For context, this is for a pretty strange use case — I was trying to preprocess user-submitted LaTeX with PHP, by replacing occurrences of a specific macro with

For context, this is for a pretty strange use case — I was trying to preprocess user-submitted LaTeX with PHP, by replacing occurrences of a specific macro with simply the value of its first argument. The macro just colour-codes its first parameter presentationally, whilst the rest of the LaTeX needs to be passed to a compute engine for solving. The compute engine doesn't know how to "solve" a colour, hence why I'm trying to strip them out beforehand.