Perl Cookbook

Reading Records with a Pattern Separator

Problem

You want to read in records separated by a pattern, but Perl doesn’t allow its input record separator variable to be a regular expression.

Many problems, most obviously those involving the parsing of complex file formats, become a lot simpler when you are easily able to extract records that might be separated by a number of different strings.

Solution

Read the whole file and use split:

undef $/;
@chunks = split(/pattern/, <FILEHANDLE>);

Discussion

Perl’s record separator must be a fixed string, not a pattern. (After all, awk has to be better at something.) To sidestep this limitation, undefine the input record separator entirely so that the next line-read operation gets 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 lines 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 gets restored after the block finishes. By using split with parentheses in the pattern, ...