# 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;
"/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.
s/\/usr\/local/\/usr\/share/g; # bad delimiter choice
s#/usr/local#/usr/share#g; # better
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 ( <> ) {
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/sgm ) { # /s makes . cross line boundaries
print "$1\n";
}
}
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 {
$. = 0 if eof; # fix $.
}
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;
}
$_= "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($$) {
my ($old, $new) = @_;
my $mask = uc $old ^ $old;
uc $new | $mask .
substr($mask, -1) x (length($new) - length($old))
}
$a = "this is a TEsT case";
$a =~ s/(test)/preserve_case($1, "success")/egi;
print "$a\n";
This prints:
this is a SUcCESS case
As an alternative, to keep the case of the replacement word if it is longer than the original, you can use this code, by Jeff Pinyan:
sub preserve_case {
my ($from, $to) = @_;
my ($lf, $lt) = map length, @_;
if ($lt < $lf) { $from = substr $from, 0, $lt }
else { $from .= substr $to, $lf }
return uc $to | ($from ^ uc $from);
}
This changes the sentence to ``this is a SUcCess case.''
Just to show that C programmers can write C in any programming language, if you prefer a more C-like solution, the following script makes the substitution have the same case, letter by letter, as the original. (It also happens to run about 240% slower than the Perlish solution runs.) If the substitution has more characters than the string being substituted, the case of the last character is used for the rest of the substitution.
# Original by Nathan Torkington, massaged by Jeffrey Friedl
#
sub preserve_case($$)
{
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;
}
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_]/".
$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.
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';
$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/;
}
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, as long as they are not spread over multiple lines using a continuation character):
s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse;
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.
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.
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/(\w+)/g) {
# do something with $word here
}
}
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 perlfaq4: ``How do I sort a hash (optionally by value instead of key)?''.
Avoid asking Perl to compile 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.
@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.
@patterns = map { qr/\b$_\b/i } qw( foo bar baz );
LINE: while( <> )
{
foreach $pattern ( @patterns )
{
print if /$pattern/i;
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.
$regex = join '|', qw( foo bar baz );
LINE: while( <> )
{
print if /\b(?:$regex)\b/i;
}
For more details on regular expression efficiency, see Mastering Regular Expressions by Jeffrey Freidl. He explains how regular expressions engine work and why some patterns are surprisingly inefficient. Once you understand how perl applies regular expressions, you can tune them for individual situations.
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
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.
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.
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.
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:
@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';
}
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.
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 $@;
}
Date: $Date: 2007-10-27 21:29:20 +0200 (Sat, 27 Oct 2007) $
See perlfaq for source control details and availability.
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.