Comment by ogogmad

Regular expressions will always remain fundamental to computer science: They characterise all of those - and only those - conditions on bytestrings (or bitstrings, or Unicode strings, etc) which are checkable in constant memory.* In other words, they characterise the set of all "regular languages", which is a name for DSPACE(O(1)). Furthermore, regular expressions can be matched in O(n) time and O(1) memory, within a single left-to-right pass, which is the highest level of efficiency mathematically possible. Since they operate on bytestrings, they can be applied to computer memory and computer state itself, which are ultimately just bytestrings, and not just to text.

To be fair, you might know all of that, but I wanted to highlight this. LLMs are a lot less efficient than regular expressions wherever both are applicable, simply because everything is less efficient than regular expressions.

* By constant memory, I mean that the memory usage has a maximum value independent of the size or the contents of the input bytestring.