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m:  # use : as delims so / in code won't hurt
  ^ # Anchor it at start, don't need to backtrack
  (? # Start a conditional test
...
    |(?!) # Simplest "T|F" - match succeed, nonmatch fails
  ) # End the condition
:x

#!/usr/bin/env perl
use Benchmark 'cmpthese';
...
pos_anch 1806441/s  168%      21%      15%       --      -5%     -67%
pos_look 1906963/s  183%      27%      21%       6%       --     -65%
3calls   5522908/s  719%     269%     252%     206%     190%       --