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=head1 NAME

perlfaq6 - Regular Expressions ($Revision: 1.30 $, $Date: 2005/02/14 18:25:48 $)

=head1 DESCRIPTION

This section is surprisingly small because the rest of the FAQ is
littered with answers involving regular expressions.  For example,
decoding a URL and checking whether something is a number are handled
with regular expressions, but those answers are found elsewhere in
this document (in L<perlfaq9>: ``How do I decode or create those %-encodings
on the web'' and L<perlfaq4>: ``How do I determine whether a scalar is
a number/whole/integer/float'', to be precise).

=head2 How can I hope to use regular expressions without creating illegible and unmaintainable code?

Three techniques can make regular expressions maintainable and
understandable.

=over 4

=item Comments Outside the Regex

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

    # turn the line into the first word, a colon, and the
    # number of characters on the rest of the line
    s/^(\w+)(.*)/ lc($1) . ":" . length($2) /meg;

=item Comments Inside the Regex

The C</x> modifier causes whitespace to be ignored in a regex pattern
(except in a character class), and also allows you to use normal
comments there, too.  As you can imagine, whitespace and comments help
a lot.

C</x> lets you turn this:

    s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs;

into this:

    s{ <                    # opening angle bracket
        (?:                 # Non-backreffing grouping paren
             [^>'"] *       # 0 or more things that are neither > nor ' nor "
                |           #    or else
             ".*?"          # a section between double quotes (stingy match)
                |           #    or else
             '.*?'          # a section between single quotes (stingy match)
        ) +                 #   all occurring one or more times
       >                    # closing angle bracket
    }{}gsx;                 # replace with nothing, i.e. delete

It's still not quite so clear as prose, but it is very useful for
describing the meaning of each part of the pattern.

=item Different Delimiters

While we normally think of patterns as being delimited with C</>
characters, they can be delimited by almost any character.  L<perlre>
describes this.  For example, the C<s///> above uses braces as
delimiters.  Selecting another delimiter can avoid quoting the
delimiter within the pattern:

    s/\/usr\/local/\/usr\/share/g;      # bad delimiter choice
    s#/usr/local#/usr/share#g;          # better

=back

=head2 I'm having trouble matching over more than one line.  What's wrong?

Either you don't have more than one line in the string you're looking
at (probably), or else you aren't using the correct modifier(s) on
your pattern (possibly).

There are many ways to get multiline data into a string.  If you want
it to happen automatically while reading input, you'll want to set $/
(probably to '' for paragraphs or C<undef> for the whole file) to
allow you to read more than one line at a time.

Read L<perlre> to help you decide which of C</s> and C</m> (or both)
you might want to use: C</s> allows dot to include newline, and C</m>
allows caret and dollar to match next to a newline, not just at the
end of the string.  You do need to make sure that you've actually
got a multiline string in there.

For example, this program detects duplicate words, even when they span
line breaks (but not paragraph ones).  For this example, we don't need
C</s> because we aren't using dot in a regular expression that we want
to cross line boundaries.  Neither do we need C</m> because we aren't
wanting caret or dollar to match at any point inside the record next
to newlines.  But it's imperative that $/ be set to something other
than the default, or else we won't actually ever have a multiline
record read in.

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

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

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

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

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

=head2 How can I pull out lines between two patterns that are themselves on different lines?

You can use Perl's somewhat exotic C<..> operator (documented in
L<perlop>):

    perl -ne 'print if /START/ .. /END/' file1 file2 ...

If you wanted text and not lines, you would use

    perl -0777 -ne 'print "$1\n" while /START(.*?)END/gs' file1 file2 ...

But if you want nested occurrences of C<START> through C<END>, you'll
run up against the problem described in the question in this section
on matching balanced text.

Here's another example of using C<..>:

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

=head2 I put a regular expression into $/ but it didn't work. What's wrong?

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

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

                         use File::Stream;
             my $stream = File::Stream->new(
                  $filehandle,
                  separator => qr/\s*,\s*/,
                  );

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

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

You can use the four argument form of sysread to continually add to
a buffer.  After you add to the buffer, you check if you have a
complete line (using your regular expression).

       local $_ = "";
       while( sysread FH, $_, 8192, length ) {
          while( s/^((?s).*?)your_pattern/ ) {
             my $record = $1;
             # do stuff here.
          }
       }

 You can do the same thing with foreach and a match using the
 c flag and the \G anchor, if you do not mind your entire file
 being in memory at the end.

       local $_ = "";
       while( sysread FH, $_, 8192, length ) {
          foreach my $record ( m/\G((?s).*?)your_pattern/gc ) {
             # do stuff here.
          }
          substr( $_, 0, pos ) = "" if pos;
       }


=head2 How do I substitute case insensitively on the LHS while preserving case on the RHS?

Here's a lovely Perlish solution by Larry Rosler.  It exploits
properties of bitwise xor on ASCII strings.

    $_= "this is a TEsT case";

    $old = 'test';
    $new = 'success';

    s{(\Q$old\E)}
     { uc $new | (uc $1 ^ $1) .
        (uc(substr $1, -1) ^ substr $1, -1) x
            (length($new) - length $1)
     }egi;

    print;

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

    sub preserve_case($$) {
        my ($old, $new) = @_;
        my $mask = uc $old ^ $old;

        uc $new | $mask .
            substr($mask, -1) x (length($new) - length($old))
    }

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

This prints:

    this is a SUcCESS case

As an alternative, to keep the case of the replacement word if it is
longer than the original, you can use this code, by Jeff Pinyan:

  sub preserve_case {
    my ($from, $to) = @_;
    my ($lf, $lt) = map length, @_;

    if ($lt < $lf) { $from = substr $from, 0, $lt }
    else { $from .= substr $to, $lf }

    return uc $to | ($from ^ uc $from);
  }

This changes the sentence to "this is a SUcCess case."

Just to show that C programmers can write C in any programming language,
if you prefer a more C-like solution, the following script makes the
substitution have the same case, letter by letter, as the original.
(It also happens to run about 240% slower than the Perlish solution runs.)
If the substitution has more characters than the string being substituted,
the case of the last character is used for the rest of the substitution.

    # Original by Nathan Torkington, massaged by Jeffrey Friedl
    #
    sub preserve_case($$)
    {
        my ($old, $new) = @_;
        my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc
        my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new));
        my ($len) = $oldlen < $newlen ? $oldlen : $newlen;

        for ($i = 0; $i < $len; $i++) {
            if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) {
                $state = 0;
            } elsif (lc $c eq $c) {
                substr($new, $i, 1) = lc(substr($new, $i, 1));
                $state = 1;
            } else {
                substr($new, $i, 1) = uc(substr($new, $i, 1));
                $state = 2;
            }
        }
        # finish up with any remaining new (for when new is longer than old)
        if ($newlen > $oldlen) {
            if ($state == 1) {
                substr($new, $oldlen) = lc(substr($new, $oldlen));
            } elsif ($state == 2) {
                substr($new, $oldlen) = uc(substr($new, $oldlen));
            }
        }
        return $new;
    }

=head2 How can I make C<\w> match national character sets?

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

See L<perllocale> for details.

=head2 How can I match a locale-smart version of C</[a-zA-Z]/>?

You can use the POSIX character class syntax C</[[:alpha:]]/>
documented in L<perlre>.

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

=head2 How can I quote a variable to use in a regex?

The Perl parser will expand $variable and @variable references in
regular expressions unless the delimiter is a single quote.  Remember,
too, that the right-hand side of a C<s///> substitution is considered
a double-quoted string (see L<perlop> for more details).  Remember
also that any regex special characters will be acted on unless you
precede the substitution with \Q.  Here's an example:

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

    $string =~ s/$regex/Polyp/;
    # $string is now "Polypacido P. Octopus"

Because C<.> is special in regular expressions, and can match any
single character, the regex C<P.> here has matched the <Pl> in the
original string.

To escape the special meaning of C<.>, we use C<\Q>:

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

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

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

=head2 What is C</o> really for?

Using a variable in a regular expression match forces a re-evaluation
(and perhaps recompilation) each time the regular expression is
encountered.  The C</o> modifier locks in the regex the first time
it's used.  This always happens in a constant regular expression, and
in fact, the pattern was compiled into the internal format at the same
time your entire program was.

Use of C</o> is irrelevant unless variable interpolation is used in
the pattern, and if so, the regex engine will neither know nor care
whether the variables change after the pattern is evaluated the I<very
first> time.

C</o> is often used to gain an extra measure of efficiency by not
performing subsequent evaluations when you know it won't matter
(because you know the variables won't change), or more rarely, when
you don't want the regex to notice if they do.

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

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

=head2 How do I use a regular expression to strip C style comments from a file?

While this actually can be done, it's much harder than you'd think.
For example, this one-liner

    perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c

will work in many but not all cases.  You see, it's too simple-minded for
certain kinds of C programs, in particular, those with what appear to be
comments in quoted strings.  For that, you'd need something like this,
created by Jeffrey Friedl and later modified by Fred Curtis.

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

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

    s{
       /\*         ##  Start of /* ... */ comment
       [^*]*\*+    ##  Non-* followed by 1-or-more *'s
       (
         [^/*][^*]*\*+
       )*          ##  0-or-more things which don't start with /
                   ##    but do end with '*'
       /           ##  End of /* ... */ comment

     |         ##     OR  various things which aren't comments:

       (
         "           ##  Start of " ... " string
         (
           \\.           ##  Escaped char
         |               ##    OR
           [^"\\]        ##  Non "\
         )*
         "           ##  End of " ... " string

       |         ##     OR

         '           ##  Start of ' ... ' string
         (
           \\.           ##  Escaped char
         |               ##    OR
           [^'\\]        ##  Non '\
         )*
         '           ##  End of ' ... ' string

       |         ##     OR

         .           ##  Anything other char
         [^/"'\\]*   ##  Chars which doesn't start a comment, string or escape
       )
     }{defined $2 ? $2 : ""}gxse;

A slight modification also removes C++ comments:

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

=head2 Can I use Perl regular expressions to match balanced text?

Historically, Perl regular expressions were not capable of matching
balanced text.  As of more recent versions of perl including 5.6.1
experimental features have been added that make it possible to do this.
Look at the documentation for the (??{ }) construct in recent perlre manual
pages to see an example of matching balanced parentheses.  Be sure to take
special notice of the  warnings present in the manual before making use
of this feature.

CPAN contains many modules that can be useful for matching text
depending on the context.  Damian Conway provides some useful
patterns in Regexp::Common.  The module Text::Balanced provides a
general solution to this problem.

One of the common applications of balanced text matching is working
with XML and HTML.  There are many modules available that support
these needs.  Two examples are HTML::Parser and XML::Parser. There
are many others.

An elaborate subroutine (for 7-bit ASCII only) to pull out balanced
and possibly nested single chars, like C<`> and C<'>, C<{> and C<}>,
or C<(> and C<)> can be found in
http://www.cpan.org/authors/id/TOMC/scripts/pull_quotes.gz .

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

=head2 What does it mean that regexes are greedy?  How can I get around it?

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

An example:

        $s1 = $s2 = "I am very very cold";
        $s1 =~ s/ve.*y //;      # I am cold
        $s2 =~ s/ve.*?y //;     # I am very cold

Notice how the second substitution stopped matching as soon as it
encountered "y ".  The C<*?> quantifier effectively tells the regular
expression engine to find a match as quickly as possible and pass
control on to whatever is next in line, like you would if you were
playing hot potato.

=head2 How do I process each word on each line?

Use the split function:

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

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

To work with only alphanumeric sequences (including underscores), you
might consider

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

=head2 How can I print out a word-frequency or line-frequency summary?

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

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

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

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

If you want these output in a sorted order, see L<perlfaq4>: ``How do I
sort a hash (optionally by value instead of key)?''.

=head2 How can I do approximate matching?

See the module String::Approx available from CPAN.

=head2 How do I efficiently match many regular expressions at once?

( contributed by brian d foy )

Avoid asking Perl to compile a regular expression every time 
you want to match it.  In this example, perl must recompile
the regular expression for every iteration of the foreach()
loop since it has no way to know what $pattern will be.

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

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

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

    LINE: while( <> ) 
        {
                foreach $pattern ( @patterns ) 
                        {
                print if /\b$pattern\b/i;
                next LINE;
                        }
                }
                
In some cases, you may be able to make several patterns into
a single regular expression.  Beware of situations that require
backtracking though.

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

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

For more details on regular expression efficiency, see Mastering
Regular Expressions by Jeffrey Freidl.  He explains how regular
expressions engine work and why some patterns are surprisingly
inefficient.  Once you understand how perl applies regular 
expressions, you can tune them for individual situations.

=head2 Why don't word-boundary searches with C<\b> work for me?

(contributed by brian d foy)

Ensure that you know what \b really does: it's the boundary between a
word character, \w, and something that isn't a word character. That
thing that isn't a word character might be \W, but it can also be the
start or end of the string.

It's not (not!) the boundary between whitespace and non-whitespace,
and it's not the stuff between words we use to create sentences.

In regex speak, a word boundary (\b) is a "zero width assertion",
meaning that it doesn't represent a character in the string, but a
condition at a certain position.

For the regular expression, /\bPerl\b/, there has to be a word
boundary before the "P" and after the "l".  As long as something other
than a word character precedes the "P" and succeeds the "l", the
pattern will match. These strings match /\bPerl\b/.

        "Perl"    # no word char before P or after l
        "Perl "   # same as previous (space is not a word char)
        "'Perl'"  # the ' char is not a word char
        "Perl's"  # no word char before P, non-word char after "l"

These strings do not match /\bPerl\b/.

        "Perl_"   # _ is a word char!
        "Perler"  # no word char before P, but one after l
        
You don't have to use \b to match words though.  You can look for
non-word characters surrrounded by word characters.  These strings
match the pattern /\b'\b/.

        "don't"   # the ' char is surrounded by "n" and "t"
        "qep'a'"  # the ' char is surrounded by "p" and "a"
        
These strings do not match /\b'\b/.

        "foo'"    # there is no word char after non-word '
        
You can also use the complement of \b, \B, to specify that there
should not be a word boundary.

In the pattern /\Bam\B/, there must be a word character before the "a"
and after the "m". These patterns match /\Bam\B/:

        "llama"   # "am" surrounded by word chars
        "Samuel"  # same
        
These strings do not match /\Bam\B/

        "Sam"      # no word boundary before "a", but one after "m"
        "I am Sam" # "am" surrounded by non-word chars


=head2 Why does using $&, $`, or $' slow my program down?

Once Perl sees that you need one of these variables anywhere in
the program, it provides 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 regex that contains capturing
parentheses.  If you never use $&, etc., in your script, then regexes
I<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.

=head2 What good is C<\G> in a regular expression?

You use the C<\G> anchor to start the next match on the same
string where the last match left off.  The regular
expression engine cannot skip over any characters to find
the next match with this anchor, so C<\G> is similar to the
beginning of string anchor, C<^>.  The C<\G> anchor is typically
used with the C<g> flag.  It uses the value of 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() value.

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

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

If you use the \G anchor, you force the match after C<22> to
start with the C<a>.  The regular expression cannot match
there since it does not find a digit, so the next match
fails and the match operator returns the pairs it already
found.

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

You can also use the C<\G> anchor in scalar context. You
still need the C<g> flag.

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

After the match fails at the letter C<a>, perl resets 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<c> flag.
Subsequent matches start where the last successful match
ended (the value of pos()) even if a match on the same
string as failed in the meantime. In this case, the match
after the while() loop starts at the C<a> (where the last
match stopped), and since it does not use any anchor it can
skip over the C<a> to find "44".

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

        print "Found $1 after while" if m/(\d\d)/g; # finds "44"

Typically you use the C<\G> anchor with the C<c> flag
when you want to try a different match if one fails,
such as in a tokenizer. Jeffrey Friedl offers this example
which works in 5.004 or later.

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

For each line, the PARSER loop first tries to match a series
of digits followed by a word boundary.  This match has to
start at the place the last match left off (or the beginning
of the string on the first match). Since C<m/ \G( \d+\b
)/gcx> uses the C<c> flag, if the string does not match that
regular expression, perl does not reset pos() and the next
match starts at the same position to try a different
pattern.

=head2 Are Perl regexes DFAs or NFAs?  Are they POSIX compliant?

While it's true that Perl's regular expressions resemble the DFAs
(deterministic finite automata) of the egrep(1) program, they are in
fact implemented as NFAs (non-deterministic finite automata) to allow
backtracking and backreferencing.  And they aren't POSIX-style either,
because those guarantee worst-case behavior for all cases.  (It seems
that some people prefer guarantees of consistency, even when what's
guaranteed is slowness.)  See the book "Mastering Regular Expressions"
(from O'Reilly) by Jeffrey Friedl for all the details you could ever
hope to know on these matters (a full citation appears in
L<perlfaq2>).

=head2 What's wrong with using grep in a void context?

The problem is that grep builds a return list, regardless of the context.
This means you're making Perl go to the trouble of building a list that
you then just throw away. If the list is large, you waste both time and space.
If your intent is to iterate over the list, then use a for loop for this
purpose.

In perls older than 5.8.1, map suffers from this problem as well.
But since 5.8.1, this has been fixed, and map is context aware - in void
context, no lists are constructed.

=head2 How can I match strings with multibyte characters?

Starting from Perl 5.6 Perl has had some level of multibyte character
support.  Perl 5.8 or later is recommended.  Supported multibyte
character repertoires include Unicode, and legacy encodings
through the Encode module.  See L<perluniintro>, L<perlunicode>,
and L<Encode>.

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

Finally, the following set of approaches was offered by Jeffrey
Friedl, whose article in issue #5 of The Perl Journal talks about
this very matter.

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

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

Now, say you want to search for the single character C</GX/>. Perl
doesn't know about Martian, so it'll find the two bytes "GX" in the "I
am CVSGXX!"  string, even though that character isn't there: it just
looks like it is because "SG" is next to "XX", but there's no real
"GX".  This is a big problem.

Here are a few ways, all painful, to deal with it:

   $martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent ``martian''
                                      # bytes are no longer adjacent.
   print "found GX!\n" if $martian =~ /GX/;

Or like this:

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

Or like this:

   while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) {  # \G probably unneeded
       print "found GX!\n", last if $1 eq 'GX';
   }

Here's another, slightly less painful, way to do it from Benjamin
Goldberg, who uses a zero-width negative look-behind assertion.

        print "found GX!\n" if  $martian =~ m/
                   (?<![A-Z])
                   (?:[A-Z][A-Z])*?
                   GX
                /x;

This succeeds if the "martian" character GX is in the string, and fails
otherwise.  If you don't like using (?<!), a zero-width negative
look-behind assertion, you can replace (?<![A-Z]) with (?:^|[^A-Z]).

It does have the drawback of putting the wrong thing in $-[0] and $+[0],
but this usually can be worked around.

=head2 How do I match a pattern that is supplied by the user?

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

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

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

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

If all you really want 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 C<\Q>...C<\E>, documented
in L<perlre>.

    $pattern = <STDIN>;

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

=head1 AUTHOR AND COPYRIGHT

Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.

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

Irrespective of its distribution, all code examples in this file
are hereby placed into the public domain.  You are permitted and
encouraged to use this code in your own programs for fun
or for profit as you see fit.  A simple comment in the code giving
credit would be courteous but is not required.