MAN page from Old RedHat 5.X perl-5.004-4.i386.rpm
PERLFAQ6
Section: Perl Programmers Reference Guide (1)
Updated: perl 5.004, patch 04
Index 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 islittered with answers involving regular expressions. For example,decoding a URL and checking whether something is a number are handledwith regular expressions, but those answers are found elsewhere inthis document (in the section on Data and the Networking one onnetworking, to be precise).
How can I hope to use regular expressions without creating illegible and unmaintainable code?
Three techniques can make regular expressions maintainable andunderstandable.
- Comments Outside the Regexp
- Describe what you're doing and how you're doing it, using normal Perlcomments.
# 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 normalcomments there, too. As you can imagine, whitespace and comments helpa 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. deleteIt's still not quite so clear as prose, but it is very useful fordescribing 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 manpagedescribes this. For example, the s/// above uses braces asdelimiters. Selecting another delimiter can avoid quoting thedelimiter 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 thecorrect
modifier(s) on your pattern.
There are many ways to get multiline data into a string. If you wantit to happen automatically while reading input, you'll want to set $/(probably to `' for paragraphs or undef for the whole file) toallow 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 /mallows caret and dollar to match next to a newline, not just at theend of the string. You do need to make sure that you've actuallygot a multiline string in there.
For example, this program detects duplicate words, even when they spanline 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 wantto cross line boundaries. Neither do we need /m because we aren'twanting caret or dollar to match at any point inside the record nextto newlines. But it's imperative that $/ be set to something otherthan the default, or else we won't actually ever have a multilinerecord 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 wouldbe 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 (documented inthe
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'llrun up against the problem described in the question in this sectionon 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 betterfor something. :-)
Actually, you could do this if you don't mind reading the whole fileinto memory:
undef $/; @records = split /your_pattern/, <FH>;
The Net::Telnet module (available from
CPAN) has the capability towait for a pattern in the input stream, or timeout if it doesn'tappear 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, but preserving case on the RHS?
It depends on what you mean by ``preserving case''. The followingscript makes the substitution have the same case, letter by letter, asthe original. If the substitution has more characters than the stringbeing substituted, the case of the last character is used for the restof 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:
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 localeyou're in. Non-alphabetics would be
/[\W\d_]/ (assuming you don'tconsider an underscore a letter).
How can I quote a variable to use in a regexp?
The Perl parser will expand
$variable and
@variable references inregular expressions unless the delimiter is a single quote. Remember,too, that the right-hand side of a
s/// substitution is considereda double-quoted string (see the
perlop manpage for more details). Rememberalso that any regexp special characters will be acted on unless youprecede 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 modifierlocks in the regexp the first time it's used. This always happens in aconstant regular expression, and in fact, the pattern was compiledinto the internal format at the same time your entire program was.
Use of /o is irrelevant unless variable interpolation is used inthe pattern, and if so, the regexp engine will neither know nor carewhether the variables change after the pattern is evaluated the veryfirst time.
/o is often used to gain an extra measure of efficiency by notperforming subsequent evaluations when you know it won't matter(because you know the variables won't change), or more rarely, whenyou don't want the regexp 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 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.cwill work in many but not all cases. You see, it's too simple-minded forcertain kinds of C programs, in particular, those with what appear to becomments 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, addingwhitespace 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 needto use non-regexp techniques to parse balanced text, such as the textenclosed between matching parentheses or braces, for example.
An elaborate subroutine (for 7-bit ASCII only) to pull out balancedand possibly nested single chars, like ` and ', { and },or ( and ) can be found inhttp://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 localgreed and immediate gratification to overall greed. To get non-greedyversions 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 itencountered ``y ``. The
*? quantifier effectively tells the regularexpression engine to find a match as quickly as possible and passcontrol on to whatever is next in line, like you would if you wereplaying 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 justchunks 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'llpretend that by word you mean chunk of alphabetics, hyphens, orapostrophes, rather than the non-whitespace chunk idea of a word givenin 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 aregular expression:
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 extensionmodules from
CPAN (which might well be overkill for your purposes),or else put together something like this, inspired from a routinein 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 }; # 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+, andthat it's the edge between whitespace characters and non-whitespacecharacters. Neither is correct.
\b is the place between a
\wcharacter and a
\W character (that is,
\b is the edge of a``word"). It's a zero-width assertion, just like
^,
$, and allthe other anchors, so it doesn't consume any characters. the
perlre manpagedescribes 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
\Bcan still be quite useful. For an example of the correct use of
\b, see the example of matching duplicate words over multiplelines.
An example of using \B is the pattern \Bis\B. This will findoccurrences of ``is'' on the insides of words only, as in ``thistle'', butnot ``this'' or ``island''.
Why does using $&, $`, or $' slow my program down?
Because once Perl sees that you need one of these variables anywherein the program, it has to provide them on each and every patternmatch. The same mechanism that handles these provides for the use of
$1,
$2, etc., so you pay the same price for each regexp that containscapturing parentheses. But if you never use $&, etc., in your script,then regexps
without capturing parentheses won't be penalized. Soavoid $&, $', and $` if you can, but if you can't (and some algorithmsreally 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 regularexpression to the point just past where the last match occurred, i.e. the
pos() point.
For example, suppose you had a line of text quoted in standard mailand Usenet notation, (that is, with leading > characters), andyou want change each leading > into a corresponding :. Youcould 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 followinglex-like example is courtesy of Jeffrey Friedl. It did not work in5.003 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 fromresetting 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 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 infact implemented as NFAs (non-deterministic finite automata) to allowbacktracking and backreferencing. And they aren't
POSIX-style either,because those guarantee worst-case behavior for all cases. (It seemsthat some people prefer guarantees of consistency, even when what'sguaranteed is slowness.) See the book ``Mastering Regular Expressions''(from O'Reilly) by Jeffrey Friedl for all the details you could everhope to know on these matters (a full citation appears inthe
perlfaq2 manpage).
What's wrong with using grep or map in a void context?
Strictly speaking, nothing. Stylistically speaking, it's not a goodway to write maintainable code. That's because you're using theseconstructs not for their return values but rather for theirside-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 supportwide characters. It pretends that a byte and a character aresynonymous. The following set of approaches was offered by JeffreyFriedl, whose article in issue #5 of The Perl Journal talks about thisvery matter.
Let's suppose you have some weird Martian encoding where pairs ofASCII uppercase letters encode single Martian letters (i.e. the twobytes ``CV'' make a single Martian letter, as do the two bytes ``SG'',``VS'', ``XX'', etc.). Other bytes represent single characters, just likeASCII.
So, the string of Martian ``I am CVSGXX!'' uses 12 bytes to encode thenine characters `I', ` `, `a', `m', ` `, `CV', `SG', `XX', `!'.
Now, say you want to search for the single character /GX/. Perldoesn't know about Martian, so it'll find the two bytes ``GX'' in the ``Iam CVSGXX!'' string, even though that character isn't there: it justlooks 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";
In addition, a sample program which converts half-width to full-widthkatakana (in Shift-
JIS or
EUC encoding) is available from
CPAN as
There are many double- (and multi-) byte encodings commonly used thesedays. 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.
Index
- NAME
- DESCRIPTION
- How can I hope to use regular expressions without creating illegible and unmaintainable code?
- I'm having trouble matching over more than one line. What's wrong?
- How can I pull out lines between two patterns that are themselves on different lines?
- I put a regular expression into $/ but it didn't work. What's wrong?
- How do I substitute case insensitively on the LHS, but preserving case on the RHS?
- How can I make \w match accented characters?
- How can I match a locale-smart version of /[a-zA-Z]/?
- How can I quote a variable to use in a regexp?
- What is /o really for?
- How do I use a regular expression to strip C style comments from a file?
- Can I use Perl regular expressions to match balanced text?
- What does it mean that regexps are greedy? How can I get around it?
- How do I process each word on each line?
- How can I print out a word-frequency or line-frequency summary?
- How can I do approximate matching?
- How do I efficiently match many regular expressions at once?
- Why don't word-boundary searches with \b work for me?
- Why does using $&, $`, or $' slow my program down?
- What good is \G in a regular expression?
- Are Perl regexps DFAs or NFAs? Are they POSIX compliant?
- What's wrong with using grep or map in a void context?
- How can I match strings with multibyte characters?
- AUTHOR AND COPYRIGHT
This document was created byman2html,using the manual pages.
