<LI><A HREF="#how can i hope to use regular expressions without creating illegible and unmaintainable code">How can I hope to use regular expressions without creating illegible and unmaintainable code?</A></LI>
<LI><A HREF="#i'm having trouble matching over more than one line. what's wrong">I'm having trouble matching over more than one line. What's wrong?</A></LI>
<LI><A HREF="#how can i pull out lines between two patterns that are themselves on different lines">How can I pull out lines between two patterns that are themselves on different lines?</A></LI>
<LI><A HREF="#i put a regular expression into $/ but it didn't work. what's wrong">I put a regular expression into $/ but it didn't work. What's wrong?</A></LI>
<LI><A HREF="#how do i substitute case insensitively on the lhs, but preserving case on the rhs">How do I substitute case insensitively on the LHS, but preserving case on the RHS?</A></LI>
<LI><A HREF="#how can i make \w match national character sets">How can I make <CODE>\w</CODE> match national character sets?</A></LI>
<LI><A HREF="#how can i match a localesmart version of /[azaz]/">How can I match a locale-smart version of <CODE>/[a-zA-Z]/</CODE>?</A></LI>
<LI><A HREF="#how can i quote a variable to use in a regex">How can I quote a variable to use in a regex?</A></LI>
<LI><A HREF="#what is /o really for">What is <CODE>/o</CODE> really for?</A></LI>
<LI><A HREF="#how do i use a regular expression to strip c style comments from a file">How do I use a regular expression to strip C style comments from a file?</A></LI>
<LI><A HREF="#can i use perl regular expressions to match balanced text">Can I use Perl regular expressions to match balanced text?</A></LI>
<LI><A HREF="#what does it mean that regexes are greedy how can i get around it">What does it mean that regexes are greedy? How can I get around it?</A></LI>
<LI><A HREF="#how do i process each word on each line">How do I process each word on each line?</A></LI>
<LI><A HREF="#how can i print out a wordfrequency or linefrequency summary">How can I print out a word-frequency or line-frequency summary?</A></LI>
<LI><A HREF="#how can i do approximate matching">How can I do approximate matching?</A></LI>
<LI><A HREF="#how do i efficiently match many regular expressions at once">How do I efficiently match many regular expressions at once?</A></LI>
<LI><A HREF="#why don't wordboundary searches with \b work for me">Why don't word-boundary searches with <CODE>\b</CODE> work for me?</A></LI>
<LI><A HREF="#why does using $&, $`, or $' slow my program down">Why does using $&, $`, or $' slow my program down?</A></LI>
<LI><A HREF="#what good is \g in a regular expression">What good is <CODE>\G</CODE> in a regular expression?</A></LI>
<LI><A HREF="#are perl regexes dfas or nfas are they posix compliant">Are Perl regexes DFAs or NFAs? Are they POSIX compliant?</A></LI>
<LI><A HREF="#what's wrong with using grep or map in a void context">What's wrong with using grep or map in a void context?</A></LI>
<LI><A HREF="#how can i match strings with multibyte characters">How can I match strings with multibyte characters?</A></LI>
<LI><A HREF="#how do i match a pattern that is supplied by the user">How do I match a pattern that is supplied by the user?</A></LI>
</UL>
<LI><A HREF="#author and copyright">AUTHOR AND COPYRIGHT</A></LI>
<P>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).</P>
<P>
<H2><A NAME="how can i hope to use regular expressions without creating illegible and unmaintainable code">How can I hope to use regular expressions without creating illegible and unmaintainable code?</A></H2>
<P>Three techniques can make regular expressions maintainable and
understandable.</P>
<DL>
<DT><STRONG><A NAME="item_Comments_Outside_the_Regex">Comments Outside the Regex</A></STRONG><BR>
<DD>
Describe what you're doing and how you're doing it, using normal Perl
comments.
<PRE>
# turn the line into the first word, a colon, and the
While we normally think of patterns as being delimited with <CODE>/</CODE>
characters, they can be delimited by almost any character. <A HREF="../../lib/Pod/perlre.html">the perlre manpage</A>
describes this. For example, the <CODE>s///</CODE> above uses braces as
delimiters. Selecting another delimiter can avoid quoting the
delimiter within the pattern:
<PRE>
s/\/usr\/local/\/usr\/share/g; # bad delimiter choice
s#/usr/local#/usr/share#g; # better</PRE>
<P></P></DL>
<P>
<H2><A NAME="i'm having trouble matching over more than one line. what's wrong">I'm having trouble matching over more than one line. What's wrong?</A></H2>
<P>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 <CODE>modifier(s)</CODE> on your
pattern (possibly).</P>
<P>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 <A HREF="../../lib/Pod/perlfunc.html#item_undef"><CODE>undef</CODE></A> for the whole file) to
allow you to read more than one line at a time.</P>
<P>Read <A HREF="../../lib/Pod/perlre.html">the perlre manpage</A> to help you decide which of <CODE>/s</CODE> and <CODE>/m</CODE> (or both)
you might want to use: <CODE>/s</CODE> allows dot to include newline, and <CODE>/m</CODE>
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.</P>
<P>For example, this program detects duplicate words, even when they span
line breaks (but not paragraph ones). For this example, we don't need
<CODE>/s</CODE> because we aren't using dot in a regular expression that we want
to cross line boundaries. Neither do we need <CODE>/m</CODE> 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.</P>
<PRE>
$/ = ''; # 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";
}
}</PRE>
<P>Here's code that finds sentences that begin with ``From '' (which would
be mangled by many mailers):</P>
<PRE>
$/ = ''; # 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";
}
}</PRE>
<P>Here's code that finds everything between START and END in a paragraph:</P>
<PRE>
undef $/; # read in whole file, not just one line or paragraph
while ( <> ) {
while ( /START(.*?)END/sm ) { # /s makes . cross line boundaries
print "$1\n";
}
}</PRE>
<P>
<H2><A NAME="how can i pull out lines between two patterns that are themselves on different lines">How can I pull out lines between two patterns that are themselves on different lines?</A></H2>
<P>You can use Perl's somewhat exotic <CODE>..</CODE> operator (documented in
perl -ne 'print if /START/ .. /END/' file1 file2 ...</PRE>
<P>If you wanted text and not lines, you would use</P>
<PRE>
perl -0777 -ne 'print "$1\n" while /START(.*?)END/gs' file1 file2 ...</PRE>
<P>But if you want nested occurrences of <CODE>START</CODE> through <CODE>END</CODE>, you'll
run up against the problem described in the question in this section
on matching balanced text.</P>
<P>Here's another example of using <CODE>..</CODE>:</P>
<PRE>
while (<>) {
$in_header = 1 .. /^$/;
$in_body = /^$/ .. eof();
# now choose between them
} continue {
reset if eof(); # fix $.
}</PRE>
<P>
<H2><A NAME="i put a regular expression into $/ but it didn't work. what's wrong">I put a regular expression into $/ but it didn't work. What's wrong?</A></H2>
<P>$/ must be a string, not a regular expression. Awk has to be better
for something. :-)</P>
<P>Actually, you could do this if you don't mind reading the whole file
into memory:</P>
<PRE>
undef $/;
@records = split /your_pattern/, <FH>;</PRE>
<P>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.</P>
<PRE>
## Create a file with three lines.
open FH, ">file";
print FH "The first line\nThe second line\nThe third line\n";
close FH;</PRE>
<PRE>
## Get a read/write filehandle to it.
$fh = new FileHandle "+<file";</PRE>
<PRE>
## Attach it to a "stream" object.
use Net::Telnet;
$file = new Net::Telnet (-fhopen => $fh);</PRE>
<PRE>
## Search for the second line and print out the third.
$file->waitfor('/second line\n/');
print $file->getline;</PRE>
<P>
<H2><A NAME="how do i substitute case insensitively on the lhs, but preserving case on the rhs">How do I substitute case insensitively on the LHS, but preserving case on the RHS?</A></H2>
<P>Here's a lovely Perlish solution by Larry Rosler. It exploits
properties of bitwise xor on ASCII strings.</P>
<PRE>
$_= "this is a TEsT case";</PRE>
<PRE>
$old = 'test';
$new = 'success';</PRE>
<PRE>
s{(\Q$old\E}
{ uc $new | (uc $1 ^ $1) .
(uc(substr $1, -1) ^ substr $1, -1) x
(length($new) - length $1)
}egi;</PRE>
<PRE>
print;</PRE>
<P>And here it is as a subroutine, modelled after the above:</P>
<PRE>
sub preserve_case($$) {
my ($old, $new) = @_;
my $mask = uc $old ^ $old;</PRE>
<PRE>
uc $new | $mask .
substr($mask, -1) x (length($new) - length($old))
}</PRE>
<PRE>
$a = "this is a TEsT case";
$a =~ s/(test)/preserve_case($1, "success")/egi;
print "$a\n";</PRE>
<P>This prints:</P>
<PRE>
this is a SUcCESS case</PRE>
<P>Just to show that C programmers can write C in any programming language,
if you prefer a more C-like solution, the following script makes the
substitution have the same case, letter by letter, as the original.
(It also happens to run about 240% slower than the Perlish solution runs.)
If the substitution has more characters than the string being substituted,
the case of the last character is used for the rest of the substitution.</P>
<PRE>
# Original by Nathan Torkington, massaged by Jeffrey Friedl
#
sub preserve_case($$)
{
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 : $newlen;</PRE>
<PRE>
for ($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)
<H2><A NAME="how can i match a localesmart version of /[azaz]/">How can I match a locale-smart version of <CODE>/[a-zA-Z]/</CODE>?</A></H2>
<P>One alphabetic character would be <CODE>/[^\W\d_]/</CODE>, no matter what locale
you're in. Non-alphabetics would be <CODE>/[\W\d_]/</CODE> (assuming you don't
consider an underscore a letter).</P>
<P>
<H2><A NAME="how can i quote a variable to use in a regex">How can I quote a variable to use in a regex?</A></H2>
<P>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 <CODE>s///</CODE> substitution is considered
a double-quoted string (see <A HREF="../../lib/Pod/perlop.html">the perlop manpage</A> 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:</P>
<PRE>
$string = "to die?";
$lhs = "die?";
$rhs = "sleep, no more";</PRE>
<PRE>
$string =~ s/\Q$lhs/$rhs/;
# $string is now "to sleep no more"</PRE>
<P>Without the \Q, the regex would also spuriously match ``di''.</P>
<P>
<H2><A NAME="what is /o really for">What is <CODE>/o</CODE> really for?</A></H2>
<P>Using a variable in a regular expression match forces a re-evaluation
(and perhaps recompilation) each time through. The <CODE>/o</CODE> 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.</P>
<P>Use of <CODE>/o</CODE> 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 <EM>very
first</EM> time.</P>
<P><CODE>/o</CODE> 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.</P>
<P>For example, here's a ``paragrep'' program:</P>
<PRE>
$/ = ''; # paragraph mode
$pat = shift;
while (<>) {
print if /$pat/o;
}</PRE>
<P>
<H2><A NAME="how do i use a regular expression to strip c style comments from a file">How do I use a regular expression to strip C style comments from a file?</A></H2>
<P>While this actually can be done, it's much harder than you'd think.
For example, this one-liner</P>
<PRE>
perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c</PRE>
<P>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.</P>
<P>The C::Scan module from CPAN contains such subs for internal usage,
but they are undocumented.</P>
<P>
<H2><A NAME="what does it mean that regexes are greedy how can i get around it">What does it mean that regexes are greedy? How can I get around it?</A></H2>
<P>Most people mean that greedy regexes match as much as they can.
Technically speaking, it's actually the quantifiers (<CODE>?</CODE>, <CODE>*</CODE>, <CODE>+</CODE>,
<CODE>{}</CODE>) 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 (<CODE>??</CODE>, <CODE>*?</CODE>, <CODE>+?</CODE>, <CODE>{}?</CODE>).</P>
<P>An example:</P>
<PRE>
$s1 = $s2 = "I am very very cold";
$s1 =~ s/ve.*y //; # I am cold
$s2 =~ s/ve.*?y //; # I am very cold</PRE>
<P>Notice how the second substitution stopped matching as soon as it
encountered ``y ''. The <CODE>*?</CODE> 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.</P>
<P>
<H2><A NAME="how do i process each word on each line">How do I process each word on each line?</A></H2>
<P>Use the split function:</P>
<PRE>
while (<>) {
foreach $word ( split ) {
# do something with $word here
}
}</PRE>
<P>Note that this isn't really a word in the English sense; it's just
chunks of consecutive non-whitespace characters.</P>
<P>To work with only alphanumeric sequences, you might consider</P>
<PRE>
while (<>) {
foreach $word (m/(\w+)/g) {
# do something with $word here
}
}</PRE>
<P>
<H2><A NAME="how can i print out a wordfrequency or linefrequency summary">How can I print out a word-frequency or line-frequency summary?</A></H2>
<P>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:</P>
<PRE>
while (<>) {
while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'"
$seen{$1}++;
}
}
while ( ($word, $count) = each %seen ) {
print "$count $word\n";
}</PRE>
<P>If you wanted to do the same thing for lines, you wouldn't need a
regular expression:</P>
<PRE>
while (<>) {
$seen{$_}++;
}
while ( ($line, $count) = each %seen ) {
print "$count $line";
}</PRE>
<P>If you want these output in a sorted order, see the section on Hashes.</P>
<P>
<H2><A NAME="how can i do approximate matching">How can I do approximate matching?</A></H2>
<P>See the module String::Approx available from CPAN.</P>
<P>
<H2><A NAME="how do i efficiently match many regular expressions at once">How do I efficiently match many regular expressions at once?</A></H2>
<P>The following is extremely inefficient:</P>
<PRE>
# slow but obvious way
@popstates = qw(CO ON MI WI MN);
while (defined($line = <>)) {
for $state (@popstates) {
if ($line =~ /\b$state\b/i) {
print $line;
last;
}
}
}</PRE>
<P>That's because Perl has to recompile all those patterns for each of
the lines of the file. As of the 5.005 release, there's a much better
approach, one which makes use of the new <CODE>qr//</CODE> operator:</P>
<PRE>
# use spiffy new qr// operator, with /i flag even
use 5.005;
@popstates = qw(CO ON MI WI MN);
@poppats = map { qr/\b$_\b/i } @popstates;
while (defined($line = <>)) {
for $patobj (@poppats) {
print $line if $line =~ /$patobj/;
}
}</PRE>
<P>
<H2><A NAME="why don't wordboundary searches with \b work for me">Why don't word-boundary searches with <CODE>\b</CODE> work for me?</A></H2>
<P>Two common misconceptions are that <CODE>\b</CODE> is a synonym for <CODE>\s+</CODE>, and
that it's the edge between whitespace characters and non-whitespace
characters. Neither is correct. <CODE>\b</CODE> is the place between a <CODE>\w</CODE>
character and a <CODE>\W</CODE> character (that is, <CODE>\b</CODE> is the edge of a
``word''). It's a zero-width assertion, just like <CODE>^</CODE>, <CODE>$</CODE>, and all
the other anchors, so it doesn't consume any characters. <A HREF="../../lib/Pod/perlre.html">the perlre manpage</A>
describes the behavior of all the regex metacharacters.</P>
<P>Here are examples of the incorrect application of <CODE>\b</CODE>, with fixes:</P>
<PRE>
"two words" =~ /(\w+)\b(\w+)/; # WRONG
"two words" =~ /(\w+)\s+(\w+)/; # right</PRE>
<PRE>
" =matchless= text" =~ /\b=(\w+)=\b/; # WRONG
" =matchless= text" =~ /=(\w+)=/; # right</PRE>
<P>Although they may not do what you thought they did, <CODE>\b</CODE> and <CODE>\B</CODE>
can still be quite useful. For an example of the correct use of
<CODE>\b</CODE>, see the example of matching duplicate words over multiple
lines.</P>
<P>An example of using <CODE>\B</CODE> is the pattern <CODE>\Bis\B</CODE>. This will find
occurrences of ``is'' on the insides of words only, as in ``thistle'', but
not ``this'' or ``island''.</P>
<P>
<H2><A NAME="why does using $&, $`, or $' slow my program down">Why does using $&, $`, or $' slow my program down?</A></H2>
<P>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 regex that contains capturing
parentheses. But if you never use $&, etc., in your script, then regexes
<EM>without</EM> 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 $&
variable is no longer ``expensive'' the way the other two are.</P>
<P>
<H2><A NAME="what good is \g in a regular expression">What good is <CODE>\G</CODE> in a regular expression?</A></H2>
<P>The notation <CODE>\G</CODE> is used in a match or substitution in conjunction the
<CODE>/g</CODE> modifier (and ignored if there's no <CODE>/g</CODE>) to anchor the regular
expression to the point just past where the last match occurred, i.e. the
<A HREF="../../lib/Pod/perlfunc.html#item_pos"><CODE>pos()</CODE></A> point. A failed match resets the position of <CODE>\G</CODE> unless the
<CODE>/c</CODE> modifier is in effect.</P>
<P>For example, suppose you had a line of text quoted in standard mail
and Usenet notation, (that is, with leading <CODE>></CODE> characters), and
you want change each leading <CODE>></CODE> into a corresponding <CODE>:</CODE>. You
could do so in this way:</P>
<PRE>
s/^(>+)/':' x length($1)/gem;</PRE>
<P>Or, using <CODE>\G</CODE>, the much simpler (and faster):</P>
<PRE>
s/\G>/:/g;</PRE>
<P>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 <CODE>/c</CODE>, which prevents a failed match with <CODE>/g</CODE> from
resetting the search position back to the beginning of the string.)</P>
