I think you'll find this will do the trick:

$rem = $W - 3;
qr/^ (?:.{$W})* .{0,$rem} (.)\1\1/x

[download]

The idea here is to minimize the branching the RE engine will have to do. The logic is pretty similar to what you might do if you had the string split into lines; just skip 0..$H1 rows, and we know we're at the beginning of a row, so from there we just match 0..$W-3 characters followed by a repeating sequence of 3 with your original regex.

Performance is the same (a few % better actually) as the plain /(.)\1\1/, and several times faster than anything I tried with unpack or split.

Edit: You can get another ~25% or so if your character set really is small like [ABCD] by unrolling (.)\1\1 into (?:AAA|BBB|CCC|DDD). If you're not just using this as a boolean test and still need the character in $1, use (AAA|BBB|CCC|DDD) instead and use substr($1,0,1) to grab the first character if you get a match. The idea here is to push the more expensive operations out of the hot loop that's called millions of times.

___________
1. $H-1, actually, but it makes no measurable difference to the efficiency, or correctness. Edit: Changed {0,$H} to *, to shave a few keystrokes. Thanks LanX!

use strict; use warnings; omitted for brevity.

In reply to Re: Regex matching on grid alignment by rjt
in thread Regex matching on grid alignment by Anonymous Monk

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