Perl Cd Bookshelf [Electronic resources] نسخه متنی

6.7.1. Problem

Perl's official record separator, the $/ variable,
must be a fixed string, not a pattern. To sidestep this limitation,
undefine the input record separator entirely so that the next
readline operation reads the rest of the file.
This is sometimes called slurp mode, because
it slurps in the whole file as one big string. Then
split that huge string using the record separating
pattern as the first argument.

Here's an example where the input stream is a text file that includes
lines consisting of ".Se",
".Ch", and ".Ss", which are
special codes in the troff macro set that this
book was developed under. These strings are the separators, and we
want to find text that falls between them.

# .Ch, .Se and .Ss divide chunks of STDIN
{
local $/ = undef;
@chunks = split(/^\.(Ch|Se|Ss)$/m, <>);
}
print "I read ", scalar(@chunks), " chunks.\n";

We create a localized version of $/ so its
previous value is restored once the block finishes. By using
split with parentheses in the pattern, captured
separators are also returned. This way data elements in the return
list alternate with elements containing "Se",
"Ch", or "Ss".

If you don't want separators returned, but still need parentheses,
use non-capturing parentheses in the pattern:
/^\.(?:Ch|Se|Ss)$/m.

To split before a pattern but include the
pattern in the return, use a lookahead assertion:
/^(?=\.(?:Ch|Se|Ss))/m. That way each chunk except
the first starts with the pattern.

Be aware that this uses a lot of memory when the file is large.
However, with today's machines and typical text files, this is less
often an issue now than it once was. Just don't try it on a 200 MB
logfile unless you have plenty of virtual memory for swapping out to
disk! Even if you do have enough swap space, you'll likely end up
thrashing.