I've written hundreds of regular expressions over the years, and I still get them wrong on the first try. Here's the debugging workflow I've settled on that consistently gets me from "why isn't this matching" to "oh, that's why" in under a minute.
Step 1: Isolate One Pattern at a Time
The most common mistake I see (and make) is testing a monster regex all at once. If you're validating an email with a complex pattern and it's not working, break it down: test just the local part, then the domain, then combine them. This alone has saved me hours of staring at a wall of backslashes and brackets.
Step 2: Check Your Quantifier Greediness
This one bites everyone. A pattern like <.*> against "<div>hello</div>" matches the entire string, not just the opening tag. The fix: add ? to make it lazy — <.*?>. I can't count how many "my regex is broken" posts on Stack Overflow boil down to this. Whenever a pattern matches too much, my first instinct is to check for greedy quantifiers.
