates capture buffers. To
-refer to the digit'th buffer use \E<lt>digitE<gt> within the
+refer to the digit'th buffer use \<digit> within the
match. Outside the match use "$" instead of "\". (The
-\E<lt>digitE<gt> notation works in certain circumstances outside
+\<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>.
if (/(.)\1/) { # find first doubled char
print "'$1' is the first doubled character\n";
}
-
+
if (/Time: (..):(..):(..)/) { # parse out values
$hours = $1;
$minutes = $2;
$seconds = $3;
}
-
+
Several special variables also refer back to portions of the previous
match. C<$+> returns whatever the last bracket match matched.
C<$&> returns the entire matched string. (At one point C<$0> did
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
+that looks like \\, \(, \), \<, \>, \{, or \} is always
interpreted as a literal character, not a metacharacter. This was
once used in a common idiom to disable or quote the special meanings
of regular expression metacharacters in a string that you want to
-use for a pattern. Simply quote all non-alphanumeric characters:
+use for a pattern. Simply quote all non-"word" characters:
$pattern =~ s/(\W)/\\$1/g;
For look-behind see below.
-=item C<(?E<lt>=pattern)>
+=item C<(?<=pattern)>
-A zero-width positive look-behind assertion. For example, C</(?E<lt>=\t)\w+/>
+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.
Better yet, use the carefully constrained evaluation within a Safe
module. See L<perlsec> for details about both these mechanisms.
-=item C<(?p{ code })>
+=item C<(??{ code })>
B<WARNING>: This extended regular expression feature is considered
highly experimental, and may be changed or deleted without notice.
(?:
(?> [^()]+ ) # Non-parens without backtracking
|
- (?p{ $re }) # Group with matching parens
+ (??{ $re }) # Group with matching parens
)*
\)
}x;
-=item C<(?E<gt>pattern)>
+=item C<< (?>pattern) >>
B<WARNING>: This extended regular expression feature is considered
highly experimental, and may be changed or deleted without notice.
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<^(?E<gt>a*)ab> will never match, since C<(?E<gt>a*)>
+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>,
C<a*ab> will match fewer characters than a standalone C<a*>, since
this makes the tail match.
-An effect similar to C<(?E<gt>pattern)> may be achieved by writing
+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<(?E<gt>...)>.
+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.)
\)
}x
-which uses C<(?E<gt>...)> matches exactly when the one above does (verifying
+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
-B<-w> saying it C<"matches the null string many times">):
+the C<use warnings> pragma or B<-w> switch saying it
+C<"matches the null string many times">):
-On simple groups, such as the pattern C<(?E<gt> [^()]+ )>, a comparable
+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.
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 appearence, C<#[ \t]*> I<is not> the correct subexpression to match
+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:
=head2 Backtracking
+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<+>,
A powerful tool for optimizing such beasts is what is known as an
"independent group",
-which does not backtrack (see L<C<(?E<gt>pattern)>>). Note also that
+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<(?E<gt>pattern)>>.
+following match, see L<C<< (?>pattern) >>>.
=head2 Version 8 Regular Expressions
first character after the "[" is "^", the class matches any character not
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 "-" 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
+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.)
+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
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
+
+=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 })>
+
+The ordering is the same as for the regular expression which is
+the result of EXPR.
+
+=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
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