title: Regex to use each letter only once?
body: <p>Is it possible to construct a PCRE-style regular expression that will only match each letter in a list only once?</p>
  
  <p>For example, if you have the letters "lrsa" and you try matching a word list against:</p>
  
  <pre><code>^[lrsa]*m[lrsa]*$
  </code></pre>
  
  <p>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".</p>
  
  <p>Is this possible with regular expressions, or should I handle it programmatically?</p>