perlfaq6.pod

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=head1 NAME

perlfaq6 - Regexps ($Revision: 1.22 $, $Date: 1998/07/16 14:01:07 $)

=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) /meg;

=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 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/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.

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?

$/ 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:

    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;

=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 national character sets?

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.