This came up on the London.pm mailing list this morning. Why does the following code do what it does:

#!/usr/local/bin/perl -w

use strict;

my $x = "wibble mmnipm";

if ($x =~ mmnipm) {
  print "match\n";
} else {
  print "no match\n";
}

[download]

A couple of people suggested that the mmnipm is being interpreted as m/nip/, but further investigation shows it's being interpreted as m/mmnipm/.

perlop says this about the binding operator:

If the right argument is an expression rather than a search pattern, substitution, or transliteration, it is interpreted as a search pattern at run time. This can be less efficient than an explicit search, because the pattern must be compiled every time the expression is evaluated.

Which explains (almost) what's going on, but the one mystery remaining is why the code doesn't trigger a bareword error under use strict 'subs'. Actually under 5.005_02, it does give an error, but that error seems to have been removed in 5.005_03.

My other question is, why isn't mmnipm interpreted as m/nip/? I'm sure that I've read that if you're using a letter as the regex delimiter then you need to put a space before it (and testing shows that m mnipm is parsed as m/nip/) but I can't find a reference anywhere to this behaviour.

--
<http://www.dave.org.uk>

"The first rule of Perl club is you do not talk about Perl club."
-- Chip Salzenberg

In reply to Bareword Regex by davorg

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