X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlfaq6.pod;h=40965d09fcb8779125c70d743c4cccb578104795;hb=2cbeaf93ebca0381b43ae9b18f99fecb381ee394;hp=d21a11157b11c80dea2ae6361fda4836226ff05d;hpb=fc36a67e8855d031b2a6921819d899eb149eee2d;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlfaq6.pod b/pod/perlfaq6.pod index d21a111..40965d0 100644 --- a/pod/perlfaq6.pod +++ b/pod/perlfaq6.pod @@ -1,6 +1,6 @@ =head1 NAME -perlfaq6 - Regexps ($Revision: 1.17 $, $Date: 1997/04/24 22:44:10 $) +perlfaq6 - Regular Expressions =head1 DESCRIPTION @@ -8,48 +8,51 @@ 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 the section on Data and the Networking one on -networking, 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? +X X +X X Three techniques can make regular expressions maintainable and understandable. =over 4 -=item Comments Outside the Regexp +=item Comments Outside the Regex 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) /ge; + # 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 Regexp +=item Comments Inside the Regex -The C modifier causes whitespace to be ignored in a regexp pattern -(except in a character class), and also allows you to use normal -comments there, too. As you can imagine, whitespace and comments help -a lot. +The C modifier causes whitespace to be ignored in a regex pattern +(except in a character class and a few other places), and also allows you to +use normal comments there, too. As you can imagine, whitespace and comments +help a lot. C 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. @@ -62,15 +65,17 @@ describes this. For example, the C 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 =back =head2 I'm having trouble matching over more than one line. What's wrong? +X X X -Either you don't have newlines in your string, or you aren't using the -correct modifier(s) on your pattern. +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 +your pattern (possibly). There are many ways to get multiline data into a string. If you want it to happen automatically while reading input, you'll want to set $/ @@ -92,218 +97,539 @@ 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 more whole paragraph, not just one line - while ( <> ) { - while ( /\b(\w\S+)(\s+\1)+\b/gi ) { - print "Duplicate $1 at paragraph $.\n"; + $/ = ''; # read in whole paragraph, not just one line + while ( <> ) { + while ( /\b([\w'-]+)(\s+\1)+\b/gi ) { # word starts alpha + print "Duplicate $1 at paragraph $.\n"; + } } - } Here's code that finds sentences that begin with "From " (which would be mangled by many mailers): - $/ = ''; # read in more 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/sm ) { # /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<..> You can use Perl's somewhat exotic C<..> operator (documented in L): - 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 -pe '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 through C, you'll run up against the problem described in the question in this section 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 $. + } + +=head2 How do I match XML, HTML, or other nasty, ugly things with a regex? +X X X X X X +X + +(contributed by brian d foy) + +If you just want to get work done, use a module and forget about the +regular expressions. The C and C 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<<
>>, 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 + +=item * Try the recursive regex features in Perl 5.10 and later. See L + +=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! + =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> + +$/ 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. + + 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 $_ = ""; + while( sysread FH, $_, 8192, length ) { + while( s/^((?s).*?)your_pattern// ) { + my $record = $1; + # do stuff here. + } + } + +You can do the same thing with foreach and a match using the +c flag and the \G anchor, if you do not mind your entire file +being in memory at the end. -$/ must be a string, not a regular expression. Awk has to be better -for something. :-) - -Actually, you could do this if you don't mind reading the whole file -into memory: - - undef $/; - @records = split /your_pattern/, ; - -The Net::Telnet module (available from CPAN) has the capability to -wait for a pattern in the input stream, or timeout if it doesn't -appear within a certain time. - - ## Create a file with three lines. - open FH, ">file"; - print FH "The first line\nThe second line\nThe third line\n"; - close FH; - - ## Get a read/write filehandle to it. - $fh = new FileHandle "+ $fh); - - ## Search for the second line and print out the third. - $file->waitfor('/second line\n/'); - print $file->getline; - -=head2 How do I substitute case insensitively on the LHS, but preserving case on the RHS? - -It depends on what you mean by "preserving case". The following -script makes the 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($$) - { - my ($old, $new) = @_; - my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc - my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new)); - my ($len) = $oldlen < $newlen ? $oldlen : $newlen; - - for ($i = 0; $i < $len; $i++) { - if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) { - $state = 0; - } elsif (lc $c eq $c) { - substr($new, $i, 1) = lc(substr($new, $i, 1)); - $state = 1; - } else { - substr($new, $i, 1) = uc(substr($new, $i, 1)); - $state = 2; - } - } - # finish up with any remaining new (for when new is longer than old) - if ($newlen > $oldlen) { - if ($state == 1) { - substr($new, $oldlen) = lc(substr($new, $oldlen)); - } elsif ($state == 2) { - substr($new, $oldlen) = uc(substr($new, $oldlen)); - } - } - return $new; + local $_ = ""; + while( sysread FH, $_, 8192, length ) { + foreach my $record ( m/\G((?s).*?)your_pattern/gc ) { + # do stuff here. + } + substr( $_, 0, pos ) = "" if pos; + } + + +=head2 How do I substitute case insensitively on the LHS while preserving case on the RHS? +X X +X X + +Here's a lovely Perlish solution by Larry Rosler. It exploits +properties of bitwise xor on ASCII strings. + + $_= "this is a TEsT case"; + + $old = 'test'; + $new = 'success'; + + s{(\Q$old\E)} + { uc $new | (uc $1 ^ $1) . + (uc(substr $1, -1) ^ substr $1, -1) x + (length($new) - length $1) + }egi; + + print; + +And here it is as a subroutine, modeled after the above: + + sub preserve_case($$) { + my ($old, $new) = @_; + my $mask = uc $old ^ $old; + + uc $new | $mask . + substr($mask, -1) x (length($new) - length($old)) } - $a = "this is a TEsT case"; - $a =~ s/(test)/preserve_case($1, "success")/gie; - print "$a\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, $lt) = map length, @_; + + if ($lt < $lf) { $from = substr $from, 0, $lt } + else { $from .= substr $to, $lf } + + return uc $to | ($from ^ uc $from); + } + +This changes the sentence to "this is a SUcCess case." + +Just to show that C programmers can write C in any programming language, +if you prefer a more C-like solution, the following script makes the +substitution have the same case, letter by letter, as the original. +(It also happens to run about 240% slower than the Perlish solution runs.) +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; + } -=head2 How can I make C<\w> match accented characters? +=head2 How can I make C<\w> match national character sets? +X<\w> -See L. +Put C in your script. The \w character class is taken +from the current locale. + +See L for details. =head2 How can I match a locale-smart version of C? +X + +You can use the POSIX character class syntax C +documented in L. -One alphabetic character would be C, no matter what locale -you're in. Non-alphabetics would be C (assuming you don't -consider an underscore a letter). +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. -=head2 How can I quote a variable to use in a regexp? +=head2 How can I quote a variable to use in a regex? +X X X The Perl parser will expand $variable and @variable references in regular expressions unless the delimiter is a single quote. Remember, too, that the right-hand side of a C substitution is considered a double-quoted string (see L for more details). Remember -also that any regexp special characters will be acted on unless you +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" - $string =~ s/\Q$lhs/$rhs/; - # $string is now "to sleep no more" +Because C<.> is special in regular expressions, and can match any +single character, the regex C here has matched the in the +original string. -Without the \Q, the regexp would also spuriously match "di". +To escape the special meaning of C<.>, we use C<\Q>: + + $string = "Placido P. Octopus"; + $regex = "P."; + + $string =~ s/\Q$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 matches a C

followed by a dot. =head2 What is C really for? +X X -Using a variable in a regular expression match forces a re-evaluation -(and perhaps recompilation) each time through. The C modifier -locks in the regexp the first time it's used. This always happens in a -constant regular expression, and in fact, the pattern was compiled -into the internal format at the same time your entire program was. +(contributed by brian d foy) -Use of C is irrelevant unless variable interpolation is used in -the pattern, and if so, the regexp engine will neither know nor care -whether the variables change after the pattern is evaluated the I time. +The C option for regular expressions (documented in L and +L) tells Perl to compile the regular expression only once. +This is only useful when the pattern contains a variable. Perls 5.6 +and later handle this automatically if the pattern does not change. -C is often used to gain an extra measure of efficiency by not -performing subsequent evaluations when you know it won't matter -(because you know the variables won't change), or more rarely, when -you don't want the regexp to notice if they do. +Since the match operator C, the substitution operator C, +and the regular expression quoting operator C are double-quotish +constructs, you can interpolate variables into the pattern. See the +answer to "How can I quote a variable to use in a regex?" for more +details. -For example, here's a "paragrep" program: +This example takes a regular expression from the argument list and +prints the lines of input that match it: - $/ = ''; # paragraph mode - $pat = shift; - while (<>) { - print if /$pat/o; - } + my $pattern = shift @ARGV; + + while( <> ) { + print if m/$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 +would prevent this by telling Perl to compile the pattern the first +time, then reuse that for subsequent iterations: + + my $pattern = shift @ARGV; + + while( <> ) { + print if m/$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 +option. It doesn't hurt, but it doesn't help either. If you want any +version of Perl to compile the regular expression only once even if +the variable changes (thus, only using its initial value), you still +need the C. + +You can watch Perl's regular expression engine at work to verify for +yourself if Perl is recompiling a regular expression. The C pragma (comes with Perl 5.005 and later) shows the details. +With Perls before 5.6, you should see C reporting that its +compiling the regular expression on each iteration. With Perl 5.6 or +later, you should only see C report that for the first iteration. + + 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/; + } =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: +created by Jeffrey Friedl and later modified by Fred Curtis. - $/ = undef; - $_ = <>; - s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|\n+|.[^/"'\\]*)#$2#g; - print; + $/ = undef; + $_ = <>; + s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse; + print; This could, of course, be more legibly written with the C modifier, adding -whitespace and comments. +whitespace and comments. Here it is expanded, courtesy of Fred Curtis. + + s{ + /\* ## Start of /* ... */ comment + [^*]*\*+ ## Non-* followed by 1-or-more *'s + ( + [^/*][^*]*\*+ + )* ## 0-or-more things which don't start with / + ## but do end with '*' + / ## End of /* ... */ comment + + | ## OR various things which aren't comments: + + ( + " ## Start of " ... " string + ( + \\. ## Escaped char + | ## OR + [^"\\] ## Non "\ + )* + " ## End of " ... " string + + | ## OR + + ' ## Start of ' ... ' string + ( + \\. ## Escaped char + | ## OR + [^'\\] ## Non '\ + )* + ' ## End of ' ... ' string + + | ## OR + + . ## Anything other char + [^/"'\\]* ## Chars which doesn't start a comment, string or escape + ) + }{defined $2 ? $2 : ""}gxse; + +A slight modification also removes C++ comments, possibly spanning multiple lines +using a continuation character: + + s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//([^\\]|[^\n][\n]?)*?\n|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $3 ? $3 : ""#gse; =head2 Can I use Perl regular expressions to match balanced text? - -Although Perl regular expressions are more powerful than "mathematical" -regular expressions, because they feature conveniences like backreferences -(C<\1> and its ilk), they still aren't powerful enough. You still need -to use non-regexp techniques to parse balanced text, such as the text -enclosed between matching parentheses or braces, for example. - -An elaborate subroutine (for 7-bit ASCII only) to pull out balanced -and possibly nested single chars, like C<`> and C<'>, C<{> and C<}>, -or C<(> and C<)> can be found in -http://www.perl.com/CPAN/authors/id/TOMC/scripts/pull_quotes.gz . - -The C::Scan module from CPAN contains such subs for internal usage, -but they are undocumented. - -=head2 What does it mean that regexps are greedy? How can I get around it? - -Most people mean that greedy regexps match as much as they can. +X X +X X X +X X X X + +(contributed by brian d foy) + +Your first try should probably be the C 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 module can +also help by providing canned patterns you can use. + +As of Perl 5.10, you can match balanced text with regular expressions +using recursive patterns. Before Perl 5.10, you had to resort to +various tricks such as using Perl code in C<(??{})> sequences. + +Here's an example using a recursive regular expression. The goal is to +capture all of the text within angle brackets, including the text in +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 > and + > > + 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 +and this example is a modified version of one in that documentation. + +First, adding the new possessive C<+> to any quantifier finds the +longest match and does not backtrack. That's important since you want +to handle any angle brackets through the recursion, not backtracking. +The group C<< [^<>]++ >> finds one or more non-angle brackets without +backtracking. + +Second, the new C<(?PARNO)> refers to the sub-pattern in the +particular capture buffer given by C. In the following regex, +the first capture buffer finds (and remembers) the balanced text, and +you need that same pattern within the first buffer to get past the +nested text. That's the recursive part. The C<(?1)> uses the pattern +in the outer capture buffer 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 > and + > > + and that's it. + HERE + + my @groups = $string =~ m/ + ( # start of capture buffer 1 + < # match an opening angle bracket + (?: + [^<>]++ # one or more non angle brackets, non backtracking + | + (?1) # found < or >, so recurse to capture buffer 1 + )* + > # match a closing angle bracket + ) # end of capture buffer 1 + /xg; + + $" = "\n\t"; + print "Found:\n\t@groups\n"; + +The output shows that Perl found the two major groups: + + Found: + > + > > + +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 +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 + + my @queue =<<"HERE"; + I have some > and + > > + 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; + + $" = "\n\t"; + + while( @queue ) + { + my $string = shift @queue; + + my @groups = $string =~ m/$regex/g; + print "Found:\n\t@groups\n\n" if @groups; + + unshift @queue, map { s/^$//; $_ } @groups; + } + +The output shows all of the groups. The outermost matches show up +first and the nested matches so up later: + + Found: + > + > > + + Found: + + + Found: + > + + Found: + + +=head2 What does it mean that regexes are greedy? How can I get around it? +X X + +Most people mean that greedy regexes match as much as they can. Technically speaking, it's actually the quantifiers (C, C<*>, C<+>, C<{}>) that are greedy rather than the whole pattern; Perl prefers local greed and immediate gratification to overall greed. To get non-greedy @@ -311,9 +637,9 @@ 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 + $s1 = $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 @@ -321,26 +647,28 @@ expression engine to find a match as quickly as possible and pass control on to whatever is next in line, like you would if you were playing hot potato. -=head2 How do I process each word on each line? +=head2 How do I process each word on each line? +X Use the split function: - while (<>) { - foreach $word ( split ) { - # do something with $word here - } - } + while (<>) { + foreach $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. -To work with only alphanumeric sequences, you might consider +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? @@ -349,175 +677,278 @@ 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}++; + while (<>) { + while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'" + $seen{$1}++; + } } - } - while ( ($word, $count) = each %seen ) { - print "$count $word\n"; - } + + while ( ($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 (<>) { - $seen{$_}++; - } - while ( ($line, $count) = each %seen ) { - print "$count $line"; - } + while (<>) { + $seen{$_}++; + } + + while ( ($line, $count) = each %seen ) { + print "$count $line"; + } -If you want these output in a sorted order, see the section on Hashes. +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? +X X See the module String::Approx available from CPAN. =head2 How do I efficiently match many regular expressions at once? +X X +X + +( contributed by brian d foy ) + +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 C +loop since it has no way to know what $pattern will be. + + @patterns = qw( foo bar baz ); + + LINE: while( ) + { + foreach $pattern ( @patterns ) + { + if( /\b$pattern\b/i ) + { + print; + next LINE; + } + } + } + +The C 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 C 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 $pattern ( @patterns ) + { + if( /$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. + + $regex = join '|', qw( foo bar baz ); + + LINE: while( <> ) + { + print if /\b(?:$regex)\b/i; + } + +For more details on regular expression efficiency, see I 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. -The following is super-inefficient: - - while () { - foreach $pat (@patterns) { - if ( /$pat/ ) { - # do something - } - } - } - -Instead, you either need to use one of the experimental Regexp extension -modules from CPAN (which might well be overkill for your purposes), -or else put together something like this, inspired from a routine -in Jeffrey Friedl's book: - - sub _bm_build { - my $condition = shift; - my @regexp = @_; # this MUST not be local(); need my() - my $expr = join $condition => map { "m/\$regexp[$_]/o" } (0..$#regexp); - my $match_func = eval "sub { $expr }"; - die if $@; # propagate $@; this shouldn't happen! - return $match_func; - } - - sub bm_and { _bm_build('&&', @_) } - sub bm_or { _bm_build('||', @_) } - - $f1 = bm_and qw{ - xterm - (?i)window - }; +=head2 Why don't word-boundary searches with C<\b> work for me? +X<\b> - $f2 = bm_or qw{ - \b[Ff]ree\b - \bBSD\B - (?i)sys(tem)?\s*[V5]\b - }; +(contributed by brian d foy) - # feed me /etc/termcap, prolly - while ( <> ) { - print "1: $_" if &$f1; - print "2: $_" if &$f2; - } +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. -=head2 Why don't word-boundary searches with C<\b> work for me? +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 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 behaviour of all the regexp metacharacters. +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. -Here are examples of the incorrect application of C<\b>, with fixes: +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/. - "two words" =~ /(\w+)\b(\w+)/; # WRONG - "two words" =~ /(\w+)\s+(\w+)/; # right + "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" - " =matchless= text" =~ /\b=(\w+)=\b/; # WRONG - " =matchless= text" =~ /=(\w+)=/; # right +These strings do not 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_" # _ is a word char! + "Perler" # no word char before P, but one after l -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". +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/. -=head2 Why does using $&, $`, or $' slow my program down? + "don't" # the ' char is surrounded by "n" and "t" + "qep'a'" # the ' char is surrounded by "p" and "a" -Because once Perl sees that you need one of these variables anywhere -in the program, it has to provide 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 regexp that contains -capturing parentheses. But if you never use $&, etc., in your script, -then regexps I capturing parentheses won't be penalized. So -avoid $&, $', and $` if you can, but if you can't (and some algorithms -really appreciate them), once you've used them once, use them at will, -because you've already paid the price. +These strings do not match /\b'\b/. -=head2 What good is C<\G> in a regular expression? + "foo'" # there is no word char after non-word ' -The notation C<\G> is used in a match or substitution in conjunction the -C modifier (and ignored if there's no C) to anchor the regular -expression to the point just past where the last match occurred, i.e. the -pos() point. +You can also use the complement of \b, \B, to specify that there +should not be a word boundary. -For example, suppose you had a line of text quoted in standard mail -and Usenet notation, (that is, with leading C> characters), and -you want change each leading C> into a corresponding C<:>. You -could do so in this way: +In the pattern /\Bam\B/, there must be a word character before the "a" +and after the "m". These patterns match /\Bam\B/: - s/^(>+)/':' x length($1)/gem; + "llama" # "am" surrounded by word chars + "Samuel" # same -Or, using C<\G>, the much simpler (and faster): +These strings do not match /\Bam\B/ - s/\G>/:/g; + "Sam" # no word boundary before "a", but one after "m" + "I am Sam" # "am" surrounded by non-word chars -A more sophisticated use might involve a tokenizer. The following -lex-like example is courtesy of Jeffrey Friedl. It did not work in -5.003 due to bugs in that release, but does work in 5.004 or better: - while (<>) { - chomp; - PARSER: { - m/ \G( \d+\b )/gx && do { print "number: $1\n"; redo; }; - m/ \G( \w+ )/gx && do { print "word: $1\n"; redo; }; - m/ \G( \s+ )/gx && do { print "space: $1\n"; redo; }; - m/ \G( [^\w\d]+ )/gx && do { print "other: $1\n"; redo; }; - } - } +=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 $& variable is no +longer "expensive" the way the other two are. + +Since Perl 5.6.1 the special variables @- and @+ can functionally replace +$`, $& and $'. These arrays contain pointers to the beginning and end +of each match (see perlvar for the full story), so they give you +essentially the same information, but without the risk of excessive +string copying. + +Perl 5.10 added three specials, C<${^MATCH}>, C<${^PREMATCH}>, and +C<${^POSTMATCH}> to do the same job but without the global performance +penalty. Perl 5.10 only sets these variables if you compile or execute the +regular expression with the C

modifier. -Of course, that could have been written as - - while (<>) { - chomp; - PARSER: { - if ( /\G( \d+\b )/gx { - print "number: $1\n"; - redo PARSER; - } - if ( /\G( \w+ )/gx { - print "word: $1\n"; - redo PARSER; - } - if ( /\G( \s+ )/gx { - print "space: $1\n"; - redo PARSER; - } - if ( /\G( [^\w\d]+ )/gx { - print "other: $1\n"; - redo PARSER; - } - } - } +=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 +expression engine cannot skip over any characters to find +the next match with this anchor, so C<\G> is similar to the +beginning of string anchor, C<^>. The C<\G> anchor is typically +used with the C flag. It uses the value of C +as the position to start the next match. As the match +operator makes successive matches, it updates C with the +position of the next character past the last match (or the +first character of the next match, depending on how you like +to look at it). Each string has its own C value. + +Suppose you want to match all of consecutive pairs of digits +in a string like "1122a44" and stop matching when you +encounter non-digits. You want to match C<11> and C<22> but +the letter shows up between C<22> and C<44> and you want +to stop at C. Simply matching pairs of digits skips over +the C and still matches C<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. The regular expression cannot match +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 ) + +You can also use the C<\G> anchor in scalar context. You +still need the C flag. + + $_ = "1122a44"; + while( m/\G(\d\d)/g ) + { + print "Found $1\n"; + } + +After the match fails at the letter C, perl resets C +and the next match on the same string starts at the beginning. + + $_ = "1122a44"; + while( m/\G(\d\d)/g ) + { + print "Found $1\n"; + } + + print "Found $1 after while" if m/(\d\d)/g; # finds "11" + +You can disable C resets on fail with the C flag, documented +in L and L. Subsequent matches start where the last +successful match ended (the value of C) even if a match on the +same string has failed in the meantime. In this case, the match after +the C loop starts at the C (where the last match stopped), +and since it does not use any anchor it can skip over the C to find +C<44>. + + $_ = "1122a44"; + while( m/\G(\d\d)/gc ) + { + print "Found $1\n"; + } + + print "Found $1 after while" if m/(\d\d)/g; # finds "44" + +Typically you use the C<\G> anchor with the 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; }; + } + } -But then you lose the vertical alignment of the regular expressions. +For each line, the C 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 +pattern. -=head2 Are Perl regexps DFAs or NFAs? Are they POSIX compliant? +=head2 Are Perl regexes DFAs or NFAs? Are they POSIX compliant? +X X X While it's true that Perl's regular expressions resemble the DFAs (deterministic finite automata) of the egrep(1) program, they are in @@ -530,22 +961,37 @@ 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? +X + +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. -Strictly speaking, nothing. Stylistically speaking, it's not a good -way to write maintainable code. That's because you're using these -constructs not for their return values but rather for their -side-effects, and side-effects can be mystifying. There's no void -grep() that's not better written as a C (well, C, -technically) loop. +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? +X X +X X X + +Starting from Perl 5.6 Perl has had some level of multibyte character +support. Perl 5.8 or later is recommended. Supported multibyte +character repertoires include Unicode, and legacy encodings +through the Encode module. See L, L, +and L. -This is hard, and there's no good way. Perl does not directly support -wide characters. It pretends that a byte and a character are -synonymous. The following set of approaches was offered by Jeffrey -Friedl, whose article in issue #5 of The Perl Journal talks about this -very matter. +If you are stuck with older Perls, you can do Unicode with the +C module, and character conversions using the +C and C 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 +Friedl, whose article in issue #5 of The Perl Journal talks about +this very matter. Let's suppose you have some weird Martian encoding where pairs of ASCII uppercase letters encode single Martian letters (i.e. the two @@ -564,40 +1010,134 @@ 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'' bytes - # are no longer adjacent. - print "found GX!\n" if $martian =~ /GX/; + # Make sure adjacent "martian" bytes are no longer adjacent. + $martian =~ s/([A-Z][A-Z])/ $1 /g; + + 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'; - } + @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 $char eq 'GX'; + } Or like this: - while ($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 + print "found GX!\n", last if $1 eq 'GX'; + } -Or like this: +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 $martian =~ m/ + (? X X X X<\Q, regex> +X<\E, regex>, X + +(contributed by brian d foy) + +We don't have to hard-code patterns into the match operator (or +anything else that works with regular expressions). We can put the +pattern in a variable for later use. + +The match operator is a double quote context, so you can interpolate +your variable just like a double quoted string. In this case, you +read the regular expression as user input and store it in C<$regex>. +Once you have the pattern in C<$regex>, you use that variable in the +match operator. + + chomp( my $regex = ); + + 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 $regex = "Unmatched ( paren"; - die "sorry, Perl doesn't (yet) have Martian support )-:\n"; + "Two parens to bind them all" =~ m/$regex/; -In addition, a sample program which converts half-width to full-width -katakana (in Shift-JIS or EUC encoding) is available from CPAN as +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: -=for Tom make it so + Unmatched ( in regex; marked by <-- HERE in m/Unmatched ( <-- HERE paren/ at script line 3. -There are many double- (and multi-) byte encodings commonly used these -days. Some versions of these have 1-, 2-, 3-, and 4-byte characters, -all mixed. +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 before you use the string. + + chomp( my $regex = ); + $regex = quotemeta( $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 +for more details). + + chomp( my $regex = ); + + if( $string =~ m/\Q$regex\E/ ) { ... } + +Alternately, you can use C, the regular expression quote operator (see +L for more details). It quotes and perhaps compiles the pattern, +and you can apply regular expression flags to the pattern. + + chomp( my $input = ); + + my $regex = qr/$input/is; + + $string =~ m/$regex/ # same as m/$input/is; + +You might also want to trap any errors by wrapping an C block +around the whole thing. + + chomp( my $input = ); + + eval { + if( $string =~ m/\Q$input\E/ ) { ... } + }; + warn $@ if $@; + +Or... + + my $regex = eval { qr/$input/is }; + if( defined $regex ) { + $string =~ m/$regex/; + } + else { + warn $@; + } =head1 AUTHOR AND COPYRIGHT -Copyright (c) 1997 Tom Christiansen and Nathan Torkington. -All rights reserved. See L for distribution information. +Copyright (c) 1997-2010 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. +Irrespective of its distribution, all code examples in this file +are hereby placed into the public domain. You are permitted and +encouraged to use this code in your own programs for fun +or for profit as you see fit. A simple comment in the code giving +credit would be courteous but is not required.