703 lines
24 KiB
Plaintext
703 lines
24 KiB
Plaintext
=head1 NAME
|
|
|
|
perlfaq6 - Regexes ($Revision: 1.27 $, $Date: 1999/05/23 16:08:30 $)
|
|
|
|
=head1 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).
|
|
|
|
=head2 How can I hope to use regular expressions without creating illegible and unmaintainable code?
|
|
|
|
Three techniques can make regular expressions maintainable and
|
|
understandable.
|
|
|
|
=over 4
|
|
|
|
=item Comments Outside the Regex
|
|
|
|
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) /meg;
|
|
|
|
=item Comments Inside the Regex
|
|
|
|
The C</x> modifier causes whitespace to be ignored in a regex 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.
|
|
|
|
C</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.
|
|
|
|
=item Different Delimiters
|
|
|
|
While we normally think of patterns as being delimited with C</>
|
|
characters, they can be delimited by almost any character. L<perlre>
|
|
describes this. For example, the C<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
|
|
|
|
=back
|
|
|
|
=head2 I'm having trouble matching over more than one line. What's wrong?
|
|
|
|
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 modifier(s) on your
|
|
pattern (possibly).
|
|
|
|
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 C<undef> for the whole file) to
|
|
allow you to read more than one line at a time.
|
|
|
|
Read L<perlre> to help you decide which of C</s> and C</m> (or both)
|
|
you might want to use: C</s> allows dot to include newline, and C</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
|
|
C</s> because we aren't using dot in a regular expression that we want
|
|
to cross line boundaries. Neither do we need C</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/sm ) { # /s makes . cross line boundaries
|
|
print "$1\n";
|
|
}
|
|
}
|
|
|
|
=head2 How can I pull out lines between two patterns that are themselves on different lines?
|
|
|
|
You can use Perl's somewhat exotic C<..> operator (documented in
|
|
L<perlop>):
|
|
|
|
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 C<START> through C<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 C<..>:
|
|
|
|
while (<>) {
|
|
$in_header = 1 .. /^$/;
|
|
$in_body = /^$/ .. eof();
|
|
# now choose between them
|
|
} continue {
|
|
reset if eof(); # fix $.
|
|
}
|
|
|
|
=head2 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);
|
|
|
|
## Search for the second line and print out the third.
|
|
$file->waitfor('/second line\n/');
|
|
print $file->getline;
|
|
|
|
=head2 How do I substitute case insensitively on the LHS, but preserving case on the RHS?
|
|
|
|
Here's a lovely Perlish solution by Larry Rosler. It exploits
|
|
properties of bitwise xor on ASCII strings.
|
|
|
|
$_= "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, modelled after the above:
|
|
|
|
sub preserve_case($$) {
|
|
my ($old, $new) = @_;
|
|
my $mask = uc $old ^ $old;
|
|
|
|
uc $new | $mask .
|
|
substr($mask, -1) x (length($new) - length($old))
|
|
}
|
|
|
|
$a = "this is a TEsT case";
|
|
$a =~ s/(test)/preserve_case($1, "success")/egi;
|
|
print "$a\n";
|
|
|
|
This prints:
|
|
|
|
this is a SUcCESS case
|
|
|
|
Just to show that C programmers can write C in any programming language,
|
|
if you prefer a more C-like solution, the following script makes the
|
|
substitution have the same case, letter by letter, as the original.
|
|
(It also happens to run about 240% slower than the Perlish solution runs.)
|
|
If the substitution has more characters than the string being substituted,
|
|
the case of the last character is used for the rest of the substitution.
|
|
|
|
# Original by Nathan Torkington, massaged by Jeffrey Friedl
|
|
#
|
|
sub preserve_case($$)
|
|
{
|
|
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;
|
|
}
|
|
|
|
=head2 How can I make C<\w> match national character sets?
|
|
|
|
See L<perllocale>.
|
|
|
|
=head2 How can I match a locale-smart version of C</[a-zA-Z]/>?
|
|
|
|
One alphabetic character would be C</[^\W\d_]/>, no matter what locale
|
|
you're in. Non-alphabetics would be C</[\W\d_]/> (assuming you don't
|
|
consider an underscore a letter).
|
|
|
|
=head2 How can I quote a variable to use in a regex?
|
|
|
|
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 C<s///> substitution is considered
|
|
a double-quoted string (see L<perlop> 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:
|
|
|
|
$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 regex would also spuriously match "di".
|
|
|
|
=head2 What is C</o> really for?
|
|
|
|
Using a variable in a regular expression match forces a re-evaluation
|
|
(and perhaps recompilation) each time through. The C</o> 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.
|
|
|
|
Use of C</o> 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 I<very
|
|
first> time.
|
|
|
|
C</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 regex to notice if they do.
|
|
|
|
For example, here's a "paragrep" program:
|
|
|
|
$/ = ''; # paragraph mode
|
|
$pat = shift;
|
|
while (<>) {
|
|
print if /$pat/o;
|
|
}
|
|
|
|
=head2 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 and later modified by Fred Curtis.
|
|
|
|
$/ = undef;
|
|
$_ = <>;
|
|
s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs
|
|
print;
|
|
|
|
This could, of course, be more legibly written with the C</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
|
|
)
|
|
}{$2}gxs;
|
|
|
|
A slight modification also removes C++ comments:
|
|
|
|
s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs;
|
|
|
|
=head2 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
|
|
(C<\1> and its ilk), they still aren't powerful enough -- with
|
|
the possible exception of bizarre and experimental features in the
|
|
development-track releases of Perl. You still need to use non-regex
|
|
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 C<`> and C<'>, C<{> and C<}>,
|
|
or C<(> and C<)> 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.
|
|
|
|
=head2 What does it mean that regexes are greedy? How can I get around it?
|
|
|
|
Most people mean that greedy regexes match as much as they can.
|
|
Technically speaking, it's actually the quantifiers (C<?>, C<*>, C<+>,
|
|
C<{}>) 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 (C<??>, C<*?>, C<+?>, C<{}?>).
|
|
|
|
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 C<*?> 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.
|
|
|
|
=head2 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
|
|
}
|
|
}
|
|
|
|
=head2 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:
|
|
|
|
while (<>) {
|
|
$seen{$_}++;
|
|
}
|
|
while ( ($line, $count) = each %seen ) {
|
|
print "$count $line";
|
|
}
|
|
|
|
If you want these output in a sorted order, see the section on Hashes.
|
|
|
|
=head2 How can I do approximate matching?
|
|
|
|
See the module String::Approx available from CPAN.
|
|
|
|
=head2 How do I efficiently match many regular expressions at once?
|
|
|
|
The following is extremely inefficient:
|
|
|
|
# slow but obvious way
|
|
@popstates = qw(CO ON MI WI MN);
|
|
while (defined($line = <>)) {
|
|
for $state (@popstates) {
|
|
if ($line =~ /\b$state\b/i) {
|
|
print $line;
|
|
last;
|
|
}
|
|
}
|
|
}
|
|
|
|
That's because Perl has to recompile all those patterns for each of
|
|
the lines of the file. As of the 5.005 release, there's a much better
|
|
approach, one which makes use of the new C<qr//> operator:
|
|
|
|
# use spiffy new qr// operator, with /i flag even
|
|
use 5.005;
|
|
@popstates = qw(CO ON MI WI MN);
|
|
@poppats = map { qr/\b$_\b/i } @popstates;
|
|
while (defined($line = <>)) {
|
|
for $patobj (@poppats) {
|
|
print $line if $line =~ /$patobj/;
|
|
}
|
|
}
|
|
|
|
=head2 Why don't word-boundary searches with C<\b> work for me?
|
|
|
|
Two common misconceptions are that C<\b> is a synonym for C<\s+>, and
|
|
that it's the edge between whitespace characters and non-whitespace
|
|
characters. Neither is correct. C<\b> is the place between a C<\w>
|
|
character and a C<\W> character (that is, C<\b> is the edge of a
|
|
"word"). It's a zero-width assertion, just like C<^>, C<$>, and all
|
|
the other anchors, so it doesn't consume any characters. L<perlre>
|
|
describes the behavior of all the regex metacharacters.
|
|
|
|
Here are examples of the incorrect application of C<\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, C<\b> and C<\B>
|
|
can still be quite useful. For an example of the correct use of
|
|
C<\b>, see the example of matching duplicate words over multiple
|
|
lines.
|
|
|
|
An example of using C<\B> is the pattern C<\Bis\B>. This will find
|
|
occurrences of "is" on the insides of words only, as in "thistle", but
|
|
not "this" or "island".
|
|
|
|
=head2 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 regex that contains capturing
|
|
parentheses. But if you never use $&, etc., in your script, then regexes
|
|
I<without> 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.
|
|
|
|
=head2 What good is C<\G> in a regular expression?
|
|
|
|
The notation C<\G> is used in a match or substitution in conjunction the
|
|
C</g> modifier (and ignored if there's no C</g>) to anchor the regular
|
|
expression to the point just past where the last match occurred, i.e. the
|
|
pos() point. A failed match resets the position of C<\G> unless the
|
|
C</c> modifier is in effect.
|
|
|
|
For example, suppose you had a line of text quoted in standard mail
|
|
and Usenet notation, (that is, with leading C<< > >> characters), and
|
|
you want change each leading C<< > >> into a corresponding C<:>. You
|
|
could do so in this way:
|
|
|
|
s/^(>+)/':' x length($1)/gem;
|
|
|
|
Or, using C<\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</c>, which prevents a failed match with C</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.
|
|
|
|
=head2 Are Perl regexes 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
|
|
L<perlfaq2>).
|
|
|
|
=head2 What's wrong with using grep or map in a void context?
|
|
|
|
Both grep and map build a return list, regardless of their context.
|
|
This means you're making Perl go to the trouble of building up a
|
|
return list that you then just ignore. That's no way to treat a
|
|
programming language, you insensitive scoundrel!
|
|
|
|
=head2 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 C</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 $1 eq 'GX';
|
|
}
|
|
|
|
Or like this:
|
|
|
|
die "sorry, Perl doesn't (yet) have Martian support )-:\n";
|
|
|
|
There are many double- (and multi-) byte encodings commonly used these
|
|
days. Some versions of these have 1-, 2-, 3-, and 4-byte characters,
|
|
all mixed.
|
|
|
|
=head2 How do I match a pattern that is supplied by the user?
|
|
|
|
Well, if it's really a pattern, then just use
|
|
|
|
chomp($pattern = <STDIN>);
|
|
if ($line =~ /$pattern/) { }
|
|
|
|
Or, since you have no guarantee that your user entered
|
|
a valid regular expression, trap the exception this way:
|
|
|
|
if (eval { $line =~ /$pattern/ }) { }
|
|
|
|
But if all you really want to search for a string, not a pattern,
|
|
then you should either use the index() function, which is made for
|
|
string searching, or if you can't be disabused of using a pattern
|
|
match on a non-pattern, then be sure to use C<\Q>...C<\E>, documented
|
|
in L<perlre>.
|
|
|
|
$pattern = <STDIN>;
|
|
|
|
open (FILE, $input) or die "Couldn't open input $input: $!; aborting";
|
|
while (<FILE>) {
|
|
print if /\Q$pattern\E/;
|
|
}
|
|
close FILE;
|
|
|
|
=head1 AUTHOR AND COPYRIGHT
|
|
|
|
Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
|
|
All rights reserved.
|
|
|
|
When included as part of the Standard Version of Perl, or as part of
|
|
its complete documentation whether printed or otherwise, this work
|
|
may be distributed only under the terms of Perl's Artistic License.
|
|
Any distribution of this file or derivatives thereof I<outside>
|
|
of that package require that special arrangements be made with
|
|
copyright holder.
|
|
|
|
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.
|