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


NAME
       perlfaq6 - Regexps ($Revision: 1.2 $, $Date: 1999/04/29
       22:52:11 $)



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 the section on Data and the Networking one on
       networking, 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 Regexp
           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 Regexp
           The /x modifier causes whitespace to be ignored in a
           regexp 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
           pattern.



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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.  the perlre manpage 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 the perlre manpage 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 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 newlines.  But it's imperative that $/ be
       set to something other than the default, or else we won't
       actually ever have a multiline record read in.

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

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






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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/sm ) { # /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 the perlop manpage):

           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?

       $/ must be a string, not a regular expression.  Awk has to
       be better for something. :-)

       Actually, you could do this if you don't mind reading the
       whole file into memory:




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           undef $/;
           @records = split /your_pattern/, <FH>;

       The Net::Telnet module (available from CPAN) has the
       capability to wait for a pattern in the input stream, or
       timeout if it doesn't appear within a certain time.

           ## Create a file with three lines.
           open FH, ">file";
           print FH "The first line\nThe second line\nThe third line\n";
           close FH;

           ## Get a read/write filehandle to it.
           $fh = new FileHandle "+<file";

           ## Attach it to a "stream" object.
           use Net::Telnet;
           $file = new Net::Telnet (-fhopen => $fh);

           ## Search for the second line and print out the third.
           $file->waitfor('/second line\n/');
           print $file->getline;


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

       It depends on what you mean by "preserving case".  The
       following script makes the substitution have the same
       case, letter by letter, as the original.  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;















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               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;
           }

           $a = "this is a TEsT case";
           $a =~ s/(test)/preserve_case($1, "success")/gie;
           print "$a\n";

       This prints:

           this is a SUcCESS case


       How can I make \w match national character sets?

       See the perllocale manpage.

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

       One alphabetic character would be /[^\W\d_]/, no matter
       what locale you're in.  Non-alphabetics would be /[\W\d_]/
       (assuming you don't consider an underscore a letter).

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

       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 the perlop manpage for more details).
       Remember also that any regexp special characters will be
       acted on unless you precede the substitution with \Q.
       Here's an example:






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           $string = "to die?";
           $lhs = "die?";
           $rhs = "sleep no more";

           $string =~ s/\Q$lhs/$rhs/;
           # $string is now "to sleep no more"

       Without the \Q, the regexp would also spuriously match
       "di".

       What is /o really for?

       Using a variable in a regular expression match forces a
       re-evaluation (and perhaps recompilation) each time
       through.  The /o modifier locks in the regexp 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 regexp engine will
       neither 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 regexp to
       notice if they do.

       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
       particular, those with what appear to be comments in
       quoted strings.  For that, you'd need something like this,
       created by Jeffrey Friedl:





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           $/ = undef;
           $_ = <>;
           s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|\n+|.[^/"'\\]*)#$2#g;
           print;

       This could, of course, be more legibly written with the /x
       modifier, adding whitespace and comments.

       Can I use Perl regular expressions to match balanced text?

       Although Perl regular expressions are more powerful than
       "mathematical" regular expressions, because they feature
       conveniences like backreferences (\1 and its ilk), they
       still aren't powerful enough. You still need to use non-
       regexp techniques to parse balanced text, such as the text
       enclosed between matching parentheses or braces, for
       example.

       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.perl.com/CPAN/authors/id/TOMC/scripts/pull_quotes.gz
       .

       The C::Scan module from CPAN contains such subs for
       internal usage, but they are undocumented.

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

       Most people mean that greedy regexps 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:




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           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, 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
       stream.  We'll pretend that by word you mean chunk of
       alphabetics, hyphens, or apostrophes, rather than the non-
       whitespace chunk idea of a word given in the previous
       question:

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

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

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

       If you want these output in a sorted order, see the
       section on Hashes.

       How can I do approximate matching?

       See the module String::Approx available from CPAN.




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       How do I efficiently match many regular expressions at
       once?

       The following is extremely inefficient:

           # slow but obvious way
           @popstates = qw(CO ON MI WI MN);
           while (defined($line = <>)) {
               for $state (@popstates) {
                   if ($line =~ /\b$state\b/i) {
                       print $line;
                       last;
                   }
               }
           }

       That's because Perl has to recompile all those patterns
       for each of the lines of the file.  As of the 5.005
       release, there's a much better approach, one which makes
       use of the new qr// operator:

           # use spiffy new qr// operator, with /i flag even
           use 5.005;
           @popstates = qw(CO ON MI WI MN);
           @poppats   = map { qr/\b$_\b/i } @popstates;
           while (defined($line = <>)) {
               for $patobj (@poppats) {
                   print $line if $line =~ /$patobj/;
               }
           }


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

       Two common misconceptions are that \b is a synonym for
       \s+, and that it's the edge between whitespace characters
       and non-whitespace characters.  Neither is correct.  \b is
       the place between a \w character and a \W character (that
       is, \b is the edge of a "word").  It's a zero-width
       assertion, just like ^, $, and all the other anchors, so
       it doesn't consume any characters.  the perlre manpage
       describes the behaviour of all the regexp metacharacters.

       Here are examples of the incorrect application of \b, with
       fixes:

           "two words" =~ /(\w+)\b(\w+)/;          # WRONG
           "two words" =~ /(\w+)\s+(\w+)/;         # right

           " =matchless= text" =~ /\b=(\w+)=\b/;   # WRONG
           " =matchless= text" =~ /=(\w+)=/;       # right

       Although they may not do what you thought they did, \b and
       \B can still be quite useful.  For an example of the



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       correct use of \b, see the example of matching duplicate
       words over multiple lines.

       An example of using \B is the pattern \Bis\B.  This will
       find occurrences of "is" on the insides of words only, as
       in "thistle", but not "this" or "island".

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

       Because once Perl sees that you need one of these
       variables anywhere in the program, it has to provide them
       on each and every pattern match.  The same mechanism that
       handles these provides for the use of $1, $2, etc., so you
       pay the same price for each regexp that contains capturing
       parentheses. But if you never use $&, etc., in your
       script, then regexps without capturing parentheses won't
       be penalized. So 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.

       What good is \G in a regular expression?

       The notation \G is used in a match or substitution in
       conjunction the /g modifier (and ignored if there's no /g)
       to anchor the regular expression to the point just past
       where the last match occurred, i.e. the pos() point.  A
       failed match resets the position of \G unless the /c
       modifier is in effect.

       For example, suppose you had a line of text quoted in
       standard mail and Usenet notation, (that is, with leading
       > characters), and you want change each leading > into a
       corresponding :.  You could do so in this way:

            s/^(>+)/':' x length($1)/gem;

       Or, using \G, the much simpler (and faster):

           s/\G>/:/g;

       A more sophisticated use might involve a tokenizer.  The
       following lex-like example is courtesy of Jeffrey Friedl.
       It did not work in 5.003 due to bugs in that release, but
       does work in 5.004 or better.  (Note the use of /c, which
       prevents a failed match with /g from resetting the search
       position back to the beginning of the string.)








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           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; };
             }
           }

       Of course, that could have been written as

           while (<>) {
             chomp;
             PARSER: {
                  if ( /\G( \d+\b    )/gcx  {
                       print "number: $1\n";
                       redo PARSER;
                  }
                  if ( /\G( \w+      )/gcx  {
                       print "word: $1\n";
                       redo PARSER;
                  }
                  if ( /\G( \s+      )/gcx  {
                       print "space: $1\n";
                       redo PARSER;
                  }
                  if ( /\G( [^\w\d]+ )/gcx  {
                       print "other: $1\n";
                       redo PARSER;
                  }
             }
           }

       But then you lose the vertical alignment of the regular
       expressions.

       Are Perl regexps 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 the perlfaq2 manpage).






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

       Both grep and map build a return list, regardless of their
       context.  This means you're making Perl go to the trouble
       of building up a return list that you then just ignore.
       That's no way to treat a programming language, you
       insensitive scoundrel!

       How can I match strings with multibyte characters?

       This is hard, and there's no good way.  Perl does not
       directly support wide characters.  It pretends that a byte
       and a character are synonymous.  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 problem.

       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';
          }




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       Or like this:

           die "sorry, Perl doesn't (yet) have Martian support )-:\n";

       There are many double- (and multi-) byte encodings
       commonly used these days.  Some versions of these have 1-,
       2-, 3-, and 4-byte characters, all mixed.

       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/) { }

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

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

       But if all you really want to search for a string, not a
       pattern, 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 the perlre manpage.

           $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) 1997-1999 Tom Christiansen and Nathan
       Torkington.  All rights reserved.

       When included as part of the Standard Version of Perl, or
       as part of its complete documentation whether printed or
       otherwise, this work may be distributed only under the
       terms of Perl's Artistic Licence.  Any distribution of
       this file or derivatives thereof outside of that package
       require that special arrangements be made with copyright
       holder.

       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.




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