Regular expressions can only parse regular languages. More powerful parser techniques can parse context-free languages or even recursively enumerable languages.

Regular expressions can only parse regular languages. More powerful parser techniques can parse context-free languages or even recursively enumerable languages. These are fundamentally distinct levels of complexity. It is fundamentally impossible to correctly parse e.g. HTML with regular expressions. See: https://stackoverflow.com/a/1732454 https://stackoverflow.com/a/1732454