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diff --git a/chromium/third_party/cygwin/lib/perl5/5.10/pods/perlfaq6.pod b/chromium/third_party/cygwin/lib/perl5/5.10/pods/perlfaq6.pod deleted file mode 100644 index 6bf1428fb5b..00000000000 --- a/chromium/third_party/cygwin/lib/perl5/5.10/pods/perlfaq6.pod +++ /dev/null @@ -1,1004 +0,0 @@ -=head1 NAME - -perlfaq6 - Regular Expressions ($Revision: 10126 $) - -=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 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 more 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"; - } - } - -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 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($$) { - my ($old, $new) = @_; - my $mask = uc $old ^ $old; - - uc $new | $mask . - substr($mask, -1) x (length($new) - length($old)) - } - - $a = "this is a TEsT case"; - $a =~ s/(test)/preserve_case($1, "success")/egi; - print "$a\n"; - -This prints: - - this is a SUcCESS case - -As an alternative, to keep the case of the replacement word if it is -longer than the original, you can use this code, by Jeff Pinyan: - - sub preserve_case { - my ($from, $to) = @_; - my ($lf, $lt) = map length, @_; - - if ($lt < $lf) { $from = substr $from, 0, $lt } - else { $from .= substr $to, $lf } - - return uc $to | ($from ^ uc $from); - } - -This changes the sentence to "this is a SUcCess case." - -Just to show that C programmers can write C in any programming language, -if you prefer a more C-like solution, the following script makes the -substitution have the same case, letter by letter, as the original. -(It also happens to run about 240% slower than the Perlish solution runs.) -If the substitution has more characters than the string being substituted, -the case of the last character is used for the rest of the substitution. - - # Original by Nathan Torkington, massaged by Jeffrey Friedl - # - sub preserve_case($$) - { - 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, as long as they are not -spread over multiple lines using a continuation character): - - s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#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> - -Historically, Perl regular expressions were not capable of matching -balanced text. As of more recent versions of perl including 5.6.1 -experimental features have been added that make it possible to do this. -Look at the documentation for the (??{ }) construct in recent perlre manual -pages to see an example of matching balanced parentheses. Be sure to take -special notice of the warnings present in the manual before making use -of this feature. - -CPAN contains many modules that can be useful for matching text -depending on the context. Damian Conway provides some useful -patterns in Regexp::Common. The module Text::Balanced provides a -general solution to this problem. - -One of the common applications of balanced text matching is working -with XML and HTML. There are many modules available that support -these needs. Two examples are HTML::Parser and XML::Parser. There -are many others. - -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.cpan.org/authors/id/TOMC/scripts/pull_quotes.gz . - -The C::Scan module from CPAN also contains such subs for internal use, -but they are undocumented. - -=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, like 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 ) - -Avoid asking Perl to compile a regular expression every time -you want to match it. In this example, perl must recompile -the regular expression for every iteration of the foreach() -loop since it has no way to know what $pattern will be. - - @patterns = qw( foo bar baz ); - - LINE: while( <DATA> ) - { - foreach $pattern ( @patterns ) - { - if( /\b$pattern\b/i ) - { - print; - next LINE; - } - } - } - -The qr// operator showed up in perl 5.005. It compiles a -regular expression, but doesn't apply it. When you use the -pre-compiled version of the regex, perl does less work. In -this example, I inserted a map() to turn each pattern into -its pre-compiled form. The rest of the script is the same, -but faster. - - @patterns = map { qr/\b$_\b/i } qw( foo bar baz ); - - LINE: while( <> ) - { - foreach $pattern ( @patterns ) - { - print if /$pattern/i; - next LINE; - } - } - -In some cases, you may be able to make several patterns into -a single regular expression. Beware of situations that require -backtracking though. - - $regex = join '|', qw( foo bar baz ); - - LINE: while( <> ) - { - print if /\b(?:$regex)\b/i; - } - -For more details on regular expression efficiency, see Mastering -Regular Expressions by Jeffrey Freidl. He explains how regular -expressions engine work and why some patterns are surprisingly -inefficient. Once you understand how perl applies regular -expressions, you can tune them for individual situations. - -=head2 Why don't word-boundary searches with C<\b> work for me? -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. - -=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 REVISION - -Revision: $Revision: 10126 $ - -Date: $Date: 2007-10-27 21:29:20 +0200 (Sat, 27 Oct 2007) $ - -See L<perlfaq> for source control details and availability. - -=head1 AUTHOR AND COPYRIGHT - -Copyright (c) 1997-2007 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. |