if (! /([smtwhfa])(?=.*?\1)/) {
    |    |         |
    |    |         +----- followed by this character
    |    +--------------- find any of these characters
    +-------------------- but take the negative of the match

[download]

bart's regex below is much better, see his reply and follow-up's.

In reply to Re: Re: Re: Regex (lookahead) Confusion by Roger
in thread Regex (lookahead) Confusion by ChrisR

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