This section is surprisingly small because the rest of the FAQ islittered with answers involving regular expressions. For example,decoding a URL and checking whether something is a number can be handledwith regular expressions, but those answers are found elsewhere inthis document (in perlfaq9: "How do I decode or create those %-encodingson the web" and perlfaq4: "How do I determine whether a scalar isa 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 andunderstandable.

Comments Outside the Regex

Describe what you're doing and how you're doing it, using normal Perlcomments.

 # 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 touse normal comments there, too. As you can imagine, whitespace and commentshelp 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 fordescribing the meaning of each part of the pattern.

Different Delimiters
While we normally think of patterns as being delimited with /characters, they can be delimited by almost any character. perlredescribes this. For example, the s/// above uses braces asdelimiters. Selecting another delimiter can avoid quoting thedelimiter within the pattern:
```
 s/\/usr\/local/\/usr\/share/g; # bad delimiter choice
  s#/usr/local#/usr/share#g; # better
 
```
Using logically paired delimiters can be even more readable:
```
 s{/usr/local/}{/usr/share}g;  # better still
 
```

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 lookingat (probably), or else you aren't using the correct modifier(s) onyour pattern (possibly).

There are many ways to get multiline data into a string. If you wantit to happen automatically while reading input, you'll want to set $/(probably to '' for paragraphs or undef for the whole file) toallow 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 /mallows caret and dollar to match next to a newline, not just at theend of the string. You do need to make sure that you've actuallygot a multiline string in there.

For example, this program detects duplicate words, even when they spanline 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 wantto cross line boundaries. Neither do we need /m because we don'twant caret or dollar to match at any point inside the record nextto newlines. But it's imperative that $/ be set to something otherthan the default, or else we won't actually ever have a multilinerecord read in.

 $/ = '';  # read in whole paragraph, not just one line
  while ( <> ) {
  while ( /\b([\w'-]+)(\s+\g1)+\b/gi ) { # word starts alpha
  print "Duplicate $1 at paragraph $.\n";
  }
  }

Here's some code that finds sentences that begin with "From " (which wouldbe mangled by many mailers):

 $/ = '';  # read in whole paragraph, not just one line
  while ( <> ) {
  while ( /^From /gm ) { # /m makes ^ match next to \n
  print "leading from in paragraph $.\n";
  }
  }

Here's code that finds everything between START and END in a paragraph:

 undef $/;  # read in whole file, not just one line or paragraph
  while ( <> ) {
  while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries
  print "$1\n";
  }
  }

How can I pull out lines between two patterns that are themselves on different lines?

You can use Perl's somewhat exotic .. operator (documented inperlop):

 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'llrun up against the problem described in the question in this sectionon matching balanced text.

Here's another example of using ..:

 while (<>) {
  my $in_header =   1  .. /^$/;
  my $in_body   = /^$/ .. eof;
  # now choose between them
  } continue {
  $. = 0 if eof; # fix $.
  }

How do I match XML, HTML, or other nasty, ugly things with a regex?

Do not use regexes. Use a module and forget about theregular expressions. The XML::LibXML, HTML::TokeParser andHTML::TreeBuilder modules are good starts, although each namespacehas other parsing modules specialized for certain tasks and differentways of doing it. Start at CPAN Search ( http://metacpan.org/ )and wonder at all the work people have done for you already! :)

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

$/ has to be a string. You can use these examples if you really need todo 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 toa buffer. After you add to the buffer, you check if you have acomplete 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 thec flag and the \G anchor, if you do not mind your entire filebeing 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 exploitsproperties of bitwise xor on ASCII strings.

 $_= "this is a TEsT case";
 
  $old = 'test';
  $new = 'success';
 
  s{(\Q$old\E)}
   { uc $new | (uc $1 ^ $1) .
   (uc(substr $1, -1) ^ substr $1, -1) x
   (length($new) - length $1)
   }egi;
 
  print;

And here it is as a subroutine, modeled after the above:

 sub preserve_case($$) {
  my ($old, $new) = @_;
  my $mask = uc $old ^ $old;
 
  uc $new | $mask .
  substr($mask, -1) x (length($new) - length($old))
  }
 
  $string = "this is a TEsT case";
  $string =~ s/(test)/preserve_case($1, "success")/egi;
  print "$string\n";

This prints:

 this is a SUcCESS case

As an alternative, to keep the case of the replacement word if it islonger 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 thesubstitution 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;
  }

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

Put use locale; in your script. The \w character class is takenfrom 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 arethe 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 withthe 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 inregular expressions unless the delimiter is a single quote. Remember,too, that the right-hand side of a s/// substitution is considereda double-quoted string (see perlop for more details). Rememberalso that any regex special characters will be acted on unless youprecede 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 anysingle character, the regex P. here has matched the <Pl> in theoriginal 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 aregular 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 andperlreref) tells Perl to compile the regular expression only once.This is only useful when the pattern contains a variable. Perls 5.6and 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-quotishconstructs, you can interpolate variables into the pattern. See theanswer to "How can I quote a variable to use in a regex?" for moredetails.

This example takes a regular expression from the argument list andprints 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 expressionfor each iteration, even if $pattern had not changed. The /owould prevent this by telling Perl to compile the pattern the firsttime, 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 expressionif the variable hasn't changed, so you probably don't need the /ooption. It doesn't hurt, but it doesn't help either. If you want anyversion of Perl to compile the regular expression only once even ifthe variable changes (thus, only using its initial value), you stillneed the /o.

You can watch Perl's regular expression engine at work to verify foryourself 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 itscompiling the regular expression on each iteration. With Perl 5.6 orlater, you should only see re report that for the first iteration.

 use re 'debug';
 
  my $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 forcertain kinds of C programs, in particular, those with what appear to becomments 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, addingwhitespace and comments. Here it is expanded, courtesy of Fred Curtis.

 s{
       /\* ##  Start of /* ... */ comment
       [^*]*\*+ ##  Non-* followed by 1-or-more *'s
       (
   [^/*][^*]*\*+
       )*  ##  0-or-more things which don't start with /
       ## but do end with '*'
       /   ##  End of /* ... */ comment
 
   | ## OR  various things which aren't comments:
 
       (
   "   ##  Start of " ... " string
   (
       \.   ##  Escaped char
   |   ## OR
       [^"\] ##  Non "\
   )*
   "   ##  End of " ... " string
 
       | ## OR
 
   '   ##  Start of ' ... ' string
   (
       \.   ##  Escaped char
   |   ## OR
       [^'\] ##  Non '\
   )*
   '   ##  End of ' ... ' string
 
       | ## OR
 
   .   ##  Anything other char
   [^/"'\]*   ##  Chars which doesn't start a comment, string or escape
       )
   }{defined $2 ? $2 : ""}gxse;

A slight modification also removes C++ comments, possibly spanning multiple linesusing 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, whichis in the Perl standard library since Perl 5.8. It has a variety offunctions to deal with tricky text. The Regexp::Common module canalso help by providing canned patterns you can use.

As of Perl 5.10, you can match balanced text with regular expressionsusing recursive patterns. Before Perl 5.10, you had to resort tovarious tricks such as using Perl code in (??{}) sequences.

Here's an example using a recursive regular expression. The goal is tocapture all of the text within angle brackets, including the text innested angle brackets. This sample text has two "major" groups: agroup with one level of nesting and a group with two levels ofnesting. There are five total groups in angle brackets:

 I have some <brackets in <nested brackets> > and
  <another group <nested once <nested twice> > >
  and that's it.

The regular expression to match the balanced text uses two new (toPerl 5.10) regular expression features. These are covered in perlreand this example is a modified version of one in that documentation.

First, adding the new possessive + to any quantifier finds thelongest match and does not backtrack. That's important since you wantto handle any angle brackets through the recursion, not backtracking.The group [^<>]++ finds one or more non-angle brackets withoutbacktracking.

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

Putting it all together, you have:

 #!/usr/local/bin/perl5.10.0
 
  my $string =<<"HERE";
  I have some <brackets in <nested brackets> > and
  <another group <nested once <nested twice> > >
  and that's it.
  HERE
 
  my @groups = $string =~ m/
  (   # start of capture group 1
  <   # match an opening angle bracket
  (?:
  [^<>]++ # one or more non angle brackets, non backtracking
   |
  (?1) # found < or >, so recurse to capture group 1
  )*
  >   # match a closing angle bracket
  )   # end of capture group 1
  /xg;
 
  $" = "\n\t"
  print "Found:\n\t@groups\n"

The output shows that Perl found the two major groups:

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

With a little extra work, you can get the all of the groups in anglebrackets even if they are in other angle brackets too. Each time youget a balanced match, remove its outer delimiter (that's the one youjust matched so don't match it again) and add it to a queue of stringsto process. Keep doing that until you get no matches:

 #!/usr/local/bin/perl5.10.0
 
  my @queue =<<"HERE";
  I have some <brackets in <nested brackets> > and
  <another group <nested once <nested twice> > >
  and that's it.
  HERE
 
  my $regex = qr/
  (   # start of bracket 1
  <   # match an opening angle bracket
  (?:
  [^<>]++ # one or more non angle brackets, non backtracking
   |
  (?1) # recurse to bracket 1
  )*
  >   # match a closing angle bracket
  )   # end of bracket 1
  /x;
 
  $" = "\n\t"
 
  while( @queue ) {
  my $string = shift @queue;
 
  my @groups = $string =~ m/$regex/g;
  print "Found:\n\t@groups\n\n" if @groups;
 
  unshift @queue, map { s/^<//; s/>$//; $_ } @groups;
  }

The output shows all of the groups. The outermost matches show upfirst and the nested matches so up later:

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

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 localgreed and immediate gratification to overall greed. To get non-greedyversions of the same quantifiers, use (??, *?, +?, {}?).

An example:

 my $s1 = my $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 itencountered "y ". The *? quantifier effectively tells the regularexpression engine to find a match as quickly as possible and passcontrol on to whatever is next in line, as you would if you wereplaying hot potato.

How do I process each word on each line?

Use the split function:

 while (<>) {
  foreach my $word ( split ) {
  # do something with $word here
  }
  }

Note that this isn't really a word in the English sense; it's justchunks of consecutive non-whitespace characters.

To work with only alphanumeric sequences (including underscores), youmight 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 stream. We'llpretend that by word you mean chunk of alphabetics, hyphens, orapostrophes, rather than the non-whitespace chunk idea of a word givenin the previous question:

 my (%seen);
  while (<>) {
  while ( /(\b[^\W_\d][\w'-]+\b)/g ) {   # misses "`sheep'"
  $seen{$1}++;
  }
  }
 
  while ( my ($word, $count) = each %seen ) {
  print "$count $word\n";
  }

If you wanted to do the same thing for lines, you wouldn't need aregular expression:

 my (%seen);
 
  while (<>) {
  $seen{$_}++;
  }
 
  while ( my ($line, $count) = each %seen ) {
  print "$count $line";
  }

If you want these output in a sorted order, see perlfaq4: "How do Isort 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)

If you have Perl 5.10 or later, this is almost trivial. You just smartmatch against an array of regular expression objects:

 my @patterns = ( qr/Fr.d/, qr/B.rn.y/, qr/W.lm./ );
 
  if( $string ~~ @patterns ) {
  ...
  };

The smart match stops when it finds a match, so it doesn't have to tryevery expression.

Earlier than Perl 5.10, you have a bit of work to do. You want toavoid compiling a regular expression every time you want to match it.In this example, perl must recompile the regular expression for everyiteration of the foreach loop since it has no way to know what$pattern will be:

 my @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 regularexpression, but doesn't apply it. When you use the pre-compiledversion of the regex, perl does less work. In this example, I inserteda map to turn each pattern into its pre-compiled form. The rest ofthe script is the same, but faster:

 my @patterns = map { qr/\b$_\b/i } qw( foo bar baz );
 
  LINE: while( <> ) {
  foreach $pattern ( @patterns ) {
  if( /$pattern/ ) {
  print;
  next LINE;
  }
  }
  }

In some cases, you may be able to make several patterns into a singleregular expression. Beware of situations that require backtrackingthough.

 my $regex = join '|', qw( foo bar baz );
 
  LINE: while( <> ) {
  print if /\b(?:$regex)\b/i;
  }

For more details on regular expression efficiency, see MasteringRegular Expressions by Jeffrey Friedl. He explains how the regularexpressions engine works and why some patterns are surprisinglyinefficient. 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 aword character, \w, and something that isn't a word character. Thatthing that isn't a word character might be \W, but it can also be thestart 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 acondition at a certain position.

For the regular expression, /\bPerl\b/, there has to be a wordboundary before the "P" and after the "l". As long as something otherthan a word character precedes the "P" and succeeds the "l", thepattern 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 fornon-word characters surrounded by word characters. These stringsmatch 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 thereshould 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 theprogram, it provides them on each and every pattern match. That meansthat on every pattern match the entire string will be copied, part of itto $`, part to $&, and part to $'. Thus the penalty is most severe withlong strings and patterns that match often. Avoid $&, $', and $` if youcan, but if you can't, once you've used them at all, use them at willbecause you've already paid the price. Remember that some algorithmsreally appreciate them. As of the 5.005 release, the $& variable is nolonger "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 endof each match (see perlvar for the full story), so they give youessentially the same information, but without the risk of excessivestring copying.

Perl 5.10 added three specials, ${^MATCH}, ${^PREMATCH}, and${^POSTMATCH} to do the same job but without the global performancepenalty. Perl 5.10 only sets these variables if you compile or execute theregular 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 samestring where the last match left off. The regularexpression engine cannot skip over any characters to findthe next match with this anchor, so \G is similar to thebeginning of string anchor, ^. The \G anchor is typicallyused with the g flag. It uses the value of pos()as the position to start the next match. As the matchoperator makes successive matches, it updates pos() with theposition of the next character past the last match (or thefirst character of the next match, depending on how you liketo look at it). Each string has its own pos() value.

Suppose you want to match all of consecutive pairs of digitsin a string like "1122a44" and stop matching when youencounter non-digits. You want to match 11 and 22 butthe letter <a> shows up between 22 and 44 and you wantto stop at a. Simply matching pairs of digits skips overthe 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 tostart with the a. The regular expression cannot matchthere since it does not find a digit, so the next matchfails and the match operator returns the pairs it alreadyfound.

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

You can also use the \G anchor in scalar context. Youstill 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 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 pos() resets on fail with the c flag, documentedin perlop and perlreref. Subsequent matches start where the lastsuccessful match ended (the value of pos()) even if a match on thesame string has failed in the meantime. In this case, the match afterthe 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 find44.

 $_ = "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 flagwhen you want to try a different match if one fails,such as in a tokenizer. Jeffrey Friedl offers this examplewhich 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 seriesof digits followed by a word boundary. This match has tostart at the place the last match left off (or the beginningof the string on the first match). Since m/ \G( \d+\b)/gcx uses the c flag, if the string does not match thatregular expression, perl does not reset pos() and the nextmatch starts at the same position to try a differentpattern.

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 infact implemented as NFAs (non-deterministic finite automata) to allowbacktracking and backreferencing. And they aren't POSIX-style either,because those guarantee worst-case behavior for all cases. (It seemsthat some people prefer guarantees of consistency, even when what'sguaranteed is slowness.) See the book "Mastering Regular Expressions"(from O'Reilly) by Jeffrey Friedl for all the details you could everhope to know on these matters (a full citation appears inperlfaq2).

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 thatyou 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 thispurpose.

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 voidcontext, no lists are constructed.

How can I match strings with multibyte characters?

Starting from Perl 5.6 Perl has had some level of multibyte charactersupport. Perl 5.8 or later is recommended. Supported multibytecharacter repertoires include Unicode, and legacy encodingsthrough the Encode module. See perluniintro, perlunicode,and Encode.

If you are stuck with older Perls, you can do Unicode with theUnicode::String module, and character conversions using theUnicode::Map8 and Unicode::Map modules. If you are usingJapanese encodings, you might try using the jperl 5.005_03.

Finally, the following set of approaches was offered by JeffreyFriedl, whose article in issue #5 of The Perl Journal talks aboutthis very matter.

Let's suppose you have some weird Martian encoding where pairs ofASCII uppercase letters encode single Martian letters (i.e. the twobytes "CV" make a single Martian letter, as do the two bytes "SG","VS", "XX", etc.). Other bytes represent single characters, just likeASCII.

So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode thenine characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'.

Now, say you want to search for the single character /GX/. Perldoesn't know about Martian, so it'll find the two bytes "GX" in the "Iam CVSGXX!" string, even though that character isn't there: it justlooks 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:

 my @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g;
  # above is conceptually similar to: my @chars = $text =~ m/(.)/g;
  #
  foreach my $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
  if ($1 eq 'GX') {
  print "found GX!\n";
  last;
  }
  }

Here's another, slightly less painful, way to do it from BenjaminGoldberg, 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 failsotherwise. If you don't like using (?<!), a zero-width negativelook-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 (oranything else that works with regular expressions). We can put thepattern in a variable for later use.

The match operator is a double quote context, so you can interpolateyour variable just like a double quoted string. In this case, youread the regular expression as user input and store it in $regex.Once you have the pattern in $regex, you use that variable in thematch operator.

 chomp( my $regex = <STDIN> );
 
  if( $string =~ m/$regex/ ) { ... }

Any regular expression special characters in $regex are stillspecial, 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 parenthesisas the start of a memory match. When it doesn't find the closingparenthesis, 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 bespecial, you can escape them with 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 \Qand \E sequences. The \Q tells Perl where to start escapingspecial characters, and the \E tells it where to stop (see perlopfor more details).

 chomp( my $regex = <STDIN> );
 
  if( $string =~ m/\Q$regex\E/ ) { ... }

Alternately, you can use qr//, the regular expression quote operator (seeperlop 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 blockaround 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 $@;
  }

AUTHOR AND COPYRIGHT

This documentation is free; you can redistribute it and/or modify itunder the same terms as Perl itself.

Irrespective of its distribution, all code examples in this fileare hereby placed into the public domain. You are permitted andencouraged to use this code in your own programs for funor for profit as you see fit. A simple comment in the code givingcredit would be courteous but is not required.

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