//proc/self/root/usr/share/perl5/pod/=head1 NAME
perlfaq6 - Regular Expressions
=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 can be handled
with regular expressions, but those answers are found elsewhere in
this document (in L: "How do I decode or create those %-encodings
on the web" and L: "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?
X X
X X
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 modifier causes whitespace to be ignored in a regex pattern
(except in a character class and a few other places), and also allows you to
use normal comments there, too. As you can imagine, whitespace and comments
help a lot.
C 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
describes this. For example, the C 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
Using logically paired delimiters can be even more readable:
s{/usr/local/}{/usr/share}g; # better still
=back
=head2 I'm having trouble matching over more than one line. What's wrong?
X X X
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 for the whole file) to
allow you to read more than one line at a time.
Read L to help you decide which of C and C (or both)
you might want to use: C allows dot to include newline, and C
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 because we aren't using dot in a regular expression that we want
to cross line boundaries. Neither do we need C because we don't
want 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 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 would
be 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";
}
}
=head2 How can I pull out lines between two patterns that are themselves on different lines?
X<..>
You can use Perl's somewhat exotic C<..> operator (documented in
L):
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 through C, 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 (<>) {
my $in_header = 1 .. /^$/;
my $in_body = /^$/ .. eof;
# now choose between them
} continue {
$. = 0 if eof; # fix $.
}
=head2 How do I match XML, HTML, or other nasty, ugly things with a regex?
X X X X X X
X
Do not use regexes. Use a module and forget about the
regular expressions. The L, L and
L modules are good starts, although each namespace
has other parsing modules specialized for certain tasks and different
ways of doing it. Start at CPAN Search ( L )
and wonder at all the work people have done for you already! :)
=head2 I put a regular expression into $/ but it didn't work. What's wrong?
X<$/, regexes in> X<$INPUT_RECORD_SEPARATOR, regexes in>
X<$RS, regexes in>
$/ has to be a string. You can use these examples if you really need to
do this.
If you have L, 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?
X X
X X
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($$) {
my ($old, $new) = @_;
my $mask = uc $old ^ $old;
uc $new | $mask .
substr($mask, -1) x (length($new) - length($old))
}
$string = "this is a TEsT case";
$string =~ s/(test)/preserve_case($1, "success")/egi;
print "$string\n";
This prints:
this is a SUcCESS case
As an alternative, to keep the case of the replacement word if it is
longer than the original, you can use this code, by Jeff Pinyan:
sub preserve_case {
my ($from, $to) = @_;
my ($lf, $lt) = map length, @_;
if ($lt < $lf) { $from = substr $from, 0, $lt }
else { $from .= substr $to, $lf }
return uc $to | ($from ^ uc $from);
}
This changes the sentence to "this is a SUcCess case."
Just to show that C programmers can write C in any programming language,
if you prefer a more C-like solution, the following script makes the
substitution have the same case, letter by letter, as the original.
(It also happens to run about 240% slower than the Perlish solution runs.)
If the substitution has more characters than the string being substituted,
the case of the last character is used for the rest of the substitution.
# Original by Nathan Torkington, massaged by Jeffrey Friedl
#
sub preserve_case($$)
{
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?
X<\w>
Put C