Here's a significantly optimized version of my previous solution, which draws heavily on anomalous's excellent work, but only bothers to capture the initial repeated component (in $1) as that's apparently all that's wanted.

my $REPETITION_PAT = qr{

    \A         # Start at the beginning of the string
    (.+?)      # Match minimal initial sequence (as $1)
    \1*+       # Rematch it exactly as often as possible (maybe zero)

    # Then verify what's left is a proper truncation...
    (?(?{ index($^N, substr($_,pos())) }) (?!) )

}xms;
[download]

It still passes BrowserUK's gauntlet, however in my own testing it's approximately 400 times faster than my previous attempt. I suspect that's still not sufficient to make it competitive with the non-regex solutions. (I know which one I'd rather maintain, though ;-)

As you can see, the key to the improvement in regex performance was—as usual—to eliminate opportunities for backtracking or capturing.

Damian

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