PERLFAQ6(1) Perl Programmers Reference Guide PERLFAQ6(1)
NAME
perlfaq6 - Regular Expressions
DESCRIPTION
This section is surprisingly small because the rest of the FAQ is
littered with answers involving regular expressions. For example,
decoding a URL and checking whether something is a number can be
handled with regular expressions, but those answers are found elsewhere
in this document (in 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).
How can I hope to use regular expressions without creating illegible and
unmaintainable code?
Three techniques can make regular expressions maintainable and
understandable.
Comments Outside the Regex
Describe what you're doing and how you're doing it, using normal
Perl comments.
# turn the line into the first word, a colon, and the
# number of characters on the rest of the line
s/^(\w+)(.*)/ lc($1) . ":" . length($2) /meg;
Comments Inside the Regex
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.
Different Delimiters
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
Using logically paired delimiters can be even more readable:
s{/usr/local/}{/usr/share}g; # better still
I'm having trouble matching over more than one line. What's wrong?
Either you don't have more than one line in the string you're looking
at (probably), or else you aren't using the correct modifier(s) on your
pattern (possibly).
There are many ways to get multiline data into a string. If you want it
to happen automatically while reading input, you'll want to set $/
(probably to '' for paragraphs or "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 don't want
caret or dollar to match at any point inside the record next to
newlines. But it's imperative that $/ be set to something other than
the default, or else we won't actually ever have a multiline record
read in.
$/ = ''; # read in whole paragraph, not just one line
while ( <> ) {
while ( /\b([\w'-]+)(\s+\g1)+\b/gi ) { # word starts alpha
print "Duplicate $1 at paragraph $.\n";
}
}
Here's some code that finds sentences that begin with "From " (which
would be mangled by many mailers):
$/ = ''; # read in whole paragraph, not just one line
while ( <> ) {
while ( /^From /gm ) { # /m makes ^ match next to \n
print "leading from in paragraph $.\n";
}
}
Here's code that finds everything between START and END in a paragraph:
undef $/; # read in whole file, not just one line or paragraph
while ( <> ) {
while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries
print "$1\n";
}
}
How can I pull out lines between two patterns that are themselves on
different lines?
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 "..":
while (<>) {
my $in_header = 1 .. /^$/;
my $in_body = /^$/ .. eof;
# now choose between them
} continue {
$. = 0 if eof; # fix $.
}
How do I match XML, HTML, or other nasty, ugly things with a regex?
Do not use regexes. Use a module and forget about the regular
expressions. The XML::LibXML, HTML::TokeParser and HTML::TreeBuilder
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://metacpan.org/>; ) and wonder at all the
work people have done for you already! :)
I put a regular expression into $/ but it didn't work. What's wrong?
$/ 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;
}
How do I substitute case-insensitively on the LHS while preserving case on
the RHS?
Here's a lovely Perlish solution by Larry Rosler. It exploits
properties of bitwise xor on ASCII strings.
$_= "this is a TEsT case";
$old = 'test';
$new = 'success';
s{(\Q$old\E)}
{ uc $new | (uc $1 ^ $1) .
(uc(substr $1, -1) ^ substr $1, -1) x
(length($new) - length $1)
}egi;
print;
And here it is as a subroutine, modeled after the above:
sub preserve_case($$) {
my ($old, $new) = @_;
my $mask = uc $old ^ $old;
uc $new | $mask .
substr($mask, -1) x (length($new) - length($old))
}
$string = "this is a TEsT case";
$string =~ s/(test)/preserve_case($1, "success")/egi;
print "$string\n";
This prints:
this is a SUcCESS case
As an alternative, to keep the case of the replacement word if it is
longer than the original, you can use this code, by Jeff Pinyan:
sub preserve_case {
my ($from, $to) = @_;
my ($lf, $lt) = map length, @_;
if ($lt < $lf) { $from = substr $from, 0, $lt }
else { $from .= substr $to, $lf }
return uc $to | ($from ^ uc $from);
}
This changes the sentence to "this is a SUcCess case."
Just to show that C programmers can write C in any programming
language, if you prefer a more C-like solution, the following script
makes the substitution have the same case, letter by letter, as the
original. (It also happens to run about 240% slower than the Perlish
solution runs.) If the substitution has more characters than the
string being substituted, the case of the last character is used for
the rest of the substitution.
# Original by Nathan Torkington, massaged by Jeffrey Friedl
#
sub preserve_case($$)
{
my ($old, $new) = @_;
my $state = 0; # 0 = no change; 1 = lc; 2 = uc
my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new));
my $len = $oldlen < $newlen ? $oldlen : $newlen;
for ($i = 0; $i < $len; $i++) {
if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) {
$state = 0;
} elsif (lc $c eq $c) {
substr($new, $i, 1) = lc(substr($new, $i, 1));
$state = 1;
} else {
substr($new, $i, 1) = uc(substr($new, $i, 1));
$state = 2;
}
}
# finish up with any remaining new (for when new is longer than old)
if ($newlen > $oldlen) {
if ($state == 1) {
substr($new, $oldlen) = lc(substr($new, $oldlen));
} elsif ($state == 2) {
substr($new, $oldlen) = uc(substr($new, $oldlen));
}
}
return $new;
}
How can I make "\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.
How can I match a locale-smart version of "/[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_]/".
How can I quote a variable to use in a regex?
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.
What is "/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:
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 $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:
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 "/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.
use re 'debug';
my $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/;
}
How do I use a regular expression to strip C-style comments from a file?
While this actually can be done, it's much harder than you'd think.
For example, this one-liner
perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c
will work in many but not all cases. You see, it's too simple-minded
for certain kinds of C programs, in particular, those with what appear
to be comments in quoted strings. For that, you'd need something like
this, created by Jeffrey Friedl and later modified by Fred Curtis.
$/ = undef;
$_ = <>;
s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse;
print;
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;
Can I use Perl regular expressions to match balanced text?
(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>
What does it mean that regexes are greedy? How can I get around it?
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:
my $s1 = my $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.
How do I process each word on each line?
Use the split function:
while (<>) {
foreach my $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
}
}
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
in the previous question:
my (%seen);
while (<>) {
while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'"
$seen{$1}++;
}
}
while ( my ($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:
my (%seen);
while (<>) {
$seen{$_}++;
}
while ( my ($line, $count) = each %seen ) {
print "$count $line";
}
If you want these output in a sorted order, see perlfaq4: "How do I
sort a hash (optionally by value instead of key)?".
How can I do approximate matching?
See the module String::Approx available from CPAN.
How do I efficiently match many regular expressions at once?
(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:
my @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:
my @patterns = map { qr/\b$_\b/i } qw( foo bar baz );
LINE: while( <> ) {
foreach $pattern ( @patterns ) {
if( /$pattern/ ) {
print;
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.
my $regex = join '|', qw( foo bar baz );
LINE: while( <> ) {
print if /\b(?:$regex)\b/i;
}
For more details on regular expression efficiency, see Mastering
Regular Expressions by Jeffrey Friedl. He explains how the regular
expressions engine works and why some patterns are surprisingly
inefficient. Once you understand how perl applies regular expressions,
you can tune them for individual situations.
Why don't word-boundary searches with "\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
Why does using $&, $`, or $' slow my program down?
(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.
What good is "\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.
$_ = "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 "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.
$_ = "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 "\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.
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 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.
Are Perl regexes DFAs or NFAs? Are they POSIX compliant?
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).
What's wrong with using grep in a void context?
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.
How can I match strings with multibyte 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 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:
my @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g;
# above is conceptually similar to: my @chars = $text =~ m/(.)/g;
#
foreach my $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
if ($1 eq 'GX') {
print "found GX!\n";
last;
}
}
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.
How do I match a regular expression that's in a variable?
(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.
chomp( my $regex = <STDIN> );
if( $string =~ m/$regex/ ) { ... }
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.
chomp( my $regex = <STDIN> );
$regex = quotemeta( $regex );
if( $string =~ m/$regex/ ) { ... }
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).
chomp( my $regex = <STDIN> );
if( $string =~ m/\Q$regex\E/ ) { ... }
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.
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 "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 $@;
}
AUTHOR AND COPYRIGHT
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.
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.
perl v5.16.3 2013-03-04 PERLFAQ6(1)
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