Mention and discourage use of term 'soft reference'

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=head1 NAME

perlfaq6 - Regexps ($Revision: 1.14 $)

=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 the section on Data and the Networking one on
networking, 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 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) /ge;

=item Comments Inside the Regexp

The C</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.

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 newlines in your string, or you aren't using the
correct modifier(s) on your pattern.

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+)(\s+\1)+\b/gi ) {
            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/sm ) { # /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 -pe '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.

=head2 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 

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

=head2 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;

        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

=head2 How can I make C<\w> match accented characters?

See L<perllocale>.

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

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

=head2 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 C<s///> substitution is considered
a double-quoted string (see L<perlop> 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:

    $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".

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

Using a variable in a regular expression match forces a re-evaluation
(and perhaps recompilation) each time through.  The C</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 C</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 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 regexp 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:

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

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

=head2 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
(C<\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 C<`> and C<'>, C<{> and C<}>,
or C<(> and C<)> 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.

=head2 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 (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, 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 the section on Hashes.

=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?

The following is super-inefficient:

    while (<FH>) {
        foreach $pat (@patterns) {
            if ( /$pat/ ) {
                # do something
            }
        }
    }

Instead, you either need to use one of the experimental Regexp extension
modules from CPAN (which might well be overkill for your purposes),
or else put together something like this, inspired from a routine
in Jeffrey Friedl's book:

    sub _bm_build {
        my $condition = shift;
        my @regexp = @_;  # this MUST not be local(); need my()
        my $expr = join $condition => map { "m/\$regexp[$_]/o" } (0..$#regexp);
        my $match_func = eval "sub { $expr }";
        die if $@;  # propagate $@; this shouldn't happen!
        return $match_func;
    }

    sub bm_and { _bm_build('&&', @_) }
    sub bm_or  { _bm_build('||', @_) }

    $f1 = bm_and qw{
            xterm
            (?i)window
    };

    $f2 = bm_or qw{
            \b[Ff]ree\b
            \bBSD\B
            (?i)sys(tem)?\s*[V5]\b
    };

    # feed me /etc/termcap, prolly
    while ( <> ) {
        print "1: $_" if &$f1;
        print "2: $_" if &$f2;
    }

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

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

Here are examples of the incorrect application of C<\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, C<\b> and C<\B>
can still be quite useful.  For an example of the correct use of
C<\b>, see the example of matching duplicate words over multiple
lines.

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

=head2 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 I<without> capturing parentheses won't be penalized. So
avoid $&, $', and $` if you can, but if you can't (and some algorithms
really appreciate them), once you've used them once, use them at will,
because you've already paid the price.

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

The notation C<\G> is used in a match or substitution in conjunction the
C</g> modifier (and ignored if there's no C</g>) to anchor the regular
expression to the point just past where the last match occurred, i.e. the
pos() point.

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

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

Or, using C<\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:

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

Of course, that could have been written as

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

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

=head2 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
L<perlfaq2>).

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

Strictly speaking, nothing.  Stylistically speaking, it's not a good
way to write maintainable code.  That's because you're using these
constructs not for their return values but rather for their
side-effects, and side-effects can be mystifying.  There's no void
grep() that's not better written as a C<for> (well, C<foreach>,
technically) loop.

=head2 How can I match strings with multi-byte 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 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 ``maritan'' 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'; 
   }

Or like this:

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

In addition, a sample program which converts half-width to full-width
katakana (in Shift-JIS or EUC encoding) is available from CPAN as 

=for Tom make it so

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.

=head1 AUTHOR AND COPYRIGHT

Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
All rights reserved.  See L<perlfaq> for distribution information.