After many years of experience, I still sometimes stumble on how to phrase a regex pattern to get exactly what I want if it is a bit unconventional. Before I'd have to look up lists of common regex's (sed commands, etc). But I've lately been feeding in sample input lines, telling it what I want to do, and it has been really good about answering correctly (and explaining each part of the regex).

Sometimes I have to clarify "But this will also output x, on line 3, but I really want it to output y". And it gives a correction based on my clarification.