perlfaq6 - Regular Expressions
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 perlfaq9: "How do I decode or create those %-encodings on the web" and perlfaq4: "How do I determine whether a scalar is a number/whole/integer/float", to be precise).
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;
The /x 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.
/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.
While we normally think of patterns as being delimited with /
characters, they can be delimited by almost any character. perlre
describes this. For example, the 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
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 undef for the whole file) to
allow you to read more than one line at a time.
Read perlre to help you decide which of /s and /m (or both)
you might want to use: /s allows dot to include newline, and /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
/s because we aren't using dot in a regular expression that we want
to cross line boundaries. Neither do we need /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 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 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:
You can use Perl's somewhat exotic ..
operator (documented in
perlop):
- 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 START
through 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 ..
:
(contributed by brian d foy)
If you just want to get work done, use a module and forget about the
regular expressions. The XML::Parser
and HTML::Parser
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 ( http://search.cpan.org ) and wonder at all the work people
have done for you already! :)
The problem with things such as XML is that they have balanced text
containing multiple levels of balanced text, but sometimes it isn't
balanced text, as in an empty tag (<br/>
, for instance). Even then,
things can occur out-of-order. Just when you think you've got a
pattern that matches your input, someone throws you a curveball.
If you'd like to do it the hard way, scratching and clawing your way toward a right answer but constantly being disappointed, besieged by bug reports, and weary from the inordinate amount of time you have to spend reinventing a triangular wheel, then there are several things you can try before you give up in frustration:
(?DEFINE) feature.
Good luck!
$/ 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.
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).
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.
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:
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:
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;
- }
\w
match national character sets?
Put use locale;
in your script. The \w character class is taken
from the current locale.
See perllocale for details.
/[a-zA-Z]/
?
You can use the POSIX character class syntax /[[:alpha:]]/
documented in perlre.
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 /[^\W\d_]/
. Its complement,
the non-alphabetics, is then everything in \W along with
the digits and the underscore, or /[\W\d_]/
.
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 s/// substitution is considered
a double-quoted string (see perlop for more details). 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.";
- $string =~ s/$regex/Polyp/;
- # $string is now "Polypacido P. Octopus"
Because . is special in regular expressions, and can match any
single character, the regex P.
here has matched the <Pl> in the
original string.
To escape the special meaning of ., we use \Q
:
- $string = "Placido P. Octopus";
- $regex = "P.";
- $string =~ s/\Q$regex/Polyp/;
- # $string is now "Placido Polyp Octopus"
The use of \Q
causes the <.> in the regex to be treated as a
regular character, so that P.
matches a P
followed by a dot.
/o really for?
(contributed by brian d foy)
The /o option for regular expressions (documented in perlop and
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 m//, the substitution operator s///,
and the regular expression quoting operator 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:
Versions of Perl prior to 5.6 would recompile the regular expression
for each iteration, even if $pattern
had not changed. The /o
would prevent this by telling Perl to compile the pattern the first
time, then reuse that for subsequent iterations:
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 /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 /o.
You can watch Perl's regular expression engine at work to verify for
yourself if Perl is recompiling a regular expression. The use re
'debug'
pragma (comes with Perl 5.005 and later) shows the details.
With Perls before 5.6, you should see re
reporting that its
compiling the regular expression on each iteration. With Perl 5.6 or
later, you should only see re
report that for the first iteration.
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 and later modified by Fred Curtis.
This could, of course, be more legibly written with the /x modifier, adding
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, possibly spanning multiple lines using a continuation character:
- s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//([^\\]|[^\n][\n]?)*?\n|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $3 ? $3 : ""#gse;
(contributed by brian d foy)
Your first try should probably be the Text::Balanced
module, which
is in the Perl standard library since Perl 5.8. It has a variety of
functions to deal with tricky text. The Regexp::Common
module can
also help by providing canned patterns you can use.
As of Perl 5.10, you can match balanced text with regular expressions
using recursive patterns. Before Perl 5.10, you had to resort to
various tricks such as using Perl code in (??{})
sequences.
Here's an example using a recursive regular expression. The goal is to capture all of the text within angle brackets, including the text in nested angle brackets. This sample text has two "major" groups: a group with one level of nesting and a group with two levels of nesting. There are five total groups in angle brackets:
- I have some <brackets in <nested brackets> > and
- <another group <nested once <nested twice> > >
- and that's it.
The regular expression to match the balanced text uses two new (to Perl 5.10) regular expression features. These are covered in perlre and this example is a modified version of one in that documentation.
First, adding the new possessive +
to any quantifier finds the
longest match and does not backtrack. That's important since you want
to handle any angle brackets through the recursion, not backtracking.
The group [^<>]++ finds one or more non-angle brackets without
backtracking.
Second, the new (?PARNO) refers to the sub-pattern in the
particular capture group given by PARNO
. In the following regex,
the first capture group finds (and remembers) the balanced text, and
you need that same pattern within the first buffer to get past the
nested text. That's the recursive part. The (?1) uses the pattern
in the outer capture group as an independent part of the regex.
Putting it all together, you have:
- #!/usr/local/bin/perl5.10.0
- my $string =<<"HERE";
- I have some <brackets in <nested brackets> > and
- <another group <nested once <nested twice> > >
- and that's it.
- HERE
- my @groups = $string =~ m/
- ( # start of capture group 1
- < # match an opening angle bracket
- (?:
- [^<>]++ # one or more non angle brackets, non backtracking
- |
- (?1) # found < or >, so recurse to capture group 1
- )*
- > # match a closing angle bracket
- ) # end of capture group 1
- /xg;
- $" = "\n\t";
- print "Found:\n\t@groups\n";
The output shows that Perl found the two major groups:
- Found:
- <brackets in <nested brackets> >
- <another group <nested once <nested twice> > >
With a little extra work, you can get the all of the groups in angle brackets even if they are in other angle brackets too. Each time you get a balanced match, remove its outer delimiter (that's the one you just matched so don't match it again) and add it to a queue of strings to process. Keep doing that until you get no matches:
- #!/usr/local/bin/perl5.10.0
- my @queue =<<"HERE";
- I have some <brackets in <nested brackets> > and
- <another group <nested once <nested twice> > >
- and that's it.
- HERE
- my $regex = qr/
- ( # start of bracket 1
- < # match an opening angle bracket
- (?:
- [^<>]++ # one or more non angle brackets, non backtracking
- |
- (?1) # recurse to bracket 1
- )*
- > # match a closing angle bracket
- ) # end of bracket 1
- /x;
- $" = "\n\t";
- while( @queue )
- {
- my $string = shift @queue;
- my @groups = $string =~ m/$regex/g;
- print "Found:\n\t@groups\n\n" if @groups;
- unshift @queue, map { s/^<//; s/>$//; $_ } @groups;
- }
The output shows all of the groups. The outermost matches show up first and the nested matches so up later:
- Found:
- <brackets in <nested brackets> >
- <another group <nested once <nested twice> > >
- Found:
- <nested brackets>
- Found:
- <nested once <nested twice> >
- Found:
- <nested twice>
Most people mean that greedy regexes match as much as they can.
Technically speaking, it's actually the quantifiers (?, *
, +
,
{}
) that are greedy rather than the whole pattern; Perl prefers local
greed and immediate gratification to overall greed. To get non-greedy
versions of the same quantifiers, use (??
, *?
, +?, {}?).
An example:
- $s1 = $s2 = "I am very very cold";
- $s1 =~ s/ve.*y //; # I am cold
- $s2 =~ s/ve.*?y //; # I am very cold
Notice how the second substitution stopped matching as soon as it
encountered "y ". The *?
quantifier effectively tells the regular
expression engine to find a match as quickly as possible and pass
control on to whatever is next in line, as you would if you were
playing hot potato.
Use the split function:
- while (<>) {
- foreach $word ( split ) {
- # do something with $word here
- }
- }
Note that this isn't really a word in the English sense; it's just chunks of consecutive non-whitespace characters.
To work with only alphanumeric sequences (including underscores), you might consider
- while (<>) {
- foreach $word (m/(\w+)/g) {
- # do something with $word here
- }
- }
To do this, you have to parse out each word in the input stream. We'll pretend that by word you mean chunk of alphabetics, hyphens, or apostrophes, rather than the non-whitespace chunk idea of a word given in the previous question:
If you wanted to do the same thing for lines, you wouldn't need a regular expression:
If you want these output in a sorted order, see perlfaq4: "How do I sort a hash (optionally by value instead of key)?".
See the module String::Approx available from CPAN.
(contributed by brian d foy)
If you have Perl 5.10 or later, this is almost trivial. You just smart match against an array of regular expression objects:
- my @patterns = ( qr/Fr.d/, qr/B.rn.y/, qr/W.lm./ );
- if( $string ~~ @patterns ) {
- ...
- };
The smart match stops when it finds a match, so it doesn't have to try every expression.
Earlier than Perl 5.10, you have a bit of work to do. You want to
avoid compiling 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:
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:
In some cases, you may be able to make several patterns into a single regular expression. Beware of situations that require backtracking though.
For more details on regular expression efficiency, see Mastering Regular Expressions by Jeffrey Friedl. 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.
\b
work for me?
(contributed by brian d foy)
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.
It's not (not!) the boundary between whitespace and non-whitespace, and it's not the stuff between words we use to create sentences.
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.
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/.
- "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"
These strings do not match /\bPerl\b/.
- "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 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.
In the pattern /\Bam\B/, there must be a word character before the "a" and after the "m". These patterns match /\Bam\B/:
- "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"
- "I am Sam" # "am" surrounded by non-word chars
(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.
Perl 5.10 added three specials, ${^MATCH}
, ${^PREMATCH}
, and
${^POSTMATCH}
to do the same job but without the global performance
penalty. Perl 5.10 only sets these variables if you compile or execute the
regular expression with the /p modifier.
\G
in a regular expression?
You use the \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 \G
is similar to the
beginning of string anchor, ^. The \G
anchor is typically
used with the g
flag. It uses the value of pos()
as the position to start the next match. As the match
operator makes successive matches, it updates 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 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 11
and 22
but
the letter <a> shows up between 22
and 44
and you want
to stop at a
. Simply matching pairs of digits skips over
the a
and still matches 44
.
- $_ = "1122a44";
- my @pairs = m/(\d\d)/g; # qw( 11 22 44 )
If you use the \G
anchor, you force the match after 22
to
start with the 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 \G
anchor in scalar context. You
still need the g
flag.
- $_ = "1122a44";
- while( m/\G(\d\d)/g )
- {
- print "Found $1\n";
- }
After the match fails at the letter a
, perl resets pos()
and the next match on the same string starts at the beginning.
You can disable pos() resets on fail with the c
flag, documented
in perlop and perlreref. Subsequent matches start where the last
successful match ended (the value of pos()) even if a match on the
same string has failed in the meantime. In this case, the match after
the while()
loop starts at the a
(where the last match stopped),
and since it does not use any anchor it can skip over the a
to find
44
.
Typically you use the \G
anchor with the 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.
For each line, the 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 m/ \G( \d+\b
)/gcx
uses the 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.
While it's true that Perl's regular expressions resemble the DFAs (deterministic finite automata) of the egrep(1) program, they are in fact implemented as NFAs (non-deterministic finite automata) to allow backtracking and backreferencing. And they aren't POSIX-style either, because those guarantee worst-case behavior for all cases. (It seems that some people prefer guarantees of consistency, even when what's guaranteed is slowness.) See the book "Mastering Regular Expressions" (from O'Reilly) by Jeffrey Friedl for all the details you could ever hope to know on these matters (a full citation appears in perlfaq2).
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.
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 perluniintro, perlunicode, and Encode.
If you are stuck with older Perls, you can do Unicode with the
Unicode::String
module, and character conversions using the
Unicode::Map8
and Unicode::Map
modules. If you are using
Japanese encodings, you might try using the jperl 5.005_03.
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.
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 /GX/
. Perl
doesn't know about Martian, so it'll find the two bytes "GX" in the "I
am CVSGXX!" string, even though that character isn't there: it just
looks like it is because "SG" is next to "XX", but there's no real
"GX". This is a big problem.
Here are a few ways, all painful, to deal with it:
- # 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:
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.
(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 $regex
.
Once you have the pattern in $regex
, you use that variable in the
match operator.
Any regular expression special characters in $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.
- 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 quotemeta before you use the string.
You can also do this directly in the match operator using the \Q
and \E
sequences. The \Q
tells Perl where to start escaping
special characters, and the \E
tells it where to stop (see perlop
for more details).
Alternately, you can use qr//, the regular expression quote operator (see
perlop for more details). It quotes and perhaps compiles the pattern,
and you can apply regular expression flags to the pattern.
You might also want to trap any errors by wrapping an eval block
around the whole thing.
Or...
Copyright (c) 1997-2010 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.
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