I find it's better to save my brain power for learning things like what I need to take into account when configuring a database for example, or what variables affect network performance and what options I have for tuning, etc.
When I reach for tools to solve problems, the most I usually remember is "I can use X to solve this class of problems" and the how is available from the manual or with a Google search.
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.
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.