What are regular expressions theoretically strong enough for? [closed]

Question

If you ask a question about parsing HTML with regex, you will certainly be referenced to this famous rant. Though there is not a canonical rant for it, I've also been told that regex aren't powerful enough to parse SQL.

I'm a self-taught programmer, so I don't know much about languages from a theoretical perspective. Practically speaking, what are examples of languages or grammars that regex can always parse successfully?

To be specific, I'd really like a few examples of languages that are used in the real world that fit in the category of regular languages, rather than some axioms or equivalent conditions, etc.

Answer 1

Regexes are great for parsing things with only repetitions. They go wrong when you have forms of recursion. I think most useful is showing the simplest language it can't parse:

n open parenthesis followed by n close parenthesis, so for instance: (()) and ((((()))))

If you know you cannot parse that, you can easily conclude that you cannot parse most programming languages.

So I think you could parse basic SQL (though not if you would allow stuff like subqueries). Other prime examples of regex-parseable strings are web adresses, email-adresses, phonenumbers, etc.

If you're looking for actual programming languages which one can parse using regexes you won't find many (though I think (from my limited knowledge of it) parsing assembly should be possible. Most uses however are found in parsing simple strings, or lines.