[sigil]

HTML can be described by a context-free grammar [0], but not by a regular grammar [1]. If a language can be described by a regular grammar, you can parse it with a regular expression -- that's where the "regular" in RegExp comes from!

Derivatives of RegExps don't automatically unlock parsing of context-free grammars, afaik. For that you need recursion. They do however unlock some very elegant parser designs.

[0] https://en.wikipedia.org/wiki/Context-free_grammar

[1] https://en.wikipedia.org/wiki/Regular_grammar