> I would have assumed that the "correct" way to write a syntax highlighter would be to parse into an AST and then […] I'm guessing that if this isn't done, it'

> I would have assumed that the "correct" way to write a syntax highlighter would be to parse into an AST and then […] I'm guessing that if this isn't done, it's for efficiency reasons It’s not only running time, but also ease of implementation. A good syntax highlighter should do a decent job highlighting both valid and invalid programs (rationale: in most (editor, language) pairs, writing a program involves going through moments where the program being written isn’t a valid program) If you decide to use an AST, that means you need to have good heuristics for turning invalid programs into valid ones that best mimic what the programmer intended. That can be difficult to achieve (good compilers have such heuristics, but even if you have such a compiler, chances are it isn’t possible to reuse them for syntax coloring) If this simpler approach gives you most of what you can get with the AST approach, why bother writing that? Also, there are languages where some programs can’t be perfectly parsed or syntax colored without running them. For those, you need this approach.