PERLFAQ6(1) PERLFAQ6(1)
NAME
perlfaq6 - Regexps ($Revision: 1.17 $, $Date: 1997/04/24 22:44:10 $)
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 are handled
with regular expressions, but those answers are found elsewhere in this
document (in the section on Data and the Networking one on networking, 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 Regexp
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) /ge;
Comments Inside the Regexp
The /x modifier causes whitespace to be ignored in a regexp 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.
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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 newlines in your string, or you aren't using the
correct modifier(s) on your pattern.
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 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+)(\s+\1)+\b/gi ) {
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";
}
}
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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 (documented in the perlop
manpage):
perl -ne 'print if /START/ .. /END/' file1 file2 ...
If you wanted text and not lines, you would use
perl -0777 -pe '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.
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 capability to wait
for a pattern in the input stream, or timeout 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);
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## 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, but preserving case on
the RHS?
It depends on what you mean by "preserving case". The following script
makes the substitution have the same case, letter by letter, as the
original. 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;
}
$a = "this is a TEsT case";
$a =~ s/(test)/preserve_case($1, "success")/gie;
print "$a\n";
This prints:
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this is a SUcCESS case
How can I make \w match accented characters?
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 regexp?
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 the perlop manpage for more details). Remember also that any
regexp 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/;
# $string is now "to sleep no more"
Without the \Q, the regexp would also spuriously match "di".
What is /o really for?
Using a variable in a regular expression match forces a re-evaluation
(and perhaps recompilation) each time through. The /o modifier locks in
the regexp the first time it's used. This always happens in a constant
regular expression, and in fact, the pattern was compiled into the
internal format at the same time your entire program was.
Use of /o is irrelevant unless variable interpolation is used in the
pattern, and if so, the regexp engine will neither know nor care whether
the variables change after the pattern is evaluated the very first time.
/o is often used to gain an extra measure of efficiency by not performing
subsequent evaluations when you know it won't matter (because you know
the variables won't change), or more rarely, when you don't want the
regexp to notice if they do.
For example, here's a "paragrep" program:
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$/ = ''; # paragraph mode
$pat = shift;
while (<>) {
print if /$pat/o;
}
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:
$/ = undef;
$_ = <>;
s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|\n+|.[^/"'\\]*)#$2#g;
print;
This could, of course, be more legibly written with the /x modifier,
adding whitespace and comments.
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. You
still need to use non-regexp techniques to parse balanced text, such as
the text enclosed between matching parentheses or braces, for example.
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 internal usage, but
they are undocumented.
What does it mean that regexps are greedy? How can I get around it?
Most people mean that greedy regexps 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 (??, *?, +?, {}?).
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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, 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:
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while (<>) {
$seen{$_}++;
}
while ( ($line, $count) = each %seen ) {
print "$count $line";
}
If you want these output in a sorted order, see the section on Hashes.
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 super-inefficient:
while (<FH>) {
foreach $pat (@patterns) {
if ( /$pat/ ) {
# do something
}
}
}
Instead, you either need to use one of the experimental Regexp extension
modules from CPAN (which might well be overkill for your purposes), or
else put together something like this, inspired from a routine in Jeffrey
Friedl's book:
sub _bm_build {
my $condition = shift;
my @regexp = @_; # this MUST not be local(); need my()
my $expr = join $condition => map { "m/\$regexp[$_]/o" } (0..$#regexp);
my $match_func = eval "sub { $expr }";
die if $@; # propagate $@; this shouldn't happen!
return $match_func;
}
sub bm_and { _bm_build('&&', @_) }
sub bm_or { _bm_build('||', @_) }
$f1 = bm_and qw{
xterm
(?i)window
};
$f2 = bm_or qw{
\b[Ff]ree\b
\bBSD\B
(?i)sys(tem)?\s*[V5]\b
};
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# feed me /etc/termcap, prolly
while ( <> ) {
print "1: $_" if &$f1;
print "2: $_" if &$f2;
}
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 behaviour of
all the regexp metacharacters.
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?
Because once Perl sees that you need one of these variables anywhere in
the program, it has to provide 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 regexp that contains capturing
parentheses. But if you never use $&, etc., in your script, then regexps
without capturing parentheses won't be penalized. So avoid $&, $', and $`
if you can, but if you can't (and some algorithms really appreciate
them), once you've used them once, use them at will, because you've
already paid the price.
What good is \G in a regular expression?
The notation \G is used in a match or substitution in conjunction the /g
modifier (and ignored if there's no /g) to anchor the regular expression
to the point just past where the last match occurred, i.e. the pos()
point.
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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.
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Are Perl regexps 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 the perlfaq2
manpage).
What's wrong with using grep or map in a void context?
Strictly speaking, nothing. Stylistically speaking, it's not a good way
to write maintainable code. That's because you're using these constructs
not for their return values but rather for their side-effects, and side-
effects can be mystifying. There's no void grep() that's not better
written as a for (well, foreach, technically) loop.
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:
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@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";
In addition, a sample program which converts half-width to full-width
katakana (in Shift-JIS or EUC encoding) is available from CPAN as
There are many double- (and multi-) byte encodings commonly used these
days. Some versions of these have 1-, 2-, 3-, and 4-byte characters, all
mixed.
AUTHOR AND COPYRIGHT
Copyright (c) 1997 Tom Christiansen and Nathan Torkington. All rights
reserved. See the perlfaq manpage for distribution information.
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