Only if you're doing a lot of string parsing. Just like learning about algorithms on graphs is a must only if you're working with graphs of a non-trivial size.
I don't agree. Regular expressions come in handy as well during your day to day work: you can save a lot of time if you know how to use them in your editor "search & replace" function, or just when a simple search in a directory gives you too many results and you need to switch to power tools (read: egrep).