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
\x1B hex char
\x{263a} wide hex char (Unicode SMILEY)
\c[ control char
+ \N{name} named char
\l lowercase next char (think vi)
\u uppercase next char (think vi)
\L lowercase till \E (think vi)
\Q quote (disable) pattern metacharacters till \E
If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
-and C<\U> is taken from the current locale. See L<perllocale>.
+and C<\U> is taken from the current locale. See L<perllocale>. For
+documentation of C<\N{name}>, see L<charnames>.
You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
An unescaped C<$> or C<@> interpolates the corresponding variable,
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:]
+
+is also available. The available classes and their backslash
+equivalents (if available) are as follows:
+
+ alpha
+ alnum
+ ascii
+ cntrl
+ digit \d
+ graph
+ lower
+ print
+ punct
+ space \s
+ upper
+ word \w
+ xdigit
+
+For example use C<[:upper:]> to match all the uppercase characters.
+Note that the C<[]> are part of the C<[::]> construct, not part of the whole
+character class. For example:
+
+ [01[:alpha:]%]
+
+matches one, zero, any alphabetic character, and the percentage sign.
+
+If the C<utf8> pragma is used, the following equivalences to Unicode
+\p{} constructs hold:
+
+ alpha IsAlpha
+ alnum IsAlnum
+ ascii IsASCII
+ cntrl IsCntrl
+ digit IsDigit
+ graph IsGraph
+ lower IsLower
+ print IsPrint
+ punct IsPunct
+ space IsSpace
+ upper IsUpper
+ word IsWord
+ xdigit IsXDigit
+
+For example C<[:lower:]> and C<\p{IsLower}> are equivalent.
+
+If the C<utf8> pragma is not used but the C<locale> pragma is, the
+classes correlate with the isalpha(3) interface (except for `word',
+which is a Perl extension, mirroring C<\w>).
+
+The assumedly non-obviously named classes are:
+
+=over 4
+
+=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 classified as control characters.
+
+=item graph
+
+Any alphanumeric or punctuation character.
+
+=item print
+
+Any alphanumeric or punctuation character or space.
+
+=item punct
+
+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.
+
+=back
+
+You can negate the [::] character classes by prefixing the class name
+with a '^'. This is a Perl extension. For example:
+
+ POSIX trad. Perl utf8 Perl
+
+ [:^digit:] \D \P{IsDigit}
+ [:^space:] \S \P{IsSpace}
+ [:^word:] \W \P{IsWord}
+
+The POSIX character classes [.cc.] and [=cc=] are recognized but
+B<not> supported and trying to use them will cause an error.
Perl defines the following zero-width assertions:
\A Match only at beginning of string
\Z Match only at end of string, or before newline at the end
\z Match only at end of string
- \G Match only where previous m//g left off (works only with /g)
+ \G Match only at pos() (e.g. at the end-of-match position
+ of prior m//g)
A word boundary (C<\b>) is a spot between two characters
that has a C<\w> on one side of it and a C<\W> on the other side
of 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;
/$unquoted\Q$quoted\E$unquoted/
+Beware that if you put literal backslashes (those not inside
+interpolated variables) between C<\Q> and C<\E>, double-quotish
+backslash interpolation may lead to confusing results. If you
+I<need> to use literal backslashes within C<\Q...\E>,
+consult L<perlop/"Gory details of parsing quoted constructs">.
+
=head2 Extended Patterns
Perl also defines a consistent extension syntax for features not
For look-behind see below.
-=item C<(?E<lt>=pattern)>
+=item C<(?<=pattern)>
-A zero-width positive look-behind assertion. For example, C</(?E<lt>=\t)\w+/>
+A zero-width positive look-behind assertion. For example, C</(?<=\t)\w+/>
matches a word that follows a tab, without including the tab in C<$&>.
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.
+A simplified version of the syntax may be introduced for commonly
+used idioms.
This is a "postponed" regular subexpression. The C<code> is evaluated
at run time, at the moment this subexpression may match. The result
of evaluation is considered as a regular expression and matched as
if it were inserted instead of this construct.
-C<code> is not interpolated. As before, the rules to determine
+The C<code> is not interpolated. As before, the rules to determine
where the C<code> ends are currently somewhat convoluted.
The following pattern matches a parenthesized group:
(?:
(?> [^()]+ ) # 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.
An "independent" subexpression, one which matches the substring
that a I<standalone> C<pattern> would match if anchored at the given
-position--but it matches no more than this substring. This
+position, and it matches I<nothing other than this substring>. This
construct is useful for optimizations of what would otherwise be
"eternal" matches, because it will not backtrack (see L<"Backtracking">).
+It may also be useful in places where the "grab all you can, and do not
+give anything back" semantic is desirable.
-For example: C<^(?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.)
m{ \(
(
- [^()]+
+ [^()]+ # x+
|
\( [^()]* \)
)+
m{ \(
(
- (?> [^()]+ )
+ (?> [^()]+ ) # change x+ above to (?> x+ )
|
\( [^()]* \)
)+
\)
}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.
+The "grab all you can, and do not give anything back" semantic is desirable
+in many situations where on the first sight a simple C<()*> looks like
+the correct solution. Suppose we parse text with comments being delimited
+by C<#> followed by some optional (horizontal) whitespace. Contrary to
+its appearance, C<#[ \t]*> I<is not> the correct subexpression to match
+the comment delimiter, because it may "give up" some whitespace if
+the remainder of the pattern can be made to match that way. The correct
+answer is either one of these:
+
+ (?>#[ \t]*)
+ #[ \t]*(?![ \t])
+
+For example, to grab non-empty comments into $1, one should use either
+one of these:
+
+ / (?> \# [ \t]* ) ( .+ ) /x;
+ / \# [ \t]* ( [^ \t] .* ) /x;
+
+Which one you pick depends on which of these expressions better reflects
+the above specification of comments.
+
=item C<(?(condition)yes-pattern|no-pattern)>
=item C<(?(condition)yes-pattern)>
=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<+>,
-C<+?>, C<{n,m}>, and C<{n,m}?>.
+C<+?>, C<{n,m}>, and C<{n,m}?>. Backtracking is often optimized
+internally, but the general principle outlined here is valid.
For a regular expression to match, the I<entire> regular expression must
match, not just part of it. So if the beginning of a pattern containing a
B<WARNING>: particularly complicated regular expressions can take
exponential time to solve because of the immense number of possible
-ways they can use backtracking to try match. For example, this will
-take a painfully long time to run
+ways they can use backtracking to try match. For example, without
+internal optimizations done by the regular expression engine, this will
+take a painfully long time to run:
- /((a{0,5}){0,5}){0,5}/
+ 'aaaaaaaaaaaa' =~ /((a{0,5}){0,5}){0,5}[c]/
And if you used C<*>'s instead of limiting it to 0 through 5 matches,
then it would take 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 look-ahead/look-behind assertions will not backtrace to make
+A powerful tool for optimizing such beasts is what is known as an
+"independent group",
+which does not backtrack (see L<C<< (?>pattern) >>>). Note also that
+zero-length look-ahead/look-behind assertions will not backtrack to make
the tail match, since they are in "logical" context: only
whether they match is considered relevant. For an example
-where side-effects of a look-ahead I<might> have influenced the
-following match, see L<C<(?E<gt>pattern)>>.
+where side-effects of look-ahead I<might> have influenced the
+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
@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
+Thus Perl allows such constructs, by I<forcefully breaking
the infinite loop>. The rules for this are different for lower-level
loops given by the greedy modifiers C<*+{}>, and for higher-level
ones like the C</g> modifier or split() operator.
The additional state of being I<matched with zero-length> is associated with
the matched string, and is reset by each assignment to pos().
+Zero-length matches at the end of the previous match are ignored
+during C<split>.
+
+=head2 Combining pieces together
+
+Each of the elementary pieces of regular expressions which were described
+before (such as C<ab> or C<\Z>) could match at most one substring
+at the given position of the input string. However, in a typical regular
+expression these elementary pieces are combined into more complicated
+patterns using combining operators C<ST>, C<S|T>, C<S*> etc
+(in these examples C<S> and C<T> are regular subexpressions).
+
+Such combinations can include alternatives, leading to a problem of choice:
+if we match a regular expression C<a|ab> against C<"abc">, will it match
+substring C<"a"> or C<"ab">? One way to describe which substring is
+actually matched is the concept of backtracking (see L<"Backtracking">).
+However, this description is too low-level and makes you think
+in terms of a particular implementation.
+
+Another description starts with notions of "better"/"worse". All the
+substrings which may be matched by the given regular expression can be
+sorted from the "best" match to the "worst" match, and it is the "best"
+match which is chosen. This substitutes the question of "what is chosen?"
+by the question of "which matches are better, and which are worse?".
+
+Again, for elementary pieces there is no such question, since at most
+one match at a given position is possible. This section describes the
+notion of better/worse for combining operators. In the description
+below C<S> and C<T> are regular subexpressions.
+
+=over
+
+=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
=head1 BUGS
-This manpage is varies from difficult to understand to completely
-and utterly opaque.
+This document varies from difficult to understand to completely
+and utterly opaque. The wandering prose riddled with jargon is
+hard to fathom in several places.
+
+This document needs a rewrite that separates the tutorial content
+from the reference content.
=head1 SEE ALSO
https://perl5.git.perl.org / perl5.git / blobdiff