MirOS Manual: perlfaq6(1)


PERLFAQ6(1)     Perl Programmers Reference Guide      PERLFAQ6(1)

NAME

     perlfaq6 - Regular Expressions

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 perlfaq9:
     "How do I decode or create those %-encodings on the web" and
     perlfaq4: "How do I determine whether a scalar is a
     number/whole/integer/float", to be precise).

     How can I hope to use regular expressions without creating
     illegible and unmaintainable code?

     Three techniques can make regular expressions maintainable
     and understandable.

     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;

     Comments Inside the Regex
         The "/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.

         "/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

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         pattern.

     Different Delimiters
         While we normally think of patterns as being delimited
         with "/" characters, they can be delimited by almost any
         character.  perlre describes this.  For example, the
         "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

     I'm having trouble matching over more than one line.  What's
     wrong?

     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
     "undef" for the whole file) to allow you to read more than
     one line at a time.

     Read perlre to help you decide which of "/s" and "/m" (or
     both) you might want to use: "/s" allows dot to include new-
     line, and "/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 "/s" because we aren't using dot in a
     regular expression that we want to cross line boundaries.
     Neither do we need "/m" because we aren't wanting caret or
     dollar to match at any point inside the record next to new-
     lines.  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):

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         $/ = '';            # 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";
             }
         }

     How can I pull out lines between two patterns that are them-
     selves on different lines?

     You can use Perl's somewhat exotic ".." operator (documented
     in 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 "START" through "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 "..":

         while (<>) {
             $in_header =   1  .. /^$/;
             $in_body   = /^$/ .. eof();
             # now choose between them
         } continue {
             reset if eof();         # fix $.
         }

     I put a regular expression into $/ but it didn't work.
     What's wrong?

     Up to Perl 5.8.0, $/ has to be a string.  This may change in
     5.10, but don't get your hopes up. Until then, you can use
     these examples if you really need to do this.

     If you have File::Stream, this is easy.

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                              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;
            }

     How do I substitute case insensitively on the LHS while
     preserving case on the RHS?

     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;

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         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 program-
     ming 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 sub-
     stituted, the case of the last character is used for the
     rest of the substitution.

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         # 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;
         }

     How can I make "\w" match national character sets?

     Put "use locale;" in your script.  The \w character class is
     taken from the current locale.

     See perllocale for details.

     How can I match a locale-smart version of "/[a-zA-Z]/"?

     You can use the POSIX character class syntax "/[[:alpha:]]/"
     documented in perlre.

     No matter which locale you are in, the alphabetic characters
     are the characters in \w without the digits and the under-
     score. As a regex, that looks like "/[^\W\d_]/".  Its com-
     plement, the non-alphabetics, is then everything in \W along
     with the digits and the underscore, or "/[\W\d_]/".

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     How can I quote a variable to use in a regex?

     The Perl parser will expand $variable and @variable refer-
     ences in regular expressions unless the delimiter is a sin-
     gle quote.  Remember, too, that the right-hand side of a
     "s///" substitution is considered a double-quoted string
     (see 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 "." is special in regular expressions, and can match
     any single character, the regex "P." here has matched the
     <Pl> in the original string.

     To escape the special meaning of ".", we use "\Q":

         $string = "Placido P. Octopus";
         $regex  = "P.";

         $string =~ s/\Q$regex/Polyp/;
         # $string is now "Placido Polyp Octopus"

     The use of "\Q" causes the <.> in the regex to be treated as
     a regular character, so that "P." matches a "P" followed by
     a dot.

     What is "/o" really for?

     Using a variable in a regular expression match forces a re-
     evaluation (and perhaps recompilation) each time the regular
     expression is encountered.  The "/o" modifier locks in the
     regex 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.

     Use of "/o" is irrelevant unless variable interpolation is
     used in the pattern, and if so, the regex engine will nei-
     ther know nor care whether the variables change after the
     pattern is evaluated the very first time.

     "/o" 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 regex to notice if they
     do.

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     For example, here's a "paragrep" program:

         $/ = '';  # paragraph mode
         $pat = shift;
         while (<>) {
             print if /$pat/o;
         }

     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 particu-
     lar, 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 "/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

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            |         ##     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:

         s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse;

     Can I use Perl regular expressions to match balanced text?

     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 "`" and "'",
     "{" and "}", or "(" and ")" 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.

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     What does it mean that regexes are greedy?  How can I get
     around it?

     Most people mean that greedy regexes match as much as they
     can. Technically speaking, it's actually the quantifiers
     ("?", "*", "+", "{}") that are greedy rather than the whole
     pattern; Perl prefers local greed and immediate gratifica-
     tion to overall greed.  To get non-greedy versions of the
     same quantifiers, use ("??", "*?", "+?", "{}?").

     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 "*?" 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.

     How do I process each word on each line?

     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 under-
     scores), you might consider

         while (<>) {
             foreach $word (m/(\w+)/g) {
                 # do something with $word here
             }
         }

     How can I print out a word-frequency or line-frequency sum-
     mary?

     To do this, you have to parse out each word in the input
     stream.  We'll pretend that by word you mean chunk of alpha-
     betics, hyphens, or apostrophes, rather than the non-
     whitespace chunk idea of a word given in the previous ques-
     tion:

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         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 perlfaq4:
     "How do I sort a hash (optionally by value instead of
     key)?".

     How can I do approximate matching?

     See the module String::Approx available from CPAN.

     How do I efficiently match many regular expressions at once?

     ( 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( <> )
             {
                     foreach $pattern ( @patterns )
                             {
                     print if /\b$pattern\b/i;
                     next LINE;
                             }
                     }

     The qr// operator showed up in perl 5.005.  It compiles a
     regular expression, but doesn't apply it.  When you use the
     pre-compiled version of the regex, perl does less work. In
     this example, I inserted a map() to turn each pattern into
     its pre-compiled form.  The rest of the script is the same,

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     but faster.

         @patterns = map { qr/\b$_\b/i } qw( foo bar baz );

         LINE: while( <> )
             {
                     foreach $pattern ( @patterns )
                             {
                     print if /\b$pattern\b/i;
                     next LINE;
                             }
                     }

     In some cases, you may be able to make several patterns into
     a single regular expression.  Beware of situations that
     require backtracking though.

             $regex = join '|', qw( foo bar baz );

         LINE: while( <> )
             {
                     print if /\b(?:$regex)\b/i;
                     }

     For more details on regular expression efficiency, see Mas-
     tering 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 indivi-
     dual situations.

     Why don't word-boundary searches with "\b" work for me?

     (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 asser-
     tion", 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

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     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

     Why does using $&, $`, or $' slow my program down?

     (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"

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     the way the other two are.

     Since Perl 5.6.1 the special variables @- and @+ can func-
     tionally 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 copy-
     ing.

     What good is "\G" in a regular expression?

     You use the "\G" anchor to start the next match on the same
     string where the last match left off.  The regular expres-
     sion engine cannot skip over any characters to find the next
     match with this anchor, so "\G" is similar to the beginning
     of string anchor, "^".  The "\G" anchor is typically used
     with the "g" flag.  It uses the value of pos() as the posi-
     tion to start the next match.  As the match operator makes
     successive matches, it updates pos() with the position of
     the next character past the last match (or the first charac-
     ter of the next match, depending on how you like to look at
     it). Each string has its own pos() value.

     Suppose you want to match all of consective pairs of digits
     in a string like "1122a44" and stop matching when you
     encounter non-digits.  You want to match 11 and 22 but the
     letter <a> shows up between 22 and 44 and you want to stop
     at "a". Simply matching pairs of digits skips over the "a"
     and still matches 44.

             $_ = "1122a44";
             my @pairs = m/(\d\d)/g;   # qw( 11 22 44 )

     If you use the \G anchor, you force the match after 22 to
     start with the "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 "\G" anchor in scalar context. You
     still need the "g" flag.

             $_ = "1122a44";
             while( m/\G(\d\d)/g )
                     {
                     print "Found $1\n";
                     }

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PERLFAQ6(1)     Perl Programmers Reference Guide      PERLFAQ6(1)

     After the match fails at the letter "a", perl resets pos()
     and the next match on the same string starts at the begin-
     ning.

             $_ = "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 pos() resets on fail with the "c" flag. Sub-
     sequent matches start where the last successful match ended
     (the value of pos()) even if a match on the same string as
     failed in the meantime. In this case, the match after the
     while() loop starts at the "a" (where the last match
     stopped), and since it does not use any anchor it can skip
     over the "a" to find "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 "\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; };
           }
         }

     For each line, the PARSER loop first tries to match a series
     of digits followed by a word boundary.  This match has to
     start at the place the last match left off (or the beginning
     of the string on the first match). Since "m/ \G( \d+\b
     )/gcx" 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 pat-
     tern.

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PERLFAQ6(1)     Perl Programmers Reference Guide      PERLFAQ6(1)

     Are Perl regexes DFAs or NFAs?  Are they POSIX compliant?

     While it's true that Perl's regular expressions resemble the
     DFAs (deterministic finite automata) of the egrep(1) pro-
     gram, 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 perlfaq2).

     What's wrong with using grep in a void context?

     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 con-
     text aware - in void context, no lists are constructed.

     How can I match strings with multibyte characters?

     Starting from Perl 5.6 Perl has had some level of multibyte
     character support.  Perl 5.8 or later is recommended.  Sup-
     ported multibyte character repertoires include Unicode, and
     legacy encodings through the Encode module.  See perluniin-
     tro, perlunicode, and Encode.

     If you are stuck with older Perls, you can do Unicode with
     the "Unicode::String" module, and character conversions
     using the "Unicode::Map8" and "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 Jef-
     frey 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.

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PERLFAQ6(1)     Perl Programmers Reference Guide      PERLFAQ6(1)

     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 "/GX/".
     Perl doesn't know about Martian, so it'll find the two bytes
     "GX" in the "I am CVSGXX!"  string, even though that charac-
     ter 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 prob-
     lem.

     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/;

     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.

perl v5.8.8                2006-06-30                          17

PERLFAQ6(1)     Perl Programmers Reference Guide      PERLFAQ6(1)

     How do I match a pattern that is supplied by the user?

     Well, if it's really a pattern, then just use

         chomp($pattern = <STDIN>);
         if ($line =~ /$pattern/) { }

     Alternatively, since you have no guarantee that your user
     entered a valid regular expression, trap the exception this
     way:

         if (eval { $line =~ /$pattern/ }) { }

     If all you really want is to search for a string, not a pat-
     tern, then you should either use the index() function, which
     is made for string searching, or, if you can't be disabused
     of using a pattern match on a non-pattern, then be sure to
     use "\Q"..."\E", documented in perlre.

         $pattern = <STDIN>;

         open (FILE, $input) or die "Couldn't open input $input: $!; aborting";
         while (<FILE>) {
             print if /\Q$pattern\E/;
         }
         close FILE;

AUTHOR AND COPYRIGHT

     Copyright (c) 1997-2006 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 per-
     mitted 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.

perl v5.8.8                2006-06-30                          18

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