I have added spoiler tags around your code for those who have not worked on this challenge yet.
There is a great tool for regex calls regex 101
It acts like a debugger for your regular expressions.
Copy and paste your regex into the input field and it will output detailed explanations of what each part means.
The . is not a part of that problem specifically, but I still need help understanding how it works. For some reason the meaning here doesn’t make sense to me. Also would appreciate a better source to check what does what if you have a better option.