title: regular expression match if groups same
body: <p>I'm trying to match iff two capture groups are the same. I could manually check after the match, but I'm wondering if there is a way I can do this in the expression itself.</p>
  
  <p>My expression is <code>(\d+)\/(\d+)</code>, so I only want to accept strings where the two numbers are equal. Is there a nice way to check this in the regular expression, or do I have to manually check groups after?</p>
  
  <p>EDIT: This was marked a duplicate but the supposed duplicate question is not related and does not in any way answer my question...</p>