I have a tendency to unintentionally assume all regular expressions follow PCRE, but that's not necessarily the case. Only knowing one flavor of regex well might not be enough if you need to use them in different language environments! Example:
"Regex language syntax varies between different platforms; it is not standard. In particular, the “$” anchor does not only match the end of the string in Python and PHP, but it does in JavaScript."
https://best.openssf.org/Correctly-Using-Regular-Expressions