It isn't working because (...) is just matching 3 characters. What your regex is saying, is basically: Match any 3 characters, then any character 0 or more times, then any 3 characters.

You're reusing the regex and not the match. Back referencing essentially means that you'll match whatever is matched the first time, rather than reusing the regex pattern.

I hope that made a little sense. Grouping, capturing, matching and backreferences is part of what makes regex tricky and very powerful. You might want to use some of the online tools, which can help explain the regex visually.