=head1 NAME
-perlfaq6 - Regexps ($Revision: 1.16 $, $Date: 1997/03/25 18:16:56 $)
+perlfaq6 - Regular Expressions ($Revision: 10126 $)
=head1 DESCRIPTION
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).
+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?
+=head2 How can I hope to use regular expressions without creating illegible and unmaintainable code?
+X<regex, legibility> X<regexp, legibility>
+X<regular expression, legibility> X</x>
Three techniques can make regular expressions maintainable and
understandable.
=over 4
-=item Comments Outside the Regexp
+=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) /ge;
+ # 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
+=item Comments Inside the Regex
-The C</x> modifier causes whitespace to be ignored in a regexp pattern
+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;
+ 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
+ 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.
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
+ 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?
+X<regex, multiline> X<regexp, multiline> X<regular expression, multiline>
-Either you don't have newlines in your string, or you aren't using the
-correct modifier(s) on your pattern.
+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 $/
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";
- }
- }
+ $/ = ''; # 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";
+ $/ = ''; # 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";
+ 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<perlop>):
- perl -ne 'print if /START/ .. /END/' file1 file2 ...
+ 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 ...
+ 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 {
+ $. = 0 if eof; # fix $.
+ }
+
=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 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?
+X<replace, case preserving> X<substitute, case preserving>
+X<substitution, case preserving> X<s, case preserving>
+
+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 $1)
+ }egi;
-$/ 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>;
-
-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($$)
- {
- my ($old, $new) = @_;
- my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc
- my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new));
- my ($len) = $oldlen < $newlen ? $oldlen : $newlen;
-
- for ($i = 0; $i < $len; $i++) {
- if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) {
- $state = 0;
- } elsif (lc $c eq $c) {
- substr($new, $i, 1) = lc(substr($new, $i, 1));
- $state = 1;
- } else {
- substr($new, $i, 1) = uc(substr($new, $i, 1));
- $state = 2;
- }
- }
- # finish up with any remaining new (for when new is longer than old)
- if ($newlen > $oldlen) {
- if ($state == 1) {
- substr($new, $oldlen) = lc(substr($new, $oldlen));
- } elsif ($state == 2) {
- substr($new, $oldlen) = uc(substr($new, $oldlen));
- }
- }
- return $new;
+ print;
+
+And here it is as a subroutine, modeled after the above:
+
+ sub preserve_case($$) {
+ 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($1, "success")/gie;
- print "$a\n";
+ $a = "this is a TEsT case";
+ $a =~ s/(test)/preserve_case($1, "success")/egi;
+ print "$a\n";
This prints:
- this is a SUcCESS case
+ 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>
-=head2 How can I make C<\w> match accented characters?
+Put C<use locale;> in your script. The \w character class is taken
+from the current locale.
-See L<perllocale>.
+See L<perllocale> for details.
=head2 How can I match a locale-smart version of C</[a-zA-Z]/>?
+X<alpha>
+
+You can use the POSIX character class syntax C</[[:alpha:]]/>
+documented in L<perlre>.
-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).
+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 regexp?
+=head2 How can I quote a variable to use in a regex?
+X<regex, escaping> X<regexp, escaping> X<regular expression, escaping>
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
+also that any regex 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 = "Placido P. Octopus";
+ $regex = "P.";
- $string =~ s/\Q$lhs/$rhs/;
- # $string is now "to sleep no more"
+ $string =~ s/$regex/Polyp/;
+ # $string is now "Polypacido P. Octopus"
-Without the \Q, the regexp would also spuriously match "di".
+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.
-=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.
+To escape the special meaning of C<.>, we use C<\Q>:
-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.
+ $string = "Placido P. Octopus";
+ $regex = "P.";
-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.
+ $string =~ s/\Q$regex/Polyp/;
+ # $string is now "Placido Polyp Octopus"
-For example, here's a "paragrep" program:
+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.
- $/ = ''; # paragraph mode
- $pat = shift;
- while (<>) {
- print if /$pat/o;
- }
+=head2 What is C</o> really for?
+X</o, regular expressions> X<compile, regular expressions>
+
+(contributed by brian d foy)
+
+The C</o> option for regular expressions (documented in L<perlop> and
+L<perlreref>) tells Perl to compile the regular expression only once.
+This is only useful when the pattern contains a variable. Perls 5.6
+and later handle this automatically if the pattern does not change.
+
+Since the match operator C<m//>, the substitution operator C<s///>,
+and the regular expression quoting operator C<qr//> are double-quotish
+constructs, you can interpolate variables into the pattern. See the
+answer to "How can I quote a variable to use in a regex?" for more
+details.
+
+This example takes a regular expression from the argument list and
+prints the lines of input that match it:
+
+ my $pattern = shift @ARGV;
+
+ while( <> ) {
+ print if m/$pattern/;
+ }
+
+Versions of Perl prior to 5.6 would recompile the regular expression
+for each iteration, even if C<$pattern> had not changed. The C</o>
+would prevent this by telling Perl to compile the pattern the first
+time, then reuse that for subsequent iterations:
+
+ my $pattern = shift @ARGV;
+
+ while( <> ) {
+ print if m/$pattern/o; # useful for Perl < 5.6
+ }
+
+In versions 5.6 and later, Perl won't recompile the regular expression
+if the variable hasn't changed, so you probably don't need the C</o>
+option. It doesn't hurt, but it doesn't help either. If you want any
+version of Perl to compile the regular expression only once even if
+the variable changes (thus, only using its initial value), you still
+need the C</o>.
+
+You can watch Perl's regular expression engine at work to verify for
+yourself if Perl is recompiling a regular expression. The C<use re
+'debug'> pragma (comes with Perl 5.005 and later) shows the details.
+With Perls before 5.6, you should see C<re> reporting that its
+compiling the regular expression on each iteration. With Perl 5.6 or
+later, you should only see C<re> report that for the first iteration.
+
+ use re 'debug';
+
+ $regex = 'Perl';
+ foreach ( qw(Perl Java Ruby Python) ) {
+ print STDERR "-" x 73, "\n";
+ print STDERR "Trying $_...\n";
+ print STDERR "\t$_ is good!\n" if m/$regex/;
+ }
=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
+ 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:
+created by Jeffrey Friedl and later modified by Fred Curtis.
- $/ = undef;
- $_ = <>;
- s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|\n+|.[^/"'\\]*)#$2#g;
- print;
+ $/ = undef;
+ $_ = <>;
+ s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse;
+ print;
This could, of course, be more legibly written with the C</x> modifier, adding
-whitespace and comments.
+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, as long as they are not
+spread over multiple lines using a continuation character):
+
+ s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse;
=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.
+X<regex, matching balanced test> X<regexp, matching balanced test>
+X<regular expression, matching balanced test>
+
+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.perl.com/CPAN/authors/id/TOMC/scripts/pull_quotes.gz .
+http://www.cpan.org/authors/id/TOMC/scripts/pull_quotes.gz .
-The C::Scan module from CPAN contains such subs for internal usage,
+The C::Scan module from CPAN also contains such subs for internal use,
but they are undocumented.
-=head2 What does it mean that regexps are greedy? How can I get around it?
+=head2 What does it mean that regexes are greedy? How can I get around it?
+X<greedy> X<greediness>
-Most people mean that greedy regexps match as much as they can.
+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
An example:
- $s1 = $s2 = "I am very very cold";
- $s1 =~ s/ve.*y //; # I am cold
- $s2 =~ s/ve.*?y //; # I am very cold
+ $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
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?
+=head2 How do I process each word on each line?
+X<word>
Use the split function:
- while (<>) {
- foreach $word ( split ) {
- # do something with $word here
- }
- }
+ 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.
+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
+To work with only alphanumeric sequences (including underscores), you
+might consider
- while (<>) {
- foreach $word (m/(\w+)/g) {
- # do something with $word here
+ 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
+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";
- }
+ 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";
- }
+ while (<>) {
+ $seen{$_}++;
+ }
+
+ while ( ($line, $count) = each %seen ) {
+ print "$count $line";
+ }
-If you want these output in a sorted order, see the section on Hashes.
+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?
+X<match, approximate> X<matching, approximate>
See the module String::Approx available from CPAN.
=head2 How do I efficiently match many regular expressions at once?
+X<regex, efficiency> X<regexp, efficiency>
+X<regular expression, efficiency>
+
+( 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( <DATA> )
+ {
+ foreach $pattern ( @patterns )
+ {
+ if( /\b$pattern\b/i )
+ {
+ print;
+ 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 $pattern ( @patterns )
+ {
+ print if /$pattern/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.
-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
- };
+=head2 Why don't word-boundary searches with C<\b> work for me?
+X<\b>
- $f2 = bm_or qw{
- \b[Ff]ree\b
- \bBSD\B
- (?i)sys(tem)?\s*[V5]\b
- };
+(contributed by brian d foy)
- # feed me /etc/termcap, prolly
- while ( <> ) {
- print "1: $_" if &$f1;
- print "2: $_" if &$f2;
- }
+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.
-=head2 Why don't word-boundary searches with C<\b> work for me?
+It's not (not!) the boundary between whitespace and non-whitespace,
+and it's not the stuff between words we use to create sentences.
-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.
+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.
-Here are examples of the incorrect application of C<\b>, with fixes:
+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/.
- "two words" =~ /(\w+)\b(\w+)/; # WRONG
- "two words" =~ /(\w+)\s+(\w+)/; # right
+ "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"
- " =matchless= text" =~ /\b=(\w+)=\b/; # WRONG
- " =matchless= text" =~ /=(\w+)=/; # right
+These strings do not match /\bPerl\b/.
-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.
+ "Perl_" # _ is a word char!
+ "Perler" # no word char before P, but one after l
-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".
+You don't have to use \b to match words though. You can look for
+non-word characters surrounded by word characters. These strings
+match the pattern /\b'\b/.
-=head2 Why does using $&, $`, or $' slow my program down?
+ "don't" # the ' char is surrounded by "n" and "t"
+ "qep'a'" # the ' char is surrounded by "p" and "a"
-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.
+These strings do not match /\b'\b/.
-=head2 What good is C<\G> in a regular expression?
+ "foo'" # there is no word char after non-word '
-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.
+You can also use the complement of \b, \B, to specify that there
+should not be a word boundary.
-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:
+In the pattern /\Bam\B/, there must be a word character before the "a"
+and after the "m". These patterns match /\Bam\B/:
- s/^(>+)/':' x length($1)/gem;
+ "llama" # "am" surrounded by word chars
+ "Samuel" # same
-Or, using C<\G>, the much simpler (and faster):
+These strings do not match /\Bam\B/
- s/\G>/:/g;
+ "Sam" # no word boundary before "a", but one after "m"
+ "I am Sam" # "am" surrounded by non-word chars
-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; };
- }
- }
+=head2 Why does using $&, $`, or $' slow my program down?
+X<$MATCH> X<$&> X<$POSTMATCH> X<$'> X<$PREMATCH> X<$`>
+
+(contributed by Anno Siegel)
+
+Once Perl sees that you need one of these variables anywhere in the
+program, it provides them on each and every pattern match. That means
+that on every pattern match the entire string will be copied, part of it
+to $`, part to $&, and part to $'. Thus the penalty is most severe with
+long strings and patterns that match often. 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 $& variable is no
+longer "expensive" the way the other two are.
+
+Since Perl 5.6.1 the special variables @- and @+ can functionally replace
+$`, $& and $'. These arrays contain pointers to the beginning and end
+of each match (see perlvar for the full story), so they give you
+essentially the same information, but without the risk of excessive
+string copying.
-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;
- }
- }
- }
+=head2 What good is C<\G> in a regular expression?
+X<\G>
+
+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 C<pos()>
+as the position to start the next match. As the match
+operator makes successive matches, it updates C<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 C<pos()> value.
+
+Suppose you want to match all of consecutive 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 C<\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 C<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 C<pos()> resets on fail with the C<c> flag, documented
+in L<perlop> and L<perlreref>. Subsequent matches start where the last
+successful match ended (the value of C<pos()>) even if a match on the
+same string has failed in the meantime. In this case, the match after
+the C<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
+C<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; };
+ }
+ }
-But then you lose the vertical alignment of the regular expressions.
+For each line, the C<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 regexps DFAs or NFAs? Are they POSIX compliant?
+=head2 Are Perl regexes DFAs or NFAs? Are they POSIX compliant?
+X<DFA> X<NFA> X<POSIX>
While it's true that Perl's regular expressions resemble the DFAs
(deterministic finite automata) of the egrep(1) program, they are in
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?
+=head2 What's wrong with using grep in a void context?
+X<grep>
+
+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.
-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 multibyte characters?
+X<regex, and multibyte characters> X<regexp, and multibyte characters>
+X<regular expression, and multibyte characters> X<martian> X<encoding, Martian>
-=head2 How can I match strings with multi-byte 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>.
-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.
+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.
-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.
+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.
-So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the nine
-characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'.
+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.
+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/;
+ # Make sure adjacent "martian" bytes are no longer adjacent.
+ $martian =~ s/([A-Z][A-Z])/ $1 /g;
+
+ 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';
- }
+ @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 $1 eq 'GX';
- }
+ while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded
+ print "found GX!\n", last if $1 eq 'GX';
+ }
-Or like this:
+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 $+[0],
+but this usually can be worked around.
+
+=head2 How do I match a regular expression that's in a variable?
+X<regex, in variable> X<eval> X<regex> X<quotemeta> X<\Q, regex>
+X<\E, regex>, X<qr//>
+
+(contributed by brian d foy)
+
+We don't have to hard-code patterns into the match operator (or
+anything else that works with regular expressions). We can put the
+pattern in a variable for later use.
+
+The match operator is a double quote context, so you can interpolate
+your variable just like a double quoted string. In this case, you
+read the regular expression as user input and store it in C<$regex>.
+Once you have the pattern in C<$regex>, you use that variable in the
+match operator.
+
+ chomp( my $regex = <STDIN> );
- die "sorry, Perl doesn't (yet) have Martian support )-:\n";
+ if( $string =~ m/$regex/ ) { ... }
-In addition, a sample program which converts half-width to full-width
-katakana (in Shift-JIS or EUC encoding) is available from CPAN as
+Any regular expression special characters in C<$regex> are still
+special, and the pattern still has to be valid or Perl will complain.
+For instance, in this pattern there is an unpaired parenthesis.
-=for Tom make it so
+ my $regex = "Unmatched ( paren";
-There are many double- (and multi-) byte encodings commonly used these
-days. Some versions of these have 1-, 2-, 3-, and 4-byte characters,
-all mixed.
+ "Two parens to bind them all" =~ m/$regex/;
+
+When Perl compiles the regular expression, it treats the parenthesis
+as the start of a memory match. When it doesn't find the closing
+parenthesis, it complains:
+
+ Unmatched ( in regex; marked by <-- HERE in m/Unmatched ( <-- HERE paren/ at script line 3.
+
+You can get around this in several ways depending on our situation.
+First, if you don't want any of the characters in the string to be
+special, you can escape them with C<quotemeta> before you use the string.
+
+ chomp( my $regex = <STDIN> );
+ $regex = quotemeta( $regex );
+
+ if( $string =~ m/$regex/ ) { ... }
+
+You can also do this directly in the match operator using the C<\Q>
+and C<\E> sequences. The C<\Q> tells Perl where to start escaping
+special characters, and the C<\E> tells it where to stop (see L<perlop>
+for more details).
+
+ chomp( my $regex = <STDIN> );
+
+ if( $string =~ m/\Q$regex\E/ ) { ... }
+
+Alternately, you can use C<qr//>, the regular expression quote operator (see
+L<perlop> for more details). It quotes and perhaps compiles the pattern,
+and you can apply regular expression flags to the pattern.
+
+ chomp( my $input = <STDIN> );
+
+ my $regex = qr/$input/is;
+
+ $string =~ m/$regex/ # same as m/$input/is;
+
+You might also want to trap any errors by wrapping an C<eval> block
+around the whole thing.
+
+ chomp( my $input = <STDIN> );
+
+ eval {
+ if( $string =~ m/\Q$input\E/ ) { ... }
+ };
+ warn $@ if $@;
+
+Or...
+
+ my $regex = eval { qr/$input/is };
+ if( defined $regex ) {
+ $string =~ m/$regex/;
+ }
+ else {
+ warn $@;
+ }
+
+=head1 REVISION
+
+Revision: $Revision: 10126 $
+
+Date: $Date: 2007-10-27 21:29:20 +0200 (Sat, 27 Oct 2007) $
+
+See L<perlfaq> for source control details and availability.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
-All rights reserved. See L<perlfaq> for distribution information.
+Copyright (c) 1997-2007 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.
https://perl5.git.perl.org / perl5.git / blobdiff