title: Match a character exactly once with regex
body: <p>How can I match a character exactly once with a regex in Java? Let's say I want to look for strings which contain exactly one time the digit 3, and it doesn't matter where it is.</p>
  
  <p>I tried to do this with ".*3{1}.*" but obviously this will also match "330" as I specified with the period that I don't care what character it is. How can I fix this?</p>