X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlfaq6.pod;h=ed0bc297412e18f35335cde2fae99a42d7c642c6;hb=f26f4a2f8b63c72a33468ddeeb9d0337f0892af6;hp=cf3a8fb7ca737822bae4e1212f58509a557623e1;hpb=49d635f9372392ae44fe4c5b62b06e41912ae0c9;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlfaq6.pod b/pod/perlfaq6.pod index cf3a8fb..ed0bc29 100644 --- a/pod/perlfaq6.pod +++ b/pod/perlfaq6.pod @@ -1,6 +1,6 @@ =head1 NAME -perlfaq6 - Regular Expressions ($Revision: 1.18 $, $Date: 2002/10/30 18:44:21 $) +perlfaq6 - Regular Expressions ($Revision: 1.35 $, $Date: 2005/08/10 15:55:08 $) =head1 DESCRIPTION @@ -8,9 +8,9 @@ This section is surprisingly small because the rest of the FAQ is 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: ``How do I decode or create those %-encodings -on the web'' and L: ``How do I determine whether a scalar is -a number/whole/integer/float'', to be precise). +this document (in L: "How do I decode or create those %-encodings +on the web" and L: "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? @@ -143,16 +143,28 @@ Here's another example of using C<..>: # now choose between them } continue { reset if eof(); # fix $. - } + } =head2 I put a regular expression into $/ but it didn't work. What's wrong? -As of Perl 5.8.0, $/ has to be a string. This may change in 5.10, +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 if you really need to do this. -Use the four argument form of sysread to continually add to -a buffer. After you add to the buffer, you check if you have a +If you have File::Stream, this is easy. + + use File::Stream; + my $stream = File::Stream->new( + $filehandle, + separator => qr/\s*,\s*/, + ); + + print "$_\n" while <$stream>; + +If you don't have File::Stream, you have to do a little more work. + +You can use the four argument form of sysread to continually add to +a buffer. After you add to the buffer, you check if you have a complete line (using your regular expression). local $_ = ""; @@ -162,11 +174,11 @@ complete line (using your regular expression). # do stuff here. } } - + You can do the same thing with foreach and a match using the c flag and the \G anchor, if you do not mind your entire file being in memory at the end. - + local $_ = ""; while( sysread FH, $_, 8192, length ) { foreach my $record ( m/\G((?s).*?)your_pattern/gc ) { @@ -201,7 +213,7 @@ And here it is as a subroutine, modeled after the above: my $mask = uc $old ^ $old; uc $new | $mask . - substr($mask, -1) x (length($new) - length($old)) + substr($mask, -1) x (length($new) - length($old)) } $a = "this is a TEsT case"; @@ -280,8 +292,8 @@ documented in L. No matter which locale you are in, the alphabetic characters are the characters in \w without the digits and the underscore. As a regex, that looks like C. Its complement, -the non-alphabetics, is then everything in \W along with -the digits and the underscore, or C. +the non-alphabetics, is then everything in \W along with +the digits and the underscore, or C. =head2 How can I quote a variable to use in a regex? @@ -292,14 +304,26 @@ a double-quoted string (see L for more details). Remember also that any regex special characters will be acted on unless you precede the substitution with \Q. Here's an example: - $string = "to die?"; - $lhs = "die?"; - $rhs = "sleep, no more"; + $string = "Placido P. Octopus"; + $regex = "P."; + + $string =~ s/$regex/Polyp/; + # $string is now "Polypacido P. Octopus" + +Because C<.> is special in regular expressions, and can match any +single character, the regex C here has matched the in the +original string. + +To escape the special meaning of C<.>, we use C<\Q>: + + $string = "Placido P. Octopus"; + $regex = "P."; - $string =~ s/\Q$lhs/$rhs/; - # $string is now "to sleep no more" + $string =~ s/\Q$regex/Polyp/; + # $string is now "Placido Polyp Octopus" -Without the \Q, the regex would also spuriously match "di". +The use of C<\Q> causes the <.> in the regex to be treated as a +regular character, so that C matches a C

followed by a dot. =head2 What is C really for? @@ -342,7 +366,7 @@ created by Jeffrey Friedl and later modified by Fred Curtis. $/ = undef; $_ = <>; - s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs + s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse; print; This could, of course, be more legibly written with the C modifier, adding @@ -383,11 +407,11 @@ whitespace and comments. Here it is expanded, courtesy of Fred Curtis. . ## Anything other char [^/"'\\]* ## Chars which doesn't start a comment, string or escape ) - }{$2}gxs; + }{defined $2 ? $2 : ""}gxse; A slight modification also removes C++ comments: - s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs; + s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse; =head2 Can I use Perl regular expressions to match balanced text? @@ -442,9 +466,9 @@ playing hot potato. Use the split function: while (<>) { - foreach $word ( split ) { + foreach $word ( split ) { # do something with $word here - } + } } Note that this isn't really a word in the English sense; it's just @@ -478,15 +502,15 @@ in the previous question: If you wanted to do the same thing for lines, you wouldn't need a regular expression: - while (<>) { + while (<>) { $seen{$_}++; } while ( ($line, $count) = each %seen ) { print "$count $line"; } -If you want these output in a sorted order, see L: ``How do I -sort a hash (optionally by value instead of key)?''. +If you want these output in a sorted order, see L: "How do I +sort a hash (optionally by value instead of key)?". =head2 How can I do approximate matching? @@ -494,72 +518,135 @@ See the module String::Approx available from CPAN. =head2 How do I efficiently match many regular expressions at once? -The following is extremely inefficient: +( contributed by brian d foy ) - # slow but obvious way - @popstates = qw(CO ON MI WI MN); - while (defined($line = <>)) { - for $state (@popstates) { - if ($line =~ /\b$state\b/i) { - print $line; - last; - } - } - } - -That's because Perl has to recompile all those patterns for each of -the lines of the file. As of the 5.005 release, there's a much better -approach, one which makes use of the new C operator: - - # use spiffy new qr// operator, with /i flag even - use 5.005; - @popstates = qw(CO ON MI WI MN); - @poppats = map { qr/\b$_\b/i } @popstates; - while (defined($line = <>)) { - for $patobj (@poppats) { - print $line if $line =~ /$patobj/; - } - } +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( <> ) + { + 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 -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. + +It's not (not!) the boundary between whitespace and non-whitespace, +and it's not the stuff between words we use to create sentences. + +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. + +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/. + + "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 -Here are examples of the incorrect application of C<\b>, with fixes: +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/. - "two words" =~ /(\w+)\b(\w+)/; # WRONG - "two words" =~ /(\w+)\s+(\w+)/; # right + "don't" # the ' char is surrounded by "n" and "t" + "qep'a'" # the ' char is surrounded by "p" and "a" - " =matchless= text" =~ /\b=(\w+)=\b/; # WRONG - " =matchless= text" =~ /=(\w+)=/; # right +These strings do not match /\b'\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. + "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 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? @@ -602,7 +689,7 @@ still need the C flag. { print "Found $1\n"; } - + After the match fails at the letter C, perl resets pos() and the next match on the same string starts at the beginning. @@ -648,7 +735,7 @@ which works in 5.004 or later. For each line, the PARSER loop first tries to match a series of digits followed by a word boundary. This match has to start at the place the last match left off (or the beginning -of the string on the first match). Since C uses the C flag, if the string does not match that regular expression, perl does not reset pos() and the next match starts at the same position to try a different @@ -667,15 +754,18 @@ guaranteed is slowness.) See the book "Mastering Regular Expressions" hope to know on these matters (a full citation appears in L). -=head2 What's wrong with using grep or map in a void context? +=head2 What's wrong with using grep in a void context? -The problem is that both grep and map build a return list, -regardless of the context. This means you're making Perl go -to the trouble of building a list that you then just throw away. -If the list is large, you waste both time and space. If your -intent is to iterate over the list then use a for loop for this +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 +you then just throw away. If the list is large, you waste both time and space. +If your intent is to iterate over the list, then use a for loop for this purpose. +In perls older than 5.8.1, map suffers from this problem as well. +But since 5.8.1, this has been fixed, and map is context aware - in void +context, no lists are constructed. + =head2 How can I match strings with multibyte characters? Starting from Perl 5.6 Perl has had some level of multibyte character @@ -710,7 +800,7 @@ looks like it is because "SG" is next to "XX", but there's no real 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/; @@ -730,17 +820,17 @@ Or like this: } Here's another, slightly less painful, way to do it from Benjamin -Goldberg: - - $martian =~ m/ - (?!<[A-Z]) - (?:[A-Z][A-Z])*? - GX - /x; - +Goldberg, who uses a zero-width negative look-behind assertion. + + print "found GX!\n" if $martian =~ m/ + (?. =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.