PERLFAQ6(1) Perl Programmers Reference Guide PERLFAQ6(1)NAMEperlfaq6 - Regexes ($Revision: 1.27 $, $Date: 1999/05/23
16:08:30 $)
DESCRIPTION
This section is surprisingly small because the rest of the
FAQ is littered with answers involving regular expres
sions. For example, decoding a URL and checking whether
something is a number are handled with regular expres
sions, but those answers are found elsewhere in this docu
ment (in the perlfaq9 manpage: ``How do I decode or create
those %-encodings on the web'' and the perfaq4 manpage:
``How do I determine whether a scalar is a num
ber/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 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. the perlre manpage 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
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 para
graphs or "undef" for the whole file) to allow you to read
more than one line at a time.
Read the perlre manpage 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 ( <> ) {
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";
}
}
Here's code that finds everything between START and END in
a paragraph:
undef $/; # read in whole file, not just one line or paragraph
while ( <> ) {
while ( /START(.*?)END/sm ) { # /s makes . cross line boundaries
print "$1\n";
}
}
How can I pull out lines between two patterns that are
themselves on different lines?
You can use Perl's somewhat exotic ".." operator (docu
mented in the perlop manpage):
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 (<>) {
$in_header = 1 .. /^$/;
$in_body = /^$/ .. eof();
# now choose between them
} continue {
reset if eof(); # fix $.
}
I put a regular expression into $/ but it didn't work.
What's wrong?
$/ must be a string, not a regular expression. Awk has to
be better for something. :-)
Actually, you could do this if you don't mind reading the
whole file into memory:
undef $/;
@records = split /your_pattern/, <FH>;
The Net::Telnet module (available from CPAN) has the capa
bility to wait for a pattern in the input stream, or time
out if it doesn't appear within a certain time.
## Create a file with three lines.
open FH, ">file";
print FH "The first line\nThe second line\nThe third line\n";
close FH;
## Get a read/write filehandle to it.
$fh = new FileHandle "+<file";
## Attach it to a "stream" object.
use Net::Telnet;
$file = new Net::Telnet (-fhopen => $fh);
## Search for the second line and print out the third.
$file->waitfor('/second line\n/');
print $file->getline;
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, modelled after the above:
sub preserve_case($$) {
my ($old, $new) = @_;
my $mask = uc $old ^ $old;
uc $new | $mask .
substr($mask, -1) x (length($new) - length($old))
}
$a = "this is a TEsT case";
$a =~ s/(test)/preserve_case($1, "success")/egi;
print "$a\n";
This prints:
this is a SUcCESS case
Just to show that C programmers can write C in any pro
gramming language, if you prefer a more C-like solution,
the following script makes the substitution have the same
case, letter by letter, as the original. (It also happens
to run about 240% slower than the Perlish solution runs.)
If the substitution has more characters than the string
being substituted, the case of the last character is used
for the rest of the substitution.
# Original by Nathan Torkington, massaged by Jeffrey Friedl
#
sub preserve_case($$)
{
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?
See the perllocale manpage.
How can I match a locale-smart version of "/[a-zA-Z]/"?
One alphabetic character would be "/[^\W\d_]/", no matter
what locale you're in. Non-alphabetics would be
"/[\W\d_]/" (assuming you don't consider an underscore a
letter).
How can I quote a variable to use in a regex?
The Perl parser will expand $variable and @variable refer
ences 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 the perlop manpage 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 = "to die?";
$lhs = "die?";
$rhs = "sleep, no more";
$string =~ s/\Q$lhs/$rhs/;
# $string is now "to sleep no more"
Without the \Q, the regex would also spuriously match
"di".
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
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 particu
lar, those with what appear to be comments in quoted
strings. For that, you'd need something like this, cre
ated 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#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs;
Can I use Perl regular expressions to match balanced text?
Although Perl regular expressions are more powerful than
"mathematical" regular expressions because they feature
conveniences like backreferences ("\1" and its ilk), they
still aren't powerful enough--with the possible exception
of bizarre and experimental features in the development-
track releases of Perl. You still need to use non-regex
techniques to parse balanced text, such as the text
enclosed between matching parentheses or braces, for exam
ple.
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.perl.com/CPAN/authors/id/TOMC/scripts/pull_quotes.gz
.
The C::Scan module from CPAN contains such subs for inter
nal 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 ver
sions 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 effec
tively 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 under
scores), 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:
while (<>) {
while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'"
$seen{$1}++;
}
}
while ( ($word, $count) = each %seen ) {
print "$count $word\n";
}
If you wanted to do the same thing for lines, you wouldn't
need a regular expression:
while (<>) {
$seen{$_}++;
}
while ( ($line, $count) = each %seen ) {
print "$count $line";
}
If you want these output in a sorted order, see the perl
faq4 manpage: ``How do I sort a hash (optionally by value
instead of key)?''.
How can I do approximate matching?
See the module String::Approx available from CPAN.
How do I efficiently match many regular expressions at
once?
The following is extremely inefficient:
# slow but obvious way
@popstates = qw(CO ON MI WI MN);
while (defined($line = <>)) {
for $state (@popstates) {
if ($line =~ /\b$state\b/i) {
print $line;
last;
}
}
}
That's because Perl has to recompile all those patterns
for each of the lines of the file. As of the 5.005
release, there's a much better approach, one which makes
use of the new "qr//" operator:
# use spiffy new qr// operator, with /i flag even
use 5.005;
@popstates = qw(CO ON MI WI MN);
@poppats = map { qr/\b$_\b/i } @popstates;
while (defined($line = <>)) {
for $patobj (@poppats) {
print $line if $line =~ /$patobj/;
}
}
Why don't word-boundary searches with "\b" work for me?
Two common misconceptions are that "\b" is a synonym for
"\s+" and that it's the edge between whitespace characters
and non-whitespace characters. Neither is correct. "\b"
is the place between a "\w" character and a "\W" character
(that is, "\b" is the edge of a "word"). It's a zero-
width assertion, just like "^", "$", and all the other
anchors, so it doesn't consume any characters. the perlre
manpage describes the behavior of all the regex metachar
acters.
Here are examples of the incorrect application of "\b",
with fixes:
"two words" =~ /(\w+)\b(\w+)/; # WRONG
"two words" =~ /(\w+)\s+(\w+)/; # right
" =matchless= text" =~ /\b=(\w+)=\b/; # WRONG
" =matchless= text" =~ /=(\w+)=/; # right
Although they may not do what you thought they did, "\b"
and "\B" can still be quite useful. For an example of the
correct use of "\b", see the example of matching duplicate
words over multiple lines.
An example of using "\B" is the pattern "\Bis\B". This
will find occurrences of "is" on the insides of words
only, as in "thistle", but not "this" or "island".
Why does using $&, $`, or $' slow my program down?
Once Perl sees that you need one of these variables any
where in the program, it provides them on each and every
pattern match. The same mechanism that handles these pro
vides 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., in your script, then regexes
without capturing parentheses won'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 $& vari
able is no longer "expensive" the way the other two are.
What good is "\G" in a regular expression?
The notation "\G" is used in a match or substitution in
conjunction with the "/g" modifier to anchor the regular
expression to the point just past where the last match
occurred, i.e. the pos() point. A failed match resets the
position of "\G" unless the "/c" modifier is in effect.
"\G" can be used in a match without the "/g" modifier; it
acts the same (i.e. still anchors at the pos() point) but
of course only matches once and does not update pos(), as
non-"/g" expressions never do. "\G" in an expression
applied to a target string that has never been matched
against a "/g" expression before or has had its pos()
reset is functionally equivalent to "\A", which matches at
the beginning of the string.
For example, suppose you had a line of text quoted in
standard mail and Usenet notation, (that is, with leading
">" characters), and you want change each leading ">" into
a corresponding ":". You could do so in this way:
s/^(>+)/':' x length($1)/gem;
Or, using "\G", the much simpler (and faster):
s/\G>/:/g;
A more sophisticated use might involve a tokenizer. The
following lex-like example is courtesy of Jeffrey Friedl.
It did not work in 5.003 due to bugs in that release, but
does work in 5.004 or better. (Note the use of "/c",
which prevents a failed match with "/g" from resetting the
search position back to the beginning of the string.)
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; };
}
}
Of course, that could have been written as
while (<>) {
chomp;
PARSER: {
if ( /\G( \d+\b )/gcx {
print "number: $1\n";
redo PARSER;
}
if ( /\G( \w+ )/gcx {
print "word: $1\n";
redo PARSER;
}
if ( /\G( \s+ )/gcx {
print "space: $1\n";
redo PARSER;
}
if ( /\G( [^\w\d]+ )/gcx {
print "other: $1\n";
redo PARSER;
}
}
}
but then you lose the vertical alignment of the regular
expressions.
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-deter
ministic finite automata) to allow backtracking and back
referencing. 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 the perlfaq2
manpage).
What's wrong with using grep or map in a void context?
Both grep and map build a return list, regardless of their
context. This means you're making Perl go to the trouble
of building up a return list that you then just ignore.
That's no way to treat a programming language, you insen
sitive scoundrel!
How can I match strings with multibyte characters?
This is hard, and there's no good way. Perl does not
directly support wide characters. It pretends that a byte
and a character are synonymous. The following set of
approaches was offered by Jeffrey Friedl, whose article in
issue #5 of The Perl Journal talks about this very matter.
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!\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 $char eq 'GX';
}
Or like this:
while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded
print "found GX!\n", last if $1 eq 'GX';
}
Or like this:
die "sorry, Perl doesn't (yet) have Martian support )-:\n";
There are many double- (and multi-) byte encodings com
monly used these days. Some versions of these have 1-,
2-, 3-, and 4-byte characters, all mixed.
How do I match a pattern that is supplied by the user?
Well, if it's really a pattern, then just use
chomp($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 pat
tern, 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"..."\E", documented in the perlre man
page.
$pattern = <STDIN>;
open (FILE, $input) or die "Couldn't open input $input: $!; aborting";
while (<FILE>) {
print if /\Q$pattern\E/;
}
close FILE;
AUTHOR AND COPYRIGHT
Copyright (c) 1997-1999 Tom Christiansen and Nathan Tork
ington. All rights reserved.
When included as part of the Standard Version of Perl, or
as part of its complete documentation whether printed or
otherwise, this work may be distributed only under the
terms of Perl's Artistic License. Any distribution of
this file or derivatives thereof outside of that package
require that special arrangements be made with copyright
holder.
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.
2001-04-07 perl v5.6.1 PERLFAQ6(1)
