=head1 NAME
-perlfaq6 - Regular Expressions ($Revision: 1.31 $, $Date: 2005/03/27 07:17:28 $)
+perlfaq6 - Regular Expressions ($Revision: 1.35 $, $Date: 2005/08/10 15:55:08 $)
=head1 DESCRIPTION
littered with answers involving regular expressions. For example,
decoding a URL and checking whether something is a number are handled
with regular expressions, but those answers are found elsewhere in
-this document (in L<perlfaq9>: ``How do I decode or create those %-encodings
-on the web'' and L<perlfaq4>: ``How do I determine whether a scalar is
-a number/whole/integer/float'', to be precise).
+this document (in L<perlfaq9>: "How do I decode or create those %-encodings
+on the web" and L<perlfaq4>: "How do I determine whether a scalar is
+a number/whole/integer/float", to be precise).
=head2 How can I hope to use regular expressions without creating illegible and unmaintainable code?
+X<regex, legibility> X<regexp, legibility>
+X<regular expression, legibility> X</x>
Three techniques can make regular expressions maintainable and
understandable.
=back
=head2 I'm having trouble matching over more than one line. What's wrong?
+X<regex, multiline> X<regexp, multiline> X<regular expression, multiline>
Either you don't have more than one line in the string you're looking
at (probably), or else you aren't using the correct modifier(s) on
}
=head2 How can I pull out lines between two patterns that are themselves on different lines?
+X<..>
You can use Perl's somewhat exotic C<..> operator (documented in
L<perlop>):
}
=head2 I put a regular expression into $/ but it didn't work. What's wrong?
+X<$/, regexes in> X<$INPUT_RECORD_SEPARATOR, regexes in>
+X<$RS, regexes in>
Up to Perl 5.8.0, $/ has to be a string. This may change in 5.10,
but don't get your hopes up. Until then, you can use these examples
=head2 How do I substitute case insensitively on the LHS while preserving case on the RHS?
+X<replace, case preserving> X<substitute, case preserving>
+X<substitution, case preserving> X<s, case preserving>
Here's a lovely Perlish solution by Larry Rosler. It exploits
properties of bitwise xor on ASCII strings.
}
=head2 How can I make C<\w> match national character sets?
+X<\w>
Put C<use locale;> in your script. The \w character class is taken
from the current locale.
See L<perllocale> for details.
=head2 How can I match a locale-smart version of C</[a-zA-Z]/>?
+X<alpha>
You can use the POSIX character class syntax C</[[:alpha:]]/>
documented in L<perlre>.
the digits and the underscore, or C</[\W\d_]/>.
=head2 How can I quote a variable to use in a regex?
+X<regex, escaping> X<regexp, escaping> X<regular expression, escaping>
The Perl parser will expand $variable and @variable references in
regular expressions unless the delimiter is a single quote. Remember,
regular character, so that C<P.> matches a C<P> followed by a dot.
=head2 What is C</o> really for?
+X</o>
Using a variable in a regular expression match forces a re-evaluation
(and perhaps recompilation) each time the regular expression is
s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse;
=head2 Can I use Perl regular expressions to match balanced text?
+X<regex, matching balanced test> X<regexp, matching balanced test>
+X<regular expression, matching balanced test>
Historically, Perl regular expressions were not capable of matching
balanced text. As of more recent versions of perl including 5.6.1
but they are undocumented.
=head2 What does it mean that regexes are greedy? How can I get around it?
+X<greedy> X<greediness>
Most people mean that greedy regexes match as much as they can.
Technically speaking, it's actually the quantifiers (C<?>, C<*>, C<+>,
playing hot potato.
=head2 How do I process each word on each line?
+X<word>
Use the split function:
print "$count $line";
}
-If you want these output in a sorted order, see L<perlfaq4>: ``How do I
-sort a hash (optionally by value instead of key)?''.
+If you want these output in a sorted order, see L<perlfaq4>: "How do I
+sort a hash (optionally by value instead of key)?".
=head2 How can I do approximate matching?
+X<match, approximate> X<matching, approximate>
See the module String::Approx available from CPAN.
=head2 How do I efficiently match many regular expressions at once?
+X<regex, efficiency> X<regexp, efficiency>
+X<regular expression, efficiency>
( contributed by brian d foy )
-Avoid asking Perl to compile a regular expression every time
+Avoid asking Perl to compile a regular expression every time
you want to match it. In this example, perl must recompile
the regular expression for every iteration of the foreach()
loop since it has no way to know what $pattern will be.
@patterns = qw( foo bar baz );
-
- LINE: while( <> )
+
+ LINE: while( <> )
{
- foreach $pattern ( @patterns )
+ foreach $pattern ( @patterns )
{
print if /\b$pattern\b/i;
next LINE;
@patterns = map { qr/\b$_\b/i } qw( foo bar baz );
- LINE: while( <> )
+ LINE: while( <> )
{
- foreach $pattern ( @patterns )
+ foreach $pattern ( @patterns )
{
print if /\b$pattern\b/i;
next LINE;
}
}
-
+
In some cases, you may be able to make several patterns into
a single regular expression. Beware of situations that require
backtracking though.
$regex = join '|', qw( foo bar baz );
- LINE: while( <> )
+ LINE: while( <> )
{
print if /\b(?:$regex)\b/i;
}
For more details on regular expression efficiency, see Mastering
Regular Expressions by Jeffrey Freidl. He explains how regular
expressions engine work and why some patterns are surprisingly
-inefficient. Once you understand how perl applies regular
+inefficient. Once you understand how perl applies regular
expressions, you can tune them for individual situations.
=head2 Why don't word-boundary searches with C<\b> work for me?
+X<\b>
(contributed by brian d foy)
"Perl_" # _ is a word char!
"Perler" # no word char before P, but one after l
-
+
You don't have to use \b to match words though. You can look for
-non-word characters surrrounded by word characters. These strings
+non-word characters surrounded by word characters. These strings
match the pattern /\b'\b/.
"don't" # the ' char is surrounded by "n" and "t"
"qep'a'" # the ' char is surrounded by "p" and "a"
-
+
These strings do not match /\b'\b/.
"foo'" # there is no word char after non-word '
-
+
You can also use the complement of \b, \B, to specify that there
should not be a word boundary.
"llama" # "am" surrounded by word chars
"Samuel" # same
-
+
These strings do not match /\Bam\B/
"Sam" # no word boundary before "a", but one after "m"
=head2 Why does using $&, $`, or $' slow my program down?
+X<$MATCH> X<$&> X<$POSTMATCH> X<$'> X<$PREMATCH> X<$`>
(contributed by Anno Siegel)
Once Perl sees that you need one of these variables anywhere in the
-program, it provides them on each and every pattern match. That means
-that on every pattern match the entire string will be copied, part of
-it to $`, part to $&, and part to $'. Thus the penalty is most severe
-with long strings and patterns that match often. Avoid $&, $', and $`
-if you can, but if you can't, once you've used them at all, use them
-at will because you've already paid the price. Remember that some
-algorithms really appreciate them. As of the 5.005 release, the $&
-variable is no longer "expensive" the way the other two are.
-
+program, it provides them on each and every pattern match. That means
+that on every pattern match the entire string will be copied, part of it
+to $`, part to $&, and part to $'. Thus the penalty is most severe with
+long strings and patterns that match often. Avoid $&, $', and $` if you
+can, but if you can't, once you've used them at all, use them at will
+because you've already paid the price. Remember that some algorithms
+really appreciate them. As of the 5.005 release, the $& variable is no
+longer "expensive" the way the other two are.
+
+Since Perl 5.6.1 the special variables @- and @+ can functionally replace
+$`, $& and $'. These arrays contain pointers to the beginning and end
+of each match (see perlvar for the full story), so they give you
+essentially the same information, but without the risk of excessive
+string copying.
+
=head2 What good is C<\G> in a regular expression?
+X<\G>
You use the C<\G> anchor to start the next match on the same
string where the last match left off. The regular
pattern.
=head2 Are Perl regexes DFAs or NFAs? Are they POSIX compliant?
+X<DFA> X<NFA> X<POSIX>
While it's true that Perl's regular expressions resemble the DFAs
(deterministic finite automata) of the egrep(1) program, they are in
L<perlfaq2>).
=head2 What's wrong with using grep in a void context?
+X<grep>
The problem is that grep builds a return list, regardless of the context.
This means you're making Perl go to the trouble of building a list that
context, no lists are constructed.
=head2 How can I match strings with multibyte characters?
+X<regex, and multibyte characters> X<regexp, and multibyte characters>
+X<regular expression, and multibyte characters>
Starting from Perl 5.6 Perl has had some level of multibyte character
support. Perl 5.8 or later is recommended. Supported multibyte
Here are a few ways, all painful, to deal with it:
- $martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent ``martian''
+ $martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent "martian"
# bytes are no longer adjacent.
print "found GX!\n" if $martian =~ /GX/;
projects / p5sagit/p5-mst-13.2.git / blobdiff