I wanted to check our codebase for where we had six repeating characters in HTML and CSS for colours, e.g. #FFFFFF for white.  Here’s the regular expression for it, basically blogged here so I can remember where to find it later:

([a-fA-F0-9])1{5}

So [a-fA-F0-9] looks for a single character that is one of the letters A – F (only need to go up to F because we’re dealing with hexadecimal numbers), or the digits 0 – 9.

The ( ) parentheses around that turns it into a backreference.

The 1 then refers to that backreferenced matched, and the {5} says to match it exactly 5 times.

So it’ll find where there’s a single matching character that is then repeated five times.

What I was doing this for was to replace all six character codes (that repeat) for colours with three character codes (because #FFF is equivalent to #FFFFFF). So to…