Others mentioned, but to make it more clear, the problem with regex is not that it gets messy/complex, but that it's formally proven that it's not possible to use regex for recursive parsing.
In fact regex has a regular grammar, and based on language theories, it's not possible to parse context-free grammar with a regular language.
In fact regex has a regular grammar, and based on language theories, it's not possible to parse context-free grammar with a regular language.
See langsec for more.