perlfaq6 - Regular Expressions ($Revision: 1.6 $, $Date:
2003/12/03 03:02:44 $)
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).
How can I hope to use regular expressions without creating [Toc] [Back]
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/^(912
Comments Inside the Regex
The "/x" modifier causes whitespace to be ignored in a
regex pattern (except in a character class), and also
allows you to use normal comments there, too. As you
can imagine, whitespace and comments help a lot.
"/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/usrlocal/usrshare/g; # bad delimiter choice
s#/usr/local#/usr/share#g; # better
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 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 more whole paragraph,
not just one line [
while ( <> ) 1
while ( /9
print2"Duplicate $1 at paragraph $.0;
}
}
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
print "leading from in paragraph $.0;
}
}
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 "$10;
}
}
How can I pull out lines between two patterns that are [Toc] [Back]
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 "$10 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 (<>) {
$in_header = 1 .. /^$/;
$in_body = /^$/ .. eof();
# now choose between them
} continue {
reset if eof(); # fix $.
}
I put a regular expression into $/ but it didn't work.
What's wrong?
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 really need to do this.
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 [Toc] [Back]
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}
{ 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($$) {
my ($old, $new) = @_;
my $mask = uc $old ^ $old;
uc $new | $mask .
substr($mask, -1) x (length($new) -
length($old))
}
$a = "this is a TEsT case";
$a =~ s/(test)/preserve_case($1, "success")/egi;
print "$a0;
This prints:
this is a SUcCESS case
As an alternative, to keep the case of the replacement
word if it is longer than the original, you can use this
code, by Jeff Pinyan:
sub preserve_case {
my ($from, $to) = @_;
my ($lf, $lt) = map length, @_;
if ($lt < $lf) { $from = substr $from, 0, $lt }
else { $from .= substr $to, $lf }
return uc $to | ($from ^ uc $from);
}
This changes the sentence to "this is a SUcCess case."
Just to show that C programmers can write C in any programming
language, if you prefer a more C-like solution,
the following script makes the substitution have the same
case, letter by letter, as the original. (It also happens
to run about 240% slower than the Perlish solution runs.)
If the substitution has more characters than the string
being substituted, the case of the last character is used
for the rest of the substitution.
# Original by Nathan Torkington, massaged by Jeffrey
Friedl
#
sub preserve_case($$)
{
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_]/) {
$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 "match national character sets?
Put "use locale;" in your script. The 216class
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 168the digits and the
underscore. As a regex, that looks like "/[^W_]/".
Its complement, the non-alphabetics, is then everything in
W along with the digits and the underscore, or
"/[W_]/".
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?
Using a variable in a regular expression match forces a
re-evaluation (and perhaps recompilation) each time the
regular expression is encountered. The "/o" modifier
locks in the regex the first time it's used. This always
happens in a constant regular expression, and in fact, the
pattern was compiled into the internal format at the same
time your entire program was.
Use of "/o" is irrelevant unless variable interpolation is
used in the pattern, and if so, the regex engine will neither
know nor care whether the variables change after the
pattern is evaluated the very first time.
"/o" is often used to gain an extra measure of efficiency
by not performing subsequent evaluations when you know it
won't matter (because you know the variables won't
change), or more rarely, when you don't want the regex to
notice if they do.
For example, here's a "paragrep" program:
$/ = ''; # paragraph mode
$pat = shift;
while (<>) {
print if /$pat/o;
}
How do I use a regular expression to strip C style com- [Toc] [Back]
ments 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#/*([^/*][^*]*)*/|("(\.|[^"\])*"|'(\.|[^'\])*'|.[^/"'\]*)#$2#gs
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
)
}{$2}gxs;
A slight modification also removes C++ comments:
s#/*([^/*][^*]*)*/|//[^0*|("(\.|[^"\])*"|'(\.|[^'\])*'|.[^/"'\]*)#$2#gs;
Can I use Perl regular expressions to match balanced text?
Historically, Perl regular expressions were not capable of
matching balanced text. As of more recent versions of
perl including 5.6.1 experimental features have been added
that make it possible to do this. Look at the documentation
for the (??{ }) construct in recent perlre manual
pages to see an example of matching balanced parentheses.
Be sure to take special notice of the warnings present in
the manual before making use of this feature.
CPAN contains many modules that can be useful for matching
text depending on the context. Damian Conway provides
some useful patterns in Regexp::Common. The module
Text::Balanced provides a general solution to this problem.
One of the common applications of balanced text matching
is working with XML and HTML. There are many modules
available that support these needs. Two examples are
HTML::Parser and XML::Parser. There are many others.
An elaborate subroutine (for 7-bit ASCII only) to pull out
balanced and possibly nested single chars, like "`" and
"'", "{" and "}", or "(" and ")" can be found in
http://www.cpan.org/authors/id/TOMC/scripts/pull_quotes.gz
.
The C::Scan module from CPAN also contains such subs for
internal use, but they are undocumented.
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:
$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, like you would if you were playing
hot potato.
How do I process each word on each line?
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/(144
# do something with $word here
}
}
How can I print out a word-frequency or line-frequency [Toc] [Back]
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 nonwhitespace
chunk idea of a word given in the previous
question:
while (<>) { W
while ( /(_600"
$seen{$1}++;
}
}
while ( ($word, $count) = each %seen ) {
print "$count $word0;
}
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 $line";
}
If you want these output in a sorted order, see perlfaq4:
``How do I sort a hash (optionally by value instead of
key)?''.
How can I do approximate matching?
See the module String::Approx available from CPAN.
How do I efficiently match many regular expressions at [Toc] [Back]
once?
The following is extremely inefficient:
# slow but obvious way
@popstates = qw(CO ON MI WI MN);
while (defined($line =s<>)) {
for $state (@popstttes) {
if ($line =~ /a
print $line;
last; e
}
}
}
That's because Perl has to recompile all those patterns
for each of the lines of the file. As of the 5.005
release, there's a much better approach, one which makes
use of the new "qr//" operator:
# use spiffy new qr// operator, with /i flag even
use 5.005;
@popstates = qw(CO ON MI WI MN);
@poppats = map { qr/_
while (defined($line = <>)) {
for $patobj (@poppats) {
print $line if $line =~ /$patobj/;
} s
} a
s b
Why don't word-boundary searches with"" work for me?
s n
Two common tisconceptions are that "o
"nd that iths the edge between whitespace characters
and non-whieespace characters. Neither is correct. "
is the place between a "408W" character
(that is, "dword"). It's a zero- f
width assertion, just like "^", "$",oand all the other
anchors, soeit doesn't consume any characters. perlre
describes the behavior of all the regex metacharacters.
f
Here are examples of the incorrect application of ",
with fixes:
perl v5.8.5 2002-11-,6 11
s
e
e
t
h
PERLFAQ6(1) Perl Programmers Reeerence Guide PERLFAQ6(1)
e
x
"two words" =~ /(48 a
"two words" =~ /(48 m
(p
" =matchless= text" =~ /7l
" =matchless= text" =~ /=e432
o
Although they may not do whatfyou thought they did, "
and "0 correct use of "m
words over multiple lines. a
t
An example of using "00ll find occurrences of "is" on the
insides of words h
only, as in "thistle", but not "this" or "island".
n
Why does using $&, $`, or $' slow my program down?
d
Once Perl sees that you need one of these variables anywhere
in the program, it provides them on each and every
pattern match. The same mechanism that handles these provides
for the use of $1, $2, etc., so you pay the same
price for each regex that contains capturing parentheses.
If you never use $&, etc., inayour script, then regexes
without capturing parenthesestwon't be penalized. So 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.
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 consective 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/()/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()/g; # qw( 11 22 )
You can also use the "G" anchor in scalar context. You
still need the "g" flag.
$_ = "1122a44";
while( m/G()/g )
{
print "Found $10;
}
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()/g )
{
print "Found $10;
}
print "Found $1 after while" if m/()/g; # finds
"11"
You can disable pos() resets on fail with the "c" flag.
Subsequent matches start where the last successful match
ended (the value of pos()) even if a match on the same
string as 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()/gc )
{
print "Found $10;
}
print "Found $1 after while" if m/()/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( + )/gcx && do { print "number: $10;
redo; };
m/ G( 1224
m/ G( )/gcx && do { print "space: $10;
redo; };
m/ G( [^1152
}
}
For each line, the PARSER loop first tries to match a
s)ries of digits followed by a word boundary. This match
h/s to start at the place the last match left off (or the
bgginning of the string on the first match). Since "m/ G(
+cuses 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:
$martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent
``martian''
# bytes are no
longer adjacent.
print "found GX!0 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!0, last if $char eq 'GX';
}
Or like this:
while ($martian =~ m/G([A-Z][A-Z]|.)/gs) { # G probably unneeded
print "found GX!0, last if $1 eq 'GX';
}
Here's another, slightly less painful, way to do it from
Benjamin Goldberg:
$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
(?!<), you can replace (?!<[A-Z]) with (?:^|[^A-Z]).
It does have the drawback of putting the wrong thing in
$-[0] and $+[0], but this usually can be worked around.
How do I match a pattern that is supplied by the user?
Well, if it's really a pattern, then just use
chomp($pattern = <STDIN>);
if ($line =~ /$pattern/) { }
Alternatively, since you have no guarantee that your user
entered a valid regular expression, trap the exception
this way:
if (eval { $line =~ /$pattern/ }) { }
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 "Q"..."
$pattern = <STDIN>;
open (FILE, $input) or die "Couldn't open input $input: $!; aborting";
while (<FILE>) {
print if /Q$pattern;
}
close FILE;
Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
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 v5.8.5 2002-11-06 16 [ Back ] |
