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diff --git a/dist/perlfaq/lib/perlfaq6.pod b/dist/perlfaq/lib/perlfaq6.pod new file mode 100644 index 0000000000..d72fee946b --- /dev/null +++ b/dist/perlfaq/lib/perlfaq6.pod @@ -0,0 +1,1149 @@ +=head1 NAME + +perlfaq6 - Regular Expressions + +=head1 DESCRIPTION + +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<perlfaq9>: "How do I decode or create those %-encodings +on the web" and L<perlfaq4>: "How do I determine whether a scalar is +a number/whole/integer/float", to be precise). + +=head2 How can I hope to use regular expressions without creating illegible and unmaintainable code? +X<regex, legibility> X<regexp, legibility> +X<regular expression, legibility> X</x> + +Three techniques can make regular expressions maintainable and +understandable. + +=over 4 + +=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) /meg; + +=item Comments Inside the Regex + +The C</x> 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</x> lets you turn this: + + 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 + +It's still not quite so clear as prose, but it is very useful for +describing the meaning of each part of the pattern. + +=item Different Delimiters + +While we normally think of patterns as being delimited with C</> +characters, they can be delimited by almost any character. L<perlre> +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 + +=back + +=head2 I'm having trouble matching over more than one line. What's wrong? +X<regex, multiline> X<regexp, multiline> X<regular expression, multiline> + +Either you don't have more than one line in the string you're looking +at (probably), or else you aren't using the correct modifier(s) on +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 $/ +(probably to '' for paragraphs or C<undef> for the whole file) to +allow you to read more than one line at a time. + +Read L<perlre> to help you decide which of C</s> and C</m> (or both) +you might want to use: C</s> allows dot to include newline, and C</m> +allows caret and dollar to match next to a newline, not just at the +end of the string. You do need to make sure that you've actually +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 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"; + } + } + +Here's 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"; + } + } + +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"; + } + } + +=head2 How can I pull out lines between two patterns that are themselves on different lines? +X<..> + +You can use Perl's somewhat exotic C<..> operator (documented in +L<perlop>): + + 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 ... + +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 +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<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! + +=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. + + 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<replace, case preserving> X<substitute, case preserving> +X<substitution, case preserving> X<s, case preserving> + +Here's a lovely Perlish solution by Larry Rosler. It exploits +properties of bitwise xor on ASCII strings. + + $_= "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($$) { + my ($old, $new) = @_; + my $mask = uc $old ^ $old; + + uc $new | $mask . + substr($mask, -1) x (length($new) - length($old)) + } + + $string = "this is a TEsT case"; + $string =~ s/(test)/preserve_case($1, "success")/egi; + print "$string\n"; + +This prints: + + this is a SUcCESS case + +As an alternative, to keep the case of the replacement word if it is +longer than the original, you can use this code, by Jeff Pinyan: + + sub preserve_case { + my ($from, $to) = @_; + my ($lf, $lt) = map length, @_; + + if ($lt < $lf) { $from = substr $from, 0, $lt } + else { $from .= substr $to, $lf } + + return uc $to | ($from ^ uc $from); + } + +This changes the sentence to "this is a SUcCess case." + +Just to show that C programmers can write C in any programming language, +if you prefer a more C-like solution, the following script makes the +substitution have the same case, letter by letter, as the original. +(It also happens to run about 240% slower than the Perlish solution runs.) +If the substitution has more characters than the string being substituted, +the case of the last character is used for the rest of the substitution. + + # Original by Nathan Torkington, massaged by Jeffrey Friedl + # + sub preserve_case($$) + { + 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 national character sets? +X<\w> + +Put C<use locale;> in your script. The \w character class is taken +from the current locale. + +See L<perllocale> for details. + +=head2 How can I match a locale-smart version of C</[a-zA-Z]/>? +X<alpha> + +You can use the POSIX character class syntax C</[[:alpha:]]/> +documented in L<perlre>. + +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</[^\W\d_]/>. Its complement, +the non-alphabetics, is then everything in \W along with +the digits and the underscore, or C</[\W\d_]/>. + +=head2 How can I quote a variable to use in a regex? +X<regex, escaping> X<regexp, escaping> X<regular expression, escaping> + +The Perl parser will expand $variable and @variable references in +regular expressions unless the delimiter is a single quote. Remember, +too, that the right-hand side of a C<s///> substitution is considered +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."; + + $string =~ s/$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 +original string. + +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<P.> matches a C<P> followed by a dot. + +=head2 What is C</o> really for? +X</o, regular expressions> X<compile, regular expressions> + +(contributed by brian d foy) + +The C</o> option for regular expressions (documented in L<perlop> and +L<perlreref>) 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. + +Since the match operator C<m//>, the substitution operator C<s///>, +and the regular expression quoting operator C<qr//> 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. + +This example takes a regular expression from the argument list and +prints the lines of input that match it: + + 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</o> +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</o> +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</o>. + +You can watch Perl's regular expression engine at work to verify for +yourself if Perl is recompiling a regular expression. The C<use re +'debug'> pragma (comes with Perl 5.005 and later) shows the details. +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'; + + $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 + +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; + +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. + + 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? +X<regex, matching balanced test> X<regexp, matching balanced test> +X<regular expression, matching balanced test> X<possessive> X<PARNO> +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 +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 +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 <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> +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 group given by C<PARNO>. In the following regex, +the first capture group 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 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"; + +The output shows that Perl found the two major groups: + + 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 +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 <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; + + $" = "\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/^<//; 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: + <nested brackets> + + Found: + <nested once <nested twice> > + + Found: + <nested twice> + +=head2 What does it mean that regexes are greedy? How can I get around it? +X<greedy> X<greediness> + +Most people mean that greedy regexes match as much as they can. +Technically speaking, it's actually the quantifiers (C<?>, C<*>, C<+>, +C<{}>) that are greedy rather than the whole pattern; Perl prefers local +greed and immediate gratification to overall greed. To get non-greedy +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 + +Notice how the second substitution stopped matching as soon as it +encountered "y ". The C<*?> quantifier effectively tells the regular +expression engine to find a match as quickly as possible and pass +control on to whatever is next in line, as you would if you were +playing hot potato. + +=head2 How do I process each word on each line? +X<word> + +Use the split function: + + 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 (including underscores), you +might consider + + while (<>) { + foreach $word (m/(\w+)/g) { + # do something with $word here + } + } + +=head2 How can I print out a word-frequency or line-frequency summary? + +To do this, you have to parse out each word in the input stream. We'll +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{$1}++; + } + } + + while ( ($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"; + } + +If you want these output in a sorted order, see L<perlfaq4>: "How do I +sort a hash (optionally by value instead of key)?". + +=head2 How can I do approximate matching? +X<match, approximate> X<matching, approximate> + +See the module String::Approx available from CPAN. + +=head2 How do I efficiently match many regular expressions at once? +X<regex, efficiency> X<regexp, efficiency> +X<regular expression, efficiency> + +(contributed by brian d foy) + +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( $string ~~ @patterns ) { + ... + }; + +The smart match stops when it finds a match, so it doesn't have to try +every expression. + +Earlier than Perl 5.10, you have a bit of work to do. You want to +avoid compiling a regular expression every time you want to match it. +In this example, perl must recompile the regular expression for every +iteration of the C<foreach> loop since it has no way to know what +C<$pattern> will be: + + my @patterns = qw( foo bar baz ); + + 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 +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 ); + + 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 $regex = join '|', qw( foo bar baz ); + + LINE: while( <> ) { + print if /\b(?:$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 +inefficient. Once you understand how perl applies regular expressions, +you can tune them for individual situations. + +=head2 Why don't word-boundary searches with C<\b> work for me? +X<\b> + +(contributed by brian d foy) + +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 + +You don't have to use \b to match words though. You can look for +non-word characters surrounded by word characters. These strings +match the pattern /\b'\b/. + + "don't" # the ' char is surrounded by "n" and "t" + "qep'a'" # the ' char is surrounded by "p" and "a" + +These strings do not match /\b'\b/. + + "foo'" # there is no word char after non-word ' + +You can also use the complement of \b, \B, to specify that there +should not be a word boundary. + +In the pattern /\Bam\B/, there must be a word character before the "a" +and after the "m". These patterns match /\Bam\B/: + + "llama" # "am" surrounded by word chars + "Samuel" # same + +These strings do not match /\Bam\B/ + + "Sam" # no word boundary before "a", but one after "m" + "I am Sam" # "am" surrounded by non-word chars + + +=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</p> modifier. + +=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<g> flag. It uses the value of C<pos()> +as the position to start the next match. As the match +operator makes successive matches, it updates C<pos()> 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<pos()> 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 <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 ) + +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 +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<g> flag. + + $_ = "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"; + } + + 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 +successful match ended (the value of C<pos()>) even if a match on the +same string has failed in the meantime. In this case, the match after +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"; + } + + 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; }; + } + } + +For each line, the C<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<m/ \G( \d+\b +)/gcx> uses the C<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 regexes DFAs or NFAs? Are they POSIX compliant? +X<DFA> X<NFA> X<POSIX> + +While it's true that Perl's regular expressions resemble the DFAs +(deterministic finite automata) of the egrep(1) program, they are in +fact implemented as NFAs (non-deterministic finite automata) to allow +backtracking and backreferencing. And they aren't POSIX-style either, +because those guarantee worst-case behavior for all cases. (It seems +that some people prefer guarantees of consistency, even when what's +guaranteed is slowness.) See the book "Mastering Regular Expressions" +(from O'Reilly) by Jeffrey Friedl for all the details you could ever +hope to know on these matters (a full citation appears in +L<perlfaq2>). + +=head2 What's wrong with using grep in a void context? +X<grep> + +The problem is that grep builds a return list, regardless of the context. +This means you're making Perl go to the trouble of building a list that +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? +X<regex, and multibyte characters> X<regexp, and multibyte characters> +X<regular expression, and multibyte characters> X<martian> X<encoding, Martian> + +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<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 +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 +bytes "CV" make a single Martian letter, as do the two bytes "SG", +"VS", "XX", etc.). Other bytes represent single characters, just like +ASCII. + +So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the +nine characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'. + +Now, say you want to search for the single character C</GX/>. Perl +doesn't know about Martian, so it'll find the two bytes "GX" in the "I +am CVSGXX!" string, even though that character isn't there: it just +looks like it is because "SG" is next to "XX", but there's no real +"GX". This is a big problem. + +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; + + 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 $char eq 'GX'; + } + +Or like this: + + while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded + print "found GX!\n", last if $1 eq 'GX'; + } + +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; + +This succeeds if the "martian" character GX is in the string, and fails +otherwise. If you don't like using (?<!), a zero-width negative +look-behind assertion, you can replace (?<![A-Z]) with (?:^|[^A-Z]). + +It does have the drawback of putting the wrong thing in $-[0] and $+[0], +but this usually can be worked around. + +=head2 How do I match a regular expression that's in a variable? +X<regex, in variable> X<eval> X<regex> X<quotemeta> X<\Q, regex> +X<\E, regex>, X<qr//> + +(contributed by brian d foy) + +We don't have to hard-code patterns into the match operator (or +anything else that works with regular expressions). We can put the +pattern in a variable for later use. + +The match operator is a double quote context, so you can interpolate +your variable just like a double quoted string. In this case, you +read the regular expression as user input and store it in C<$regex>. +Once you have the pattern in C<$regex>, you use that variable in the +match operator. + + chomp( my $regex = <STDIN> ); + + 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 $regex = "Unmatched ( paren"; + + "Two parens to bind them all" =~ m/$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. + +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 ); + + 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> ); + + 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> ); + + 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<eval> block +around the whole thing. + + chomp( my $input = <STDIN> ); + + 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) 1997-2010 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. |