- The really easy ones. A simple string search/split will do and a regex would be overkill. - The really hard ones. You'll need to fully parse this and using a regex will result in fragile/hard to understand code.
Please don't use regexes in production software. Learn how to write simple parsing code.
I've been writing code for over 15 years and if I said I had reached for regular expressions maybe 10 times in that entire time, I'm pretty sure I wouldn't be off even by an order of magnitude.
There are very few cases where they're the appropriate tool, I think. They don't compose(unless you use a higher level library that lets you construct them declaratively maybe), they are hard to read and hard to debug and nearly impossible to extend.
In my opinion, parser combinators are state of the art for building parsers. They can read like prose, are very easy to extend and usually easy to debug as well since you can easily get detailed error messages.
Regex can indeed lead to issues [1] but so could any other piece of code. So, I disagree that regex shouldn't be used in production. This article [2] by Jeff Atwood gives a balanced view of when to use regex and some nice tips.
[1] https://new.blog.cloudflare.com/details-of-the-cloudflare-ou...
[2] https://blog.codinghorror.com/regular-expressions-now-you-ha...
Quickly looking at the python standard lib (urlparse, shlex, etc) and Python packages (NLTK Treebank tokenizer), a lot of packages related to slicing, dicing and parsing strings use a mashup of regex and rule based code.
Plus the memoized Packrat algorithm allows throwing in functions with custom conditions. (Somewhat like parser combinators, they also support custom logic.)
Sure, if you dont know what you're doing.