@BeAware Just in case anyone else comes across this, I found the regex at the linked page didn't really match super well with the text of the page.
This is one that seems to make a lot more sense to me:
regex101: build, test, and debug regex
Regular expression tester with syntax highlighting, explanation, cheat sheet for PHP/PCRE, Python, GO, JavaScript, Java, C#/.NET, Rust.
regex101 (regex101.com)