while (<DATA>) {
    chomp;
    if (! /([smtwhfa])(?=.*?\1)/) {
       print "$_ : OK\n";
    } else {
       print "$_ : Not OK\n";
    }
}


__DATA__
smsa
smta
stmwhas

[download]
And the output -
smsa : Not OK
smta : OK
stmwhas : Not OK

[download]

Update: Ah! Thanks people for pointing out that my regex didn't check for non-allowed characters. bart's solution is perfect.

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

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