title: Substituting a regex only when it doesn't match another regex (Python)
body: <p>Long story short, I have two regex patterns. One pattern matches things that I want to replace, and the other pattern matches a special case of those patterns that should not be replace. For a simple example, imagine that the first one is "\{.*\}" and the second one is "\{\{.*\}\}". Then "{this}" should be replaced, but "{{this}}" should not. Is there an easy way to take a string and say "substitute all instances of the first string with "hello" so long as it is not matching the second string"?</p>
  
  <p>In other words, is there a way to make a regex that is "matches the first string but not the second" easily without modifying the first string? I know that I could modify my first regex by hand to never match instances of the second, but as the first regex gets more complex, that gets very difficult.</p>