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Diffstat (limited to 'Master/tlpkg/tlperl/lib/pods/perlfaq6.pod')
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1 files changed, 372 insertions, 397 deletions
diff --git a/Master/tlpkg/tlperl/lib/pods/perlfaq6.pod b/Master/tlpkg/tlperl/lib/pods/perlfaq6.pod index 3066187fa60..40c2b07c3dc 100644 --- a/Master/tlpkg/tlperl/lib/pods/perlfaq6.pod +++ b/Master/tlpkg/tlperl/lib/pods/perlfaq6.pod @@ -6,7 +6,7 @@ perlfaq6 - Regular Expressions 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 +decoding a URL and checking whether something is a number can be 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 @@ -26,9 +26,9 @@ understandable. Describe what you're doing and how you're doing it, using normal Perl comments. - # turn the line into the first word, a colon, and the - # number of characters on the rest of the line - s/^(\w+)(.*)/ lc($1) . ":" . length($2) /meg; + # turn the line into the first word, a colon, and the + # number of characters on the rest of the line + s/^(\w+)(.*)/ lc($1) . ":" . length($2) /meg; =item Comments Inside the Regex @@ -39,20 +39,20 @@ help a lot. C</x> lets you turn this: - s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs; + s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs; into this: - s{ < # opening angle bracket - (?: # Non-backreffing grouping paren - [^>'"] * # 0 or more things that are neither > nor ' nor " - | # or else - ".*?" # a section between double quotes (stingy match) - | # or else - '.*?' # a section between single quotes (stingy match) - ) + # all occurring one or more times - > # closing angle bracket - }{}gsx; # replace with nothing, i.e. delete + s{ < # opening angle bracket + (?: # Non-backreffing grouping paren + [^>'"] * # 0 or more things that are neither > nor ' nor " + | # or else + ".*?" # a section between double quotes (stingy match) + | # or else + '.*?' # a section between single quotes (stingy match) + ) + # all occurring one or more times + > # closing angle bracket + }{}gsx; # replace with nothing, i.e. delete It's still not quite so clear as prose, but it is very useful for describing the meaning of each part of the pattern. @@ -65,8 +65,12 @@ describes this. For example, the C<s///> above uses braces as delimiters. Selecting another delimiter can avoid quoting the delimiter within the pattern: - s/\/usr\/local/\/usr\/share/g; # bad delimiter choice - s#/usr/local#/usr/share#g; # better + s/\/usr\/local/\/usr\/share/g; # bad delimiter choice + s#/usr/local#/usr/share#g; # better + +Using logically paired delimiters can be even more readable: + + s{/usr/local/}{/usr/share}g; # better still =back @@ -91,37 +95,37 @@ got a multiline string in there. For example, this program detects duplicate words, even when they span line breaks (but not paragraph ones). For this example, we don't need C</s> because we aren't using dot in a regular expression that we want -to cross line boundaries. Neither do we need C</m> because we aren't -wanting caret or dollar to match at any point inside the record next +to cross line boundaries. Neither do we need C</m> because we don't +want caret or dollar to match at any point inside the record next to newlines. But it's imperative that $/ be set to something other than the default, or else we won't actually ever have a multiline record read in. - $/ = ''; # read in whole paragraph, not just one line - while ( <> ) { - while ( /\b([\w'-]+)(\s+\g1)+\b/gi ) { # word starts alpha - print "Duplicate $1 at paragraph $.\n"; - } - } + $/ = ''; # read in whole paragraph, not just one line + while ( <> ) { + while ( /\b([\w'-]+)(\s+\g1)+\b/gi ) { # word starts alpha + print "Duplicate $1 at paragraph $.\n"; + } + } -Here's code that finds sentences that begin with "From " (which would +Here's some code that finds sentences that begin with "From " (which would be mangled by many mailers): - $/ = ''; # read in whole paragraph, not just one line - while ( <> ) { - while ( /^From /gm ) { # /m makes ^ match next to \n - print "leading from in paragraph $.\n"; - } - } + $/ = ''; # read in whole paragraph, not just one line + while ( <> ) { + while ( /^From /gm ) { # /m makes ^ match next to \n + print "leading from in paragraph $.\n"; + } + } Here's code that finds everything between START and END in a paragraph: - undef $/; # read in whole file, not just one line or paragraph - while ( <> ) { - while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries - print "$1\n"; - } - } + undef $/; # read in whole file, not just one line or paragraph + while ( <> ) { + while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries + print "$1\n"; + } + } =head2 How can I pull out lines between two patterns that are themselves on different lines? X<..> @@ -129,11 +133,11 @@ X<..> You can use Perl's somewhat exotic C<..> operator (documented in L<perlop>): - perl -ne 'print if /START/ .. /END/' file1 file2 ... + perl -ne 'print if /START/ .. /END/' file1 file2 ... If you wanted text and not lines, you would use - perl -0777 -ne 'print "$1\n" while /START(.*?)END/gs' file1 file2 ... + perl -0777 -ne 'print "$1\n" while /START(.*?)END/gs' file1 file2 ... But if you want nested occurrences of C<START> through C<END>, you'll run up against the problem described in the question in this section @@ -141,54 +145,24 @@ on matching balanced text. Here's another example of using C<..>: - while (<>) { - $in_header = 1 .. /^$/; - $in_body = /^$/ .. eof; - # now choose between them - } continue { - $. = 0 if eof; # fix $. - } + while (<>) { + my $in_header = 1 .. /^$/; + my $in_body = /^$/ .. eof; + # now choose between them + } continue { + $. = 0 if eof; # fix $. + } =head2 How do I match XML, HTML, or other nasty, ugly things with a regex? X<regex, XML> X<regex, HTML> X<XML> X<HTML> X<pain> X<frustration> X<sucking out, will to live> -(contributed by brian d foy) - -If you just want to get work done, use a module and forget about the -regular expressions. The C<XML::Parser> and C<HTML::Parser> modules -are good starts, although each namespace has other parsing modules -specialized for certain tasks and different ways of doing it. Start at -CPAN Search ( http://search.cpan.org ) and wonder at all the work people -have done for you already! :) - -The problem with things such as XML is that they have balanced text -containing multiple levels of balanced text, but sometimes it isn't -balanced text, as in an empty tag (C<< <br/> >>, for instance). Even then, -things can occur out-of-order. Just when you think you've got a -pattern that matches your input, someone throws you a curveball. - -If you'd like to do it the hard way, scratching and clawing your way -toward a right answer but constantly being disappointed, besieged by -bug reports, and weary from the inordinate amount of time you have to -spend reinventing a triangular wheel, then there are several things -you can try before you give up in frustration: - -=over 4 - -=item * Solve the balanced text problem from another question in L<perlfaq6> - -=item * Try the recursive regex features in Perl 5.10 and later. See L<perlre> - -=item * Try defining a grammar using Perl 5.10's C<(?DEFINE)> feature. - -=item * Break the problem down into sub-problems instead of trying to use a single regex - -=item * Convince everyone not to use XML or HTML in the first place - -=back - -Good luck! +Do not use regexes. Use a module and forget about the +regular expressions. The L<XML::LibXML>, L<HTML::TokeParser> and +L<HTML::TreeBuilder> modules are good starts, although each namespace +has other parsing modules specialized for certain tasks and different +ways of doing it. Start at CPAN Search ( L<http://metacpan.org/> ) +and wonder at all the work people have done for you already! :) =head2 I put a regular expression into $/ but it didn't work. What's wrong? X<$/, regexes in> X<$INPUT_RECORD_SEPARATOR, regexes in> @@ -197,16 +171,16 @@ X<$RS, regexes in> $/ has to be a string. You can use these examples if you really need to do this. -If you have File::Stream, this is easy. +If you have L<File::Stream>, this is easy. - use File::Stream; + use File::Stream; - my $stream = File::Stream->new( - $filehandle, - separator => qr/\s*,\s*/, - ); + my $stream = File::Stream->new( + $filehandle, + separator => qr/\s*,\s*/, + ); - print "$_\n" while <$stream>; + print "$_\n" while <$stream>; If you don't have File::Stream, you have to do a little more work. @@ -214,25 +188,25 @@ 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 $_ = ""; - while( sysread FH, $_, 8192, length ) { - while( s/^((?s).*?)your_pattern// ) { - my $record = $1; - # do stuff here. - } - } + local $_ = ""; + while( sysread FH, $_, 8192, length ) { + while( s/^((?s).*?)your_pattern// ) { + my $record = $1; + # 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 ) { - # do stuff here. - } - substr( $_, 0, pos ) = "" if pos; - } + local $_ = ""; + while( sysread FH, $_, 8192, length ) { + foreach my $record ( m/\G((?s).*?)your_pattern/gc ) { + # do stuff here. + } + substr( $_, 0, pos ) = "" if pos; + } =head2 How do I substitute case-insensitively on the LHS while preserving case on the RHS? @@ -242,49 +216,49 @@ 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. - $_= "this is a TEsT case"; + $_= "this is a TEsT case"; - $old = 'test'; - $new = 'success'; + $old = 'test'; + $new = 'success'; - s{(\Q$old\E)} - { uc $new | (uc $1 ^ $1) . - (uc(substr $1, -1) ^ substr $1, -1) x - (length($new) - length $1) - }egi; + s{(\Q$old\E)} + { uc $new | (uc $1 ^ $1) . + (uc(substr $1, -1) ^ substr $1, -1) x + (length($new) - length $1) + }egi; - print; + print; And here it is as a subroutine, modeled after the above: - sub preserve_case($$) { - my ($old, $new) = @_; - my $mask = uc $old ^ $old; + sub preserve_case($$) { + my ($old, $new) = @_; + my $mask = uc $old ^ $old; - uc $new | $mask . - substr($mask, -1) x (length($new) - length($old)) + uc $new | $mask . + substr($mask, -1) x (length($new) - length($old)) } - $string = "this is a TEsT case"; - $string =~ s/(test)/preserve_case($1, "success")/egi; - print "$string\n"; + $string = "this is a TEsT case"; + $string =~ s/(test)/preserve_case($1, "success")/egi; + print "$string\n"; This prints: - this is a SUcCESS case + this is a SUcCESS case As an alternative, to keep the case of the replacement word if it is longer than the original, you can use this code, by Jeff Pinyan: - sub preserve_case { - my ($from, $to) = @_; - my ($lf, $lt) = map length, @_; + sub preserve_case { + my ($from, $to) = @_; + my ($lf, $lt) = map length, @_; - if ($lt < $lf) { $from = substr $from, 0, $lt } - else { $from .= substr $to, $lf } + if ($lt < $lf) { $from = substr $from, 0, $lt } + else { $from .= substr $to, $lf } - return uc $to | ($from ^ uc $from); - } + return uc $to | ($from ^ uc $from); + } This changes the sentence to "this is a SUcCess case." @@ -295,36 +269,36 @@ substitution have the same case, letter by letter, as the original. If the substitution has more characters than the string being substituted, the case of the last character is used for the rest of the substitution. - # Original by Nathan Torkington, massaged by Jeffrey Friedl - # - sub preserve_case($$) - { - my ($old, $new) = @_; - my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc - my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new)); - my ($len) = $oldlen < $newlen ? $oldlen : $newlen; - - for ($i = 0; $i < $len; $i++) { - if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) { - $state = 0; - } elsif (lc $c eq $c) { - substr($new, $i, 1) = lc(substr($new, $i, 1)); - $state = 1; - } else { - substr($new, $i, 1) = uc(substr($new, $i, 1)); - $state = 2; - } - } - # finish up with any remaining new (for when new is longer than old) - if ($newlen > $oldlen) { - if ($state == 1) { - substr($new, $oldlen) = lc(substr($new, $oldlen)); - } elsif ($state == 2) { - substr($new, $oldlen) = uc(substr($new, $oldlen)); - } - } - return $new; - } + # Original by Nathan Torkington, massaged by Jeffrey Friedl + # + sub preserve_case($$) + { + my ($old, $new) = @_; + my $state = 0; # 0 = no change; 1 = lc; 2 = uc + my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new)); + my $len = $oldlen < $newlen ? $oldlen : $newlen; + + for ($i = 0; $i < $len; $i++) { + if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) { + $state = 0; + } elsif (lc $c eq $c) { + substr($new, $i, 1) = lc(substr($new, $i, 1)); + $state = 1; + } else { + substr($new, $i, 1) = uc(substr($new, $i, 1)); + $state = 2; + } + } + # finish up with any remaining new (for when new is longer than old) + if ($newlen > $oldlen) { + if ($state == 1) { + substr($new, $oldlen) = lc(substr($new, $oldlen)); + } elsif ($state == 2) { + substr($new, $oldlen) = uc(substr($new, $oldlen)); + } + } + return $new; + } =head2 How can I make C<\w> match national character sets? X<\w> @@ -356,11 +330,11 @@ a double-quoted string (see L<perlop> 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 = "Placido P. Octopus"; - $regex = "P."; + $string = "Placido P. Octopus"; + $regex = "P."; - $string =~ s/$regex/Polyp/; - # $string is now "Polypacido P. Octopus" + $string =~ s/$regex/Polyp/; + # $string is now "Polypacido P. Octopus" Because C<.> is special in regular expressions, and can match any single character, the regex C<P.> here has matched the <Pl> in the @@ -368,11 +342,11 @@ original string. To escape the special meaning of C<.>, we use C<\Q>: - $string = "Placido P. Octopus"; - $regex = "P."; + $string = "Placido P. Octopus"; + $regex = "P."; - $string =~ s/\Q$regex/Polyp/; - # $string is now "Placido Polyp Octopus" + $string =~ s/\Q$regex/Polyp/; + # $string is now "Placido Polyp Octopus" The use of C<\Q> causes the <.> in the regex to be treated as a regular character, so that C<P.> matches a C<P> followed by a dot. @@ -396,22 +370,22 @@ details. This example takes a regular expression from the argument list and prints the lines of input that match it: - my $pattern = shift @ARGV; + my $pattern = shift @ARGV; - while( <> ) { - print if m/$pattern/; - } + while( <> ) { + print if m/$pattern/; + } Versions of Perl prior to 5.6 would recompile the regular expression for each iteration, even if C<$pattern> had not changed. The C</o> would prevent this by telling Perl to compile the pattern the first time, then reuse that for subsequent iterations: - my $pattern = shift @ARGV; + my $pattern = shift @ARGV; - while( <> ) { - print if m/$pattern/o; # useful for Perl < 5.6 - } + while( <> ) { + print if m/$pattern/o; # useful for Perl < 5.6 + } In versions 5.6 and later, Perl won't recompile the regular expression if the variable hasn't changed, so you probably don't need the C</o> @@ -427,31 +401,31 @@ With Perls before 5.6, you should see C<re> reporting that its compiling the regular expression on each iteration. With Perl 5.6 or later, you should only see C<re> report that for the first iteration. - use re 'debug'; + use re 'debug'; - $regex = 'Perl'; - foreach ( qw(Perl Java Ruby Python) ) { - print STDERR "-" x 73, "\n"; - print STDERR "Trying $_...\n"; - print STDERR "\t$_ is good!\n" if m/$regex/; - } + my $regex = 'Perl'; + foreach ( qw(Perl Java Ruby Python) ) { + print STDERR "-" x 73, "\n"; + print STDERR "Trying $_...\n"; + print STDERR "\t$_ is good!\n" if m/$regex/; + } =head2 How do I use a regular expression to strip C-style comments from a file? While this actually can be done, it's much harder than you'd think. For example, this one-liner - perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c + perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c will work in many but not all cases. You see, it's too simple-minded for certain kinds of C programs, in particular, those with what appear to be comments in quoted strings. For that, you'd need something like this, created by Jeffrey Friedl and later modified by Fred Curtis. - $/ = undef; - $_ = <>; - s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse; - print; + $/ = undef; + $_ = <>; + s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse; + print; This could, of course, be more legibly written with the C</x> modifier, adding whitespace and comments. Here it is expanded, courtesy of Fred Curtis. @@ -505,9 +479,9 @@ X<Text::Balanced> X<Regexp::Common> X<backtracking> X<recursion> (contributed by brian d foy) -Your first try should probably be the C<Text::Balanced> module, which +Your first try should probably be the L<Text::Balanced> module, which is in the Perl standard library since Perl 5.8. It has a variety of -functions to deal with tricky text. The C<Regexp::Common> module can +functions to deal with tricky text. The L<Regexp::Common> module can also help by providing canned patterns you can use. As of Perl 5.10, you can match balanced text with regular expressions @@ -520,9 +494,9 @@ nested angle brackets. This sample text has two "major" groups: a group with one level of nesting and a group with two levels of nesting. There are five total groups in angle brackets: - I have some <brackets in <nested brackets> > and - <another group <nested once <nested twice> > > - and that's it. + I have some <brackets in <nested brackets> > and + <another group <nested once <nested twice> > > + and that's it. The regular expression to match the balanced text uses two new (to Perl 5.10) regular expression features. These are covered in L<perlre> @@ -543,34 +517,34 @@ in the outer capture group as an independent part of the regex. Putting it all together, you have: - #!/usr/local/bin/perl5.10.0 - - my $string =<<"HERE"; - I have some <brackets in <nested brackets> > and - <another group <nested once <nested twice> > > - and that's it. - HERE - - my @groups = $string =~ m/ - ( # start of capture group 1 - < # match an opening angle bracket - (?: - [^<>]++ # one or more non angle brackets, non backtracking - | - (?1) # found < or >, so recurse to capture group 1 - )* - > # match a closing angle bracket - ) # end of capture group 1 - /xg; - - $" = "\n\t"; - print "Found:\n\t@groups\n"; + #!/usr/local/bin/perl5.10.0 + + my $string =<<"HERE"; + I have some <brackets in <nested brackets> > and + <another group <nested once <nested twice> > > + and that's it. + HERE + + my @groups = $string =~ m/ + ( # start of capture group 1 + < # match an opening angle bracket + (?: + [^<>]++ # one or more non angle brackets, non backtracking + | + (?1) # found < or >, so recurse to capture group 1 + )* + > # match a closing angle bracket + ) # end of capture group 1 + /xg; + + $" = "\n\t"; + print "Found:\n\t@groups\n"; The output shows that Perl found the two major groups: - Found: - <brackets in <nested brackets> > - <another group <nested once <nested twice> > > + Found: + <brackets in <nested brackets> > + <another group <nested once <nested twice> > > With a little extra work, you can get the all of the groups in angle brackets even if they are in other angle brackets too. Each time you @@ -578,53 +552,52 @@ get a balanced match, remove its outer delimiter (that's the one you just matched so don't match it again) and add it to a queue of strings to process. Keep doing that until you get no matches: - #!/usr/local/bin/perl5.10.0 + #!/usr/local/bin/perl5.10.0 - my @queue =<<"HERE"; - I have some <brackets in <nested brackets> > and - <another group <nested once <nested twice> > > - and that's it. - HERE + my @queue =<<"HERE"; + I have some <brackets in <nested brackets> > and + <another group <nested once <nested twice> > > + and that's it. + HERE - my $regex = qr/ - ( # start of bracket 1 - < # match an opening angle bracket - (?: - [^<>]++ # one or more non angle brackets, non backtracking - | - (?1) # recurse to bracket 1 - )* - > # match a closing angle bracket - ) # end of bracket 1 - /x; + my $regex = qr/ + ( # start of bracket 1 + < # match an opening angle bracket + (?: + [^<>]++ # one or more non angle brackets, non backtracking + | + (?1) # recurse to bracket 1 + )* + > # match a closing angle bracket + ) # end of bracket 1 + /x; - $" = "\n\t"; + $" = "\n\t"; - while( @queue ) - { - my $string = shift @queue; + while( @queue ) { + my $string = shift @queue; - my @groups = $string =~ m/$regex/g; - print "Found:\n\t@groups\n\n" if @groups; + my @groups = $string =~ m/$regex/g; + print "Found:\n\t@groups\n\n" if @groups; - unshift @queue, map { s/^<//; s/>$//; $_ } @groups; - } + unshift @queue, map { s/^<//; s/>$//; $_ } @groups; + } The output shows all of the groups. The outermost matches show up first and the nested matches so up later: - Found: - <brackets in <nested brackets> > - <another group <nested once <nested twice> > > + Found: + <brackets in <nested brackets> > + <another group <nested once <nested twice> > > - Found: - <nested brackets> + Found: + <nested brackets> - Found: - <nested once <nested twice> > + Found: + <nested once <nested twice> > - Found: - <nested twice> + Found: + <nested twice> =head2 What does it mean that regexes are greedy? How can I get around it? X<greedy> X<greediness> @@ -637,9 +610,9 @@ versions of the same quantifiers, use (C<??>, C<*?>, C<+?>, C<{}?>). An example: - $s1 = $s2 = "I am very very cold"; - $s1 =~ s/ve.*y //; # I am cold - $s2 =~ s/ve.*?y //; # I am very cold + my $s1 = my $s2 = "I am very very cold"; + $s1 =~ s/ve.*y //; # I am cold + $s2 =~ s/ve.*?y //; # I am very cold Notice how the second substitution stopped matching as soon as it encountered "y ". The C<*?> quantifier effectively tells the regular @@ -652,11 +625,11 @@ X<word> Use the split function: - while (<>) { - foreach $word ( split ) { - # do something with $word here - } - } + while (<>) { + foreach my $word ( split ) { + # do something with $word here + } + } Note that this isn't really a word in the English sense; it's just chunks of consecutive non-whitespace characters. @@ -664,11 +637,11 @@ chunks of consecutive non-whitespace characters. To work with only alphanumeric sequences (including underscores), you might consider - while (<>) { - foreach $word (m/(\w+)/g) { - # do something with $word here - } - } + while (<>) { + foreach $word (m/(\w+)/g) { + # do something with $word here + } + } =head2 How can I print out a word-frequency or line-frequency summary? @@ -677,26 +650,29 @@ pretend that by word you mean chunk of alphabetics, hyphens, or apostrophes, rather than the non-whitespace chunk idea of a word given in the previous question: - while (<>) { - while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'" - $seen{$1}++; - } - } + my (%seen); + while (<>) { + while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'" + $seen{$1}++; + } + } - while ( ($word, $count) = each %seen ) { - print "$count $word\n"; - } + while ( my ($word, $count) = each %seen ) { + print "$count $word\n"; + } If you wanted to do the same thing for lines, you wouldn't need a regular expression: - while (<>) { - $seen{$_}++; - } + my (%seen); - while ( ($line, $count) = each %seen ) { - print "$count $line"; - } + while (<>) { + $seen{$_}++; + } + + while ( my ($line, $count) = each %seen ) { + print "$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)?". @@ -704,7 +680,7 @@ 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. +See the module L<String::Approx> available from CPAN. =head2 How do I efficiently match many regular expressions at once? X<regex, efficiency> X<regexp, efficiency> @@ -715,11 +691,11 @@ X<regular expression, efficiency> If you have Perl 5.10 or later, this is almost trivial. You just smart match against an array of regular expression objects: - my @patterns = ( qr/Fr.d/, qr/B.rn.y/, qr/W.lm./ ); - - if( $string ~~ @patterns ) { - ... - }; + my @patterns = ( qr/Fr.d/, qr/B.rn.y/, qr/W.lm./ ); + + if( $string ~~ @patterns ) { + ... + }; The smart match stops when it finds a match, so it doesn't have to try every expression. @@ -730,16 +706,16 @@ In this example, perl must recompile the regular expression for every iteration of the C<foreach> loop since it has no way to know what C<$pattern> will be: - my @patterns = qw( foo bar baz ); + my @patterns = qw( foo bar baz ); - LINE: while( <DATA> ) { - foreach $pattern ( @patterns ) { - if( /\b$pattern\b/i ) { - print; - next LINE; - } - } - } + LINE: while( <DATA> ) { + foreach $pattern ( @patterns ) { + if( /\b$pattern\b/i ) { + print; + next LINE; + } + } + } The C<qr//> operator showed up in perl 5.005. It compiles a regular expression, but doesn't apply it. When you use the pre-compiled @@ -747,31 +723,30 @@ version of the regex, perl does less work. In this example, I inserted a C<map> to turn each pattern into its pre-compiled form. The rest of the script is the same, but faster: - my @patterns = map { qr/\b$_\b/i } qw( foo bar baz ); + my @patterns = map { qr/\b$_\b/i } qw( foo bar baz ); - LINE: while( <> ) { - foreach $pattern ( @patterns ) { - if( /$pattern/ ) - { - print; - next LINE; - } - } - } + LINE: while( <> ) { + foreach $pattern ( @patterns ) { + if( /$pattern/ ) { + print; + 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. - my $regex = join '|', qw( foo bar baz ); + my $regex = join '|', qw( foo bar baz ); - LINE: while( <> ) { - print if /\b(?:$regex)\b/i; - } + LINE: while( <> ) { + print if /\b(?:$regex)\b/i; + } For more details on regular expression efficiency, see I<Mastering -Regular Expressions> by Jeffrey Friedl. He explains how regular -expressions engine work and why some patterns are surprisingly +Regular Expressions> by Jeffrey Friedl. He explains how the regular +expressions engine works and why some patterns are surprisingly inefficient. Once you understand how perl applies regular expressions, you can tune them for individual situations. @@ -797,26 +772,26 @@ 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" + "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 + "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" + "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 ' + "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. @@ -824,13 +799,13 @@ 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 + "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 + "Sam" # no word boundary before "a", but one after "m" + "I am Sam" # "am" surrounded by non-word chars =head2 Why does using $&, $`, or $' slow my program down? @@ -881,8 +856,8 @@ the letter <a> shows up between C<22> and C<44> and you want to stop at C<a>. Simply matching pairs of digits skips over the C<a> and still matches C<44>. - $_ = "1122a44"; - my @pairs = m/(\d\d)/g; # qw( 11 22 44 ) + $_ = "1122a44"; + my @pairs = m/(\d\d)/g; # qw( 11 22 44 ) If you use the C<\G> anchor, you force the match after C<22> to start with the C<a>. The regular expression cannot match @@ -890,28 +865,26 @@ there since it does not find a digit, so the next match fails and the match operator returns the pairs it already found. - $_ = "1122a44"; - my @pairs = m/\G(\d\d)/g; # qw( 11 22 ) + $_ = "1122a44"; + my @pairs = m/\G(\d\d)/g; # qw( 11 22 ) You can also use the C<\G> anchor in scalar context. You still need the C<g> flag. - $_ = "1122a44"; - while( m/\G(\d\d)/g ) - { - print "Found $1\n"; - } + $_ = "1122a44"; + while( m/\G(\d\d)/g ) { + print "Found $1\n"; + } After the match fails at the letter C<a>, perl resets C<pos()> and the next match on the same string starts at the beginning. - $_ = "1122a44"; - while( m/\G(\d\d)/g ) - { - print "Found $1\n"; - } + $_ = "1122a44"; + while( m/\G(\d\d)/g ) { + print "Found $1\n"; + } - print "Found $1 after while" if m/(\d\d)/g; # finds "11" + print "Found $1 after while" if m/(\d\d)/g; # finds "11" You can disable C<pos()> resets on fail with the C<c> flag, documented in L<perlop> and L<perlreref>. Subsequent matches start where the last @@ -921,28 +894,27 @@ the C<while()> loop starts at the C<a> (where the last match stopped), and since it does not use any anchor it can skip over the C<a> to find C<44>. - $_ = "1122a44"; - while( m/\G(\d\d)/gc ) - { - print "Found $1\n"; - } + $_ = "1122a44"; + while( m/\G(\d\d)/gc ) { + print "Found $1\n"; + } - print "Found $1 after while" if m/(\d\d)/g; # finds "44" + print "Found $1 after while" if m/(\d\d)/g; # finds "44" Typically you use the C<\G> anchor with the C<c> flag when you want to try a different match if one fails, such as in a tokenizer. Jeffrey Friedl offers this example which works in 5.004 or later. - while (<>) { - chomp; - PARSER: { - m/ \G( \d+\b )/gcx && do { print "number: $1\n"; redo; }; - m/ \G( \w+ )/gcx && do { print "word: $1\n"; redo; }; - m/ \G( \s+ )/gcx && do { print "space: $1\n"; redo; }; - m/ \G( [^\w\d]+ )/gcx && do { print "other: $1\n"; redo; }; - } - } + while (<>) { + chomp; + PARSER: { + m/ \G( \d+\b )/gcx && do { print "number: $1\n"; redo; }; + m/ \G( \w+ )/gcx && do { print "word: $1\n"; redo; }; + m/ \G( \s+ )/gcx && do { print "space: $1\n"; redo; }; + m/ \G( [^\w\d]+ )/gcx && do { print "other: $1\n"; redo; }; + } + } For each line, the C<PARSER> loop first tries to match a series of digits followed by a word boundary. This match has to @@ -991,8 +963,8 @@ through the Encode module. See L<perluniintro>, L<perlunicode>, and L<Encode>. If you are stuck with older Perls, you can do Unicode with the -C<Unicode::String> module, and character conversions using the -C<Unicode::Map8> and C<Unicode::Map> modules. If you are using +L<Unicode::String> module, and character conversions using the +L<Unicode::Map8> and L<Unicode::Map> modules. If you are using Japanese encodings, you might try using the jperl 5.005_03. Finally, the following set of approaches was offered by Jeffrey @@ -1016,34 +988,37 @@ 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: - # Make sure adjacent "martian" bytes are no longer adjacent. - $martian =~ s/([A-Z][A-Z])/ $1 /g; + # Make sure adjacent "martian" bytes are no longer adjacent. + $martian =~ s/([A-Z][A-Z])/ $1 /g; - print "found GX!\n" if $martian =~ /GX/; + print "found GX!\n" if $martian =~ /GX/; Or like this: - @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g; - # above is conceptually similar to: @chars = $text =~ m/(.)/g; - # - foreach $char (@chars) { - print "found GX!\n", last if $char eq 'GX'; - } + my @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g; + # above is conceptually similar to: my @chars = $text =~ m/(.)/g; + # + foreach my $char (@chars) { + print "found GX!\n", last if $char eq 'GX'; + } Or like this: - while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded - print "found GX!\n", last if $1 eq 'GX'; - } + while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded + if ($1 eq 'GX') { + print "found GX!\n"; + last; + } + } Here's another, slightly less painful, way to do it from Benjamin Goldberg, who uses a zero-width negative look-behind assertion. - print "found GX!\n" if $martian =~ m/ - (?<![A-Z]) - (?:[A-Z][A-Z])*? - GX - /x; + print "found GX!\n" if $martian =~ m/ + (?<![A-Z]) + (?:[A-Z][A-Z])*? + GX + /x; This succeeds if the "martian" character GX is in the string, and fails otherwise. If you don't like using (?<!), a zero-width negative @@ -1054,7 +1029,7 @@ but this usually can be worked around. =head2 How do I match a regular expression that's in a variable? X<regex, in variable> X<eval> X<regex> X<quotemeta> X<\Q, regex> -X<\E, regex>, X<qr//> +X<\E, regex> X<qr//> (contributed by brian d foy) @@ -1068,71 +1043,71 @@ read the regular expression as user input and store it in C<$regex>. Once you have the pattern in C<$regex>, you use that variable in the match operator. - chomp( my $regex = <STDIN> ); + chomp( my $regex = <STDIN> ); - if( $string =~ m/$regex/ ) { ... } + if( $string =~ m/$regex/ ) { ... } Any regular expression special characters in C<$regex> are still special, and the pattern still has to be valid or Perl will complain. For instance, in this pattern there is an unpaired parenthesis. - my $regex = "Unmatched ( paren"; + my $regex = "Unmatched ( paren"; - "Two parens to bind them all" =~ m/$regex/; + "Two parens to bind them all" =~ m/$regex/; When Perl compiles the regular expression, it treats the parenthesis as the start of a memory match. When it doesn't find the closing parenthesis, it complains: - Unmatched ( in regex; marked by <-- HERE in m/Unmatched ( <-- HERE paren/ at script line 3. + Unmatched ( in regex; marked by <-- HERE in m/Unmatched ( <-- HERE paren/ at script line 3. You can get around this in several ways depending on our situation. First, if you don't want any of the characters in the string to be special, you can escape them with C<quotemeta> before you use the string. - chomp( my $regex = <STDIN> ); - $regex = quotemeta( $regex ); + chomp( my $regex = <STDIN> ); + $regex = quotemeta( $regex ); - if( $string =~ m/$regex/ ) { ... } + if( $string =~ m/$regex/ ) { ... } You can also do this directly in the match operator using the C<\Q> and C<\E> sequences. The C<\Q> tells Perl where to start escaping special characters, and the C<\E> tells it where to stop (see L<perlop> for more details). - chomp( my $regex = <STDIN> ); + chomp( my $regex = <STDIN> ); - if( $string =~ m/\Q$regex\E/ ) { ... } + if( $string =~ m/\Q$regex\E/ ) { ... } Alternately, you can use C<qr//>, the regular expression quote operator (see L<perlop> for more details). It quotes and perhaps compiles the pattern, and you can apply regular expression flags to the pattern. - chomp( my $input = <STDIN> ); + chomp( my $input = <STDIN> ); - my $regex = qr/$input/is; + my $regex = qr/$input/is; - $string =~ m/$regex/ # same as m/$input/is; + $string =~ m/$regex/ # same as m/$input/is; You might also want to trap any errors by wrapping an C<eval> block around the whole thing. - chomp( my $input = <STDIN> ); + chomp( my $input = <STDIN> ); - eval { - if( $string =~ m/\Q$input\E/ ) { ... } - }; - warn $@ if $@; + eval { + if( $string =~ m/\Q$input\E/ ) { ... } + }; + warn $@ if $@; Or... - my $regex = eval { qr/$input/is }; - if( defined $regex ) { - $string =~ m/$regex/; - } - else { - warn $@; - } + my $regex = eval { qr/$input/is }; + if( defined $regex ) { + $string =~ m/$regex/; + } + else { + warn $@; + } =head1 AUTHOR AND COPYRIGHT |