As others have said, the speed difference can likely be neglected. But you should ask yourself if \d is really what you want. \d matches all Unicode characters that are digits, including many non-ASCII characters. Can your processing code later on handle those?

If not, use [0-9] instead.

It might be a tad faster to match it all in one capture, and unpack the individual pieces later, but you should really benchmark it before using it. 

use strict;
use warnings;
use 5.010;

my $str = '20091224';
if ($str =~ m/([0-9]{8})/) {
   say join '|', unpack "A4 A2 A2", $1;
}

[download]

But much more than such micro optimizations are gained by anchors: If you know something about the relative position of the searched string (like that it's at the beginning or end of a string), an anchor can avoid much backtracking and searching.

In reply to Re: better (faster) way of writing regexp by moritz
in thread better (faster) way of writing regexp by jodaka

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