=head1 NAME
-perlfaq6 - Regular Expressions ($Revision: 1.31 $, $Date: 2005/03/27 07:17:28 $)
+perlfaq6 - Regular Expressions ($Revision: 7910 $)
=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 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).
+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?
+X<regex, legibility> X<regexp, legibility>
+X<regular expression, legibility> X</x>
Three techniques can make regular expressions maintainable and
understandable.
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;
+ # 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
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 more than one line in the string you're looking
at (probably), or else you aren't using the correct modifier(s) on
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";
+ $/ = ''; # 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/sgm ) { # /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 -ne '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
Here's another example of using C<..>:
- while (<>) {
- $in_header = 1 .. /^$/;
- $in_body = /^$/ .. eof();
+ while (<>) {
+ $in_header = 1 .. /^$/;
+ $in_body = /^$/ .. eof;
# now choose between them
- } continue {
- reset if eof(); # fix $.
- }
+ } 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>
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 have File::Stream, this is easy.
- use File::Stream;
- my $stream = File::Stream->new(
- $filehandle,
- separator => qr/\s*,\s*/,
- );
+ use File::Stream;
- print "$_\n" while <$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.
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.
- }
- }
+ 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;
- }
+ 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";
+ $_= "this is a TEsT case";
- $old = 'test';
- $new = 'success';
+ $old = 'test';
+ $new = 'success';
- s{(\Q$old\E)}
- { uc $new | (uc $1 ^ $1) .
- (uc(substr $1, -1) ^ substr $1, -1) x
- (length($new) - length $1)
- }egi;
+ s{(\Q$old\E)}
+ { uc $new | (uc $1 ^ $1) .
+ (uc(substr $1, -1) ^ substr $1, -1) x
+ (length($new) - length $1)
+ }egi;
- print;
+ print;
And here it is as a subroutine, modeled after the above:
- sub preserve_case($$) {
- my ($old, $new) = @_;
- my $mask = uc $old ^ $old;
+ sub preserve_case($$) {
+ my ($old, $new) = @_;
+ my $mask = uc $old ^ $old;
- uc $new | $mask .
- substr($mask, -1) x (length($new) - length($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";
+ $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, $lt) = map length, @_;
+ sub preserve_case {
+ my ($from, $to) = @_;
+ my ($lf, $lt) = map length, @_;
- if ($lt < $lf) { $from = substr $from, 0, $lt }
- else { $from .= substr $to, $lf }
+ if ($lt < $lf) { $from = substr $from, 0, $lt }
+ else { $from .= substr $to, $lf }
- return uc $to | ($from ^ uc $from);
- }
+ return uc $to | ($from ^ uc $from);
+ }
This changes the sentence to "this is a SUcCess case."
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($$)
- {
- my ($old, $new) = @_;
- my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc
- my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new));
- my ($len) = $oldlen < $newlen ? $oldlen : $newlen;
-
- for ($i = 0; $i < $len; $i++) {
- if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) {
- $state = 0;
- } elsif (lc $c eq $c) {
- substr($new, $i, 1) = lc(substr($new, $i, 1));
- $state = 1;
- } else {
- substr($new, $i, 1) = uc(substr($new, $i, 1));
- $state = 2;
- }
- }
- # finish up with any remaining new (for when new is longer than old)
- if ($newlen > $oldlen) {
- if ($state == 1) {
- substr($new, $oldlen) = lc(substr($new, $oldlen));
- } elsif ($state == 2) {
- substr($new, $oldlen) = uc(substr($new, $oldlen));
- }
- }
- return $new;
- }
+ # Original by Nathan Torkington, massaged by Jeffrey Friedl
+ #
+ sub preserve_case($$)
+ {
+ 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<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]/>?
+X<alpha>
You can use the POSIX character class syntax C</[[:alpha:]]/>
documented in L<perlre>.
the digits and the underscore, or C</[\W\d_]/>.
=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,
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 = "Placido P. Octopus";
+ $regex = "P.";
- $string =~ s/$regex/Polyp/;
- # $string is now "Polypacido P. Octopus"
+ $string =~ s/$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
To escape the special meaning of C<.>, we use C<\Q>:
- $string = "Placido P. Octopus";
- $regex = "P.";
+ $string = "Placido P. Octopus";
+ $regex = "P.";
- $string =~ s/\Q$regex/Polyp/;
- # $string is now "Placido Polyp Octopus"
+ $string =~ s/\Q$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?
+X</o>
Using a variable in a regular expression match forces a re-evaluation
(and perhaps recompilation) each time the regular expression is
For example, here's a "paragrep" program:
- $/ = ''; # paragraph mode
- $pat = shift;
- while (<>) {
- print if /$pat/o;
- }
+ $/ = ''; # 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
+ 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;
+ $/ = 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.
)
}{defined $2 ? $2 : ""}gxse;
-A slight modification also removes C++ comments:
+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;
+ s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse;
=head2 Can I use Perl regular expressions to match balanced text?
+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
but they are undocumented.
=head2 What does it mean that regexes are greedy? How can I get around it?
+X<greedy> X<greediness>
Most people mean that greedy regexes match as much as they can.
Technically speaking, it's actually the quantifiers (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
+ $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
playing hot potato.
=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.
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?
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 (<>) {
+ while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'"
+ $seen{$1}++;
+ }
}
- }
- while ( ($word, $count) = each %seen ) {
- print "$count $word\n";
- }
+
+ while ( ($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 L<perlfaq4>: ``How do I
-sort a hash (optionally by value instead of key)?''.
+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
+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 )
+ @patterns = qw( foo bar baz );
+
+ LINE: while( <DATA> )
+ {
+ foreach $pattern ( @patterns )
{
- print if /\b$pattern\b/i;
- next LINE;
- }
+ if( /\b$pattern\b/i )
+ {
+ print;
+ next LINE;
+ }
+ }
}
The qr// operator showed up in perl 5.005. It compiles a
its pre-compiled form. The rest of the script is the same,
but faster.
- @patterns = map { qr/\b$_\b/i } qw( foo bar baz );
+ @patterns = map { qr/\b$_\b/i } qw( foo bar baz );
- LINE: while( <> )
- {
- foreach $pattern ( @patterns )
+ LINE: while( <> )
+ {
+ foreach $pattern ( @patterns )
{
- print if /\b$pattern\b/i;
- next LINE;
- }
+ 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.
$regex = join '|', qw( foo bar baz );
- LINE: while( <> )
- {
+ LINE: while( <> )
+ {
print if /\b(?:$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
+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?
+X<\b>
(contributed by brian d foy)
"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
+non-word characters surrounded 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.
"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"
=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.
-
+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.
+
=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
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; };
- }
- }
+ 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
pattern.
=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
L<perlfaq2>).
=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
context, no lists are constructed.
=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>
Starting from Perl 5.6 Perl has had some level of multibyte character
support. Perl 5.8 or later is recommended. Supported multibyte
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';
+ }
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
+ (?<![A-Z])
+ (?:[A-Z][A-Z])*?
+ GX
/x;
This succeeds if the "martian" character GX is in the string, and fails
It does have the drawback of putting the wrong thing in $-[0] and $+[0],
but this usually can be worked around.
-=head2 How do I match a pattern that is supplied by the user?
+=head2 How do I match a regular expression that's in a variable?
+X<regex, in variable> X<eval> X<regex> X<quotemeta> X<\Q, regex>
+X<\E, regex>, X<qr//>
-Well, if it's really a pattern, then just use
+(contributed by brian d foy)
- chomp($pattern = <STDIN>);
- if ($line =~ /$pattern/) { }
+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.
-Alternatively, since you have no guarantee that your user entered
-a valid regular expression, trap the exception this way:
+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.
- if (eval { $line =~ /$pattern/ }) { }
+ chomp( my $regex = <STDIN> );
-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>.
+ if( $string =~ m/$regex/ ) { ... }
- $pattern = <STDIN>;
+Any regular expression special characters in C<$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.
- open (FILE, $input) or die "Couldn't open input $input: $!; aborting";
- while (<FILE>) {
- print if /\Q$pattern\E/;
- }
- close FILE;
+ my $regex = "Unmatched ( paren";
+
+ "Two parens to bind them all" =~ m/$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: 7910 $
+
+Date: $Date: 2006-10-07 22:38:54 +0200 (sam, 07 oct 2006) $
+
+See L<perlfaq> for source control details and availability.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and
+Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
https://perl5.git.perl.org / perl5.git / blobdiff