=head1 NAME
-perlfaq6 - Regular Expressions ($Revision: 1.27 $, $Date: 2004/11/03 22:52:16 $)
+perlfaq6 - Regular Expressions ($Revision: 1.35 $, $Date: 2005/08/10 15:55:08 $)
=head1 DESCRIPTION
littered with answers involving regular expressions. For example,
decoding a URL and checking whether something is a number are handled
with regular expressions, but those answers are found elsewhere in
-this document (in L<perlfaq9>: ``How do I decode or create those %-encodings
-on the web'' and L<perlfaq4>: ``How do I determine whether a scalar is
-a number/whole/integer/float'', to be precise).
+this document (in L<perlfaq9>: "How do I decode or create those %-encodings
+on the web" and L<perlfaq4>: "How do I determine whether a scalar is
+a number/whole/integer/float", to be precise).
=head2 How can I hope to use regular expressions without creating illegible and unmaintainable code?
print "$count $line";
}
-If you want these output in a sorted order, see L<perlfaq4>: ``How do I
-sort a hash (optionally by value instead of key)?''.
+If you want these output in a sorted order, see L<perlfaq4>: "How do I
+sort a hash (optionally by value instead of key)?".
=head2 How can I do approximate matching?
=head2 How do I efficiently match many regular expressions at once?
-The following is extremely inefficient:
+( contributed by brian d foy )
- # slow but obvious way
- @popstates = qw(CO ON MI WI MN);
- while (defined($line = <>)) {
- for $state (@popstates) {
- if ($line =~ /\b$state\b/i) {
- print $line;
- last;
- }
- }
- }
+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 $pattern will be.
-That's because Perl has to recompile all those patterns for each of
-the lines of the file. As of the 5.005 release, there's a much better
-approach, one which makes use of the new C<qr//> operator:
-
- # use spiffy new qr// operator, with /i flag even
- use 5.005;
- @popstates = qw(CO ON MI WI MN);
- @poppats = map { qr/\b$_\b/i } @popstates;
- while (defined($line = <>)) {
- for $patobj (@poppats) {
- print $line if $line =~ /$patobj/;
- }
- }
+ @patterns = qw( foo bar baz );
+
+ LINE: while( <> )
+ {
+ foreach $pattern ( @patterns )
+ {
+ print if /\b$pattern\b/i;
+ next LINE;
+ }
+ }
+
+The qr// operator showed up in perl 5.005. It compiles a
+regular expression, but doesn't apply it. When you use the
+pre-compiled version of the regex, perl does less work. In
+this example, I inserted a map() to turn each pattern into
+its pre-compiled form. The rest of the script is the same,
+but faster.
+
+ @patterns = map { qr/\b$_\b/i } qw( foo bar baz );
+
+ LINE: while( <> )
+ {
+ 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.
+
+ $regex = join '|', qw( foo bar baz );
+
+ LINE: while( <> )
+ {
+ print if /\b(?:$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
+expressions, you can tune them for individual situations.
=head2 Why don't word-boundary searches with C<\b> work for me?
-Two common misconceptions are that C<\b> is a synonym for C<\s+> and
-that it's the edge between whitespace characters and non-whitespace
-characters. Neither is correct. C<\b> is the place between a C<\w>
-character and a C<\W> character (that is, C<\b> is the edge of a
-"word"). It's a zero-width assertion, just like C<^>, C<$>, and all
-the other anchors, so it doesn't consume any characters. L<perlre>
-describes the behavior of all the regex metacharacters.
+(contributed by brian d foy)
+
+Ensure that you know what \b really does: it's the boundary between a
+word character, \w, and something that isn't a word character. That
+thing that isn't a word character might be \W, but it can also be the
+start or end of the string.
-Here are examples of the incorrect application of C<\b>, with fixes:
+It's not (not!) the boundary between whitespace and non-whitespace,
+and it's not the stuff between words we use to create sentences.
- "two words" =~ /(\w+)\b(\w+)/; # WRONG
- "two words" =~ /(\w+)\s+(\w+)/; # right
+In regex speak, a word boundary (\b) is a "zero width assertion",
+meaning that it doesn't represent a character in the string, but a
+condition at a certain position.
- " =matchless= text" =~ /\b=(\w+)=\b/; # WRONG
- " =matchless= text" =~ /=(\w+)=/; # right
+For the regular expression, /\bPerl\b/, there has to be a word
+boundary before the "P" and after the "l". As long as something other
+than a word character precedes the "P" and succeeds the "l", the
+pattern will match. These strings match /\bPerl\b/.
-Although they may not do what you thought they did, C<\b> and C<\B>
-can still be quite useful. For an example of the correct use of
-C<\b>, see the example of matching duplicate words over multiple
-lines.
+ "Perl" # no word char before P or after l
+ "Perl " # same as previous (space is not a word char)
+ "'Perl'" # the ' char is not a word char
+ "Perl's" # no word char before P, non-word char after "l"
+
+These strings do not match /\bPerl\b/.
+
+ "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 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.
+
+In the pattern /\Bam\B/, there must be a word character before the "a"
+and after the "m". These patterns match /\Bam\B/:
+
+ "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"
+ "I am Sam" # "am" surrounded by non-word chars
-An example of using C<\B> is the pattern C<\Bis\B>. This will find
-occurrences of "is" on the insides of words only, as in "thistle", but
-not "this" or "island".
=head2 Why does using $&, $`, or $' slow my program down?
-Once Perl sees that you need one of these variables anywhere in
-the program, it provides them on each and every pattern match.
-The same mechanism that handles these provides for the use of $1, $2,
-etc., so you pay the same price for each regex that contains capturing
-parentheses. If you never use $&, etc., in your script, then regexes
-I<without> capturing parentheses won't be penalized. So 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.
+(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 $& variable is no
+longer "expensive" the way the other two are.
+
+Since Perl 5.6.1 the special variables @- and @+ can functionally replace
+$`, $& 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?
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/;
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
-All rights reserved.
+Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and
+other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.
projects / p5sagit/p5-mst-13.2.git / blobdiff