Write a pattern, set your flags, and watch matches highlight live as you type — with capture groups broken out.
Regular expressions are famously hard to get right by reading alone — you really need to see them run. Enter a pattern and some test text and every match highlights instantly, with a running count and the contents of any capture groups listed below. Adjust the flags to change behaviour: g finds all matches, i makes it case-insensitive, m treats each line separately, and s lets the dot match newlines.
The fastest way to build a tricky pattern is incrementally: start broad, watch what matches, then tighten. Capture groups (the parts in parentheses) are shown separately so you can confirm you're extracting exactly the pieces you want — invaluable when you're using a regex for find-and-replace or pulling fields out of text. This uses your browser's own JavaScript regex engine, so what you see here is exactly how it'll behave in JavaScript code.