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# 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;
/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.
/
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
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"; } }
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"; } }
..
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.
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);
## Search for the second line and print out the third. $file->waitfor('/second line\n/'); print $file->getline;
# Original by Nathan Torkington, massaged by Jeffrey Friedl # { my ($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
\w
match accented characters?
/[a-zA-Z]/
?
/[^\W\d_]/
, no matter what locale you're in. Non-alphabetics would be /[\W\d_]/
(assuming you don't consider an underscore a letter).
$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''.
/o
really for?
/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:
$/ = ''; # paragraph mode $pat = shift; while (<>) { print if /$pat/o; }
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.
\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.
?
, *
, +
,
{}
) 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 (??
, *?
, +?
, {}?
).
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, 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 the section on Hashes.
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 };
# feed me /etc/termcap, prolly while ( <> ) { print "1: $_" if &$f1; print "2: $_" if &$f2; }
\b
work for me?
\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''.
\G
in a regular expression?
\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.
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.
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).
grep()
that's not better
written as a for
(well, foreach
, technically) loop.
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";
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.
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