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perlfaq6 (1)


perlfaq6 - Regular Expressions


Please see following description for synopsis


Perl Programmers Reference Guide                      PERLFAQ6(1)

     perlfaq6 - Regular Expressions

     This section is surprisingly small because the rest of the
     FAQ is littered with answers involving regular expressions.
     For example, decoding a URL and checking whether something
     is a number are handled 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 a few
         other places), 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:


         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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     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
     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
     newline, 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

     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
     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+\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 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

             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
     themselves 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 {
                     $. = 0 if eof;  # fix $.

  How do I match XML, HTML, or other nasty, ugly things with a
     (contributed by brian d foy)

     If you just want to get work done, use a module and forget
     about the regular expressions. The "XML::Parser" and
     "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 ( ) and wonder at all the work people

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     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
     ("<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:

     o   Solve the balanced text problem from another question in

     o   Try the recursive regex features in Perl 5.10 and later.
         See perlre

     o   Try defining a grammar using Perl 5.10's "(?DEFINE)"

     o   Break the problem down into sub-problems instead of
         trying to use a single regex

     o   Convince everyone not to use XML or HTML in the first

     Good luck!

  I put a regular expression into $/ but it didn't work. What's
     $/ 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(
                     separator => qr/\s*,\s*/,

             print "$_\n" while <$stream>;

     If you don't have File::Stream, you have to do a little more

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     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';

             { uc $new | (uc $1 ^ $1) .
                     (uc(substr $1, -1) ^ substr $1, -1) x
                     (length($new) - length $1)


     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))

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             $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;

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                     # 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
     underscore.  As a regex, that looks like "/[^\W\d_]/".  Its
     complement, the non-alphabetics, is then everything in \W
     along with the digits and the underscore, or "/[\W\d_]/".

  How can I quote a variable to use in a regex?
     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
     "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.";

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             $string =~ s/\Q$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?
     (contributed by brian d foy)

     The "/o" option for regular expressions (documented in
     perlop and 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 "m//", the substitution operator
     "s///", and the regular expression quoting operator "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 $pattern had not
     changed. The "/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 "/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

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     You can watch Perl's regular expression engine at work to
     verify for yourself if Perl is recompiling a regular
     expression. The "use re 'debug'" pragma (comes with Perl
     5.005 and later) shows the details.  With Perls before 5.6,
     you should see "re" reporting that its compiling the regular
     expression on each iteration. With Perl 5.6 or later, you
     should only see "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/;

  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;

     This could, of course, be more legibly written with the "/x"
     modifier, adding whitespace and comments.  Here it is
     expanded, courtesy of Fred Curtis.

            /\*         ##  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

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                \\.           ##  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;

  Can I use Perl regular expressions to match balanced text?
     (contributed by brian d foy)

     Your first try should probably be the "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 "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
     "(??{})" 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:

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             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 perlre and this example is a modified version of
     one in that documentation.

     First, adding the new possessive "+" 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 "[^<>]++" finds one
     or more non-angle brackets without backtracking.

     Second, the new "(?PARNO)" refers to the sub-pattern in the
     particular capture buffer given by "PARNO". In the following
     regex, the first capture buffer finds (and remembers) the
     balanced text, and you  need that same pattern within the
     first buffer to get past the nested text. That's the
     recursive part. The "(?1)" uses the pattern in the outer
     capture buffer as an independent part of the regex.

     Putting it all together, you have:


             my $string =<<"HERE";
             I have some <brackets in <nested brackets> > and
             <another group <nested once <nested twice> > >
             and that's it.

             my @groups = $string =~ m/
                             (                   # start of capture buffer 1
                             <                   # match an opening angle bracket
                                             [^<>]++     # one or more non angle brackets, non backtracking
                                             (?1)        # found < or >, so recurse to capture buffer 1
                             >                   # match a closing angle bracket
                             )                   # end of capture buffer 1

             $" = "\n\t";
             print "Found:\n\t@groups\n";

     The output shows that Perl found the two major groups:

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                     <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:


             my @queue =<<"HERE";
             I have some <brackets in <nested brackets> > and
             <another group <nested once <nested twice> > >
             and that's it.

             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

             $" = "\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:

                     <brackets in <nested brackets> >
                     <another group <nested once <nested twice> > >

                     <nested brackets>

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                     <nested once <nested twice> >

                     <nested twice>

  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
     gratification 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
     underscores), 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 summary?

     To do this, you have to parse out each word in the input

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     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

             while (<>) {
                     while ( /(\b[^\W_\d][\w'-]+\b)/g ) {   # misses "`sheep'"

             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 (<>) {

             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

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

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     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 $pattern ( @patterns )
                             if( /$pattern/ )
                                     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.

  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.

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     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

             "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)

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     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

     Perl 5.10 added three specials, "${^MATCH}", "${^PREMATCH}",
     and "${^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 "/p" modifier.

  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
     expression 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 position 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 character of the next match, depending on how
     you like to look at it). Each string has its own "pos()"

     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 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

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     fails and the match operator returns the pairs it already

             $_ = "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";

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

             $_ = "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,
     documented in perlop and perlreref. Subsequent matches start
     where the last successful match ended (the value of "pos()")
     even if a match on the same string has 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

             $_ = "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.

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             while (<>) {
                     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

  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)
     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 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
     context 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.
     Supported multibyte character repertoires include Unicode,
     and legacy encodings through the Encode module.  See
     perluniintro, perlunicode, and Encode.

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     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
     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 "/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

     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.

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             print "found GX!\n" if  $martian =~ m/

     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.

  How do I match a regular expression that's in a variable? ,

     (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 $regex.  Once you have the pattern in
     $regex, you use that variable in the match operator.

             chomp( my $regex = <STDIN> );

             if( $string =~ m/$regex/ ) { ... }

     Any regular expression special characters in $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
     "quotemeta" before you use the string.

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             chomp( my $regex = <STDIN> );
             $regex = quotemeta( $regex );

             if( $string =~ m/$regex/ ) { ... }

     You can also do this directly in the match operator using
     the "\Q" and "\E" sequences. The "\Q" tells Perl where to
     start escaping special characters, and the "\E" tells it
     where to stop (see perlop for more details).

             chomp( my $regex = <STDIN> );

             if( $string =~ m/\Q$regex\E/ ) { ... }

     Alternately, you can use "qr//", the regular expression
     quote operator (see 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 "eval"
     block around the whole thing.

             chomp( my $input = <STDIN> );

             eval {
                     if( $string =~ m/\Q$input\E/ ) { ... }
             warn $@ if $@;


             my $regex = eval { qr/$input/is };
             if( defined $regex ) {
                     $string =~ m/$regex/;
             else {
                     warn $@;

     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.

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     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.

     See attributes(5) for descriptions of the following

     |Availability   | runtime/perl-512 |
     |Stability      | Uncommitted      |
     This software was built from source available at  The original
     community source was downloaded from

     Further information about this software can be found on the
     open source community website at

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