At the risk of getting rebuked again while commenting on a japhy regex ;-P

Overlapping matches seem to cause a problem... For example, the last four characters in 'abcabc' and 'caWcabc' match, yet the function only returns the last three.

print longest_common_substr('abcabc','caWcabc');  # 'abc' not 'cabc'
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I think it might be as simple as removing the /g (but thats the part I don't fully comprehend....) The code forces each match to be bigger than the previous one, with greedy matching helping us rachet up several steps at a time. The /g might be doing something else tricky, but I don't see it.

Update: Other word pairs that fail similarly are sense/tense and onion/union.

-Blake

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