Is it possible to construct a PCRE-style regular expression that will only match each letter in a list only once? For example, if you have the letters "lrsa" and you try matching a word list against: ^[lrsa]*m[lrsa]*$ you're going to match "lams" (valid), but also "lamas" (invalid for our purposes because you only had one "a"). If your letter set was "lrsaa", you would want to match "lamas". Is this possible with regular expressions, or should I handle it programmatically?