Eh, is there a point in memorizing it if you don't use it enough? The best way to learn something is repeated practice, so while I use regex enough to not need to look things up normally, I need a quick refresh anytime I need to do anything in matplotlib. There are people who are fluent with the library, but I don't have any more than basic syntax memorized since it's maybe once per year that I need it.
Although if you really do need to learn regex, I highly recommend https://regexone.com
At least for me, the way it was taught Wes super-helpful in retaining the syntax knowledge.
I don't even remember when or why I learned it. Was it 10 years ago? 15? 20? Was it the xkcd.com/208 ? Was it curiosity? All I know is I've been using it ever since and solved so many problems with it and yes, there are places where finding the right expression becomes a problem in itself, but online regex testers largely solve that problem by letting you visually test your regex. (see rubular, pythex)
People have huge reservations about parsing non-regular languages (like xml) with regular expressions because xml could have an attribute with a string with another xml in it or something, but the whole point is moot because regex is mainly used for partial matches and the possibility of having a string inside the html that could match whatever you are looking for is minuscule and will probably show up in tests anyway.
if you design your regex for the XML you're using it on, this shouldnt be an issue - I use it frequently to mung config files as needed for work stuff - we use XML or XML like files for all sorts of stuff.
Although if you really do need to learn regex, I highly recommend https://regexone.com At least for me, the way it was taught Wes super-helpful in retaining the syntax knowledge.