[:class:]
-is also available. The available classes and their \-equivalents
-(if any) are as follows:
+is also available. The available classes and their backslash
+equivalents (if available) are as follows:
alpha
alnum
word \w
xdigit
-Note that the [] are part of the [::] construct, not part of the whole
+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.
-The exact meanings of the above classes depend from many things:
-if the C<utf8> pragma is used, the following equivalences to Unicode
+If the C<utf8> pragma is used, the following equivalences to Unicode
\p{} constructs hold:
alpha IsAlpha
word IsWord
xdigit IsXDigit
-For example, [:lower:] and \p{IsLower} are equivalent.
+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 \w).
+which is a Perl extension, mirroring C<\w>).
The assumedly non-obviously named classes are:
[:^space:] \S \P{IsSpace}
[:^word:] \W \P{IsWord}
-The POSIX character classes [.cc.] and [=cc=] are B<not> supported
-and trying to use them will cause an error.
+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
/$unquoted\Q$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
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
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*)>
(anchored at the beginning of string, as above) will match I<all>
m{ \(
(
- [^()]+
+ [^()]+ # x+
|
\( [^()]* \)
)+
m{ \(
(
- (?> [^()]+ )
+ (?> [^()]+ ) # change x+ above to (?> x+ )
|
\( [^()]* \)
)+
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 appearence, 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)>
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<(?E<gt>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
+where side-effects of look-ahead I<might> have influenced the
following match, see L<C<(?E<gt>pattern)>>.
=head2 Version 8 Regular Expressions
@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 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
projects / p5sagit/p5-mst-13.2.git / blobdiff