Thanks. You're right, using the * instead of the + allows the regex to return true if the string is blank. I did want to be sure that string contained at least one of the allowed characters. 
$string =~ /^(?:([smtwhfa])(?!.*\1))+$/

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

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