I think the depth of your knowledge should be proportional to the frequency and quality of how you use a particular thing. And quite often, it naturally follows that trend.
Let's say for example regex. I know the basics so that as I use the easiest cases almost daily, I get by. As soon as I need anything more complex (and have decided that regex is the best way to go), I google, add tests, use different online regex platforms.
Every time I go deeper, I end up learning a bit more. So my circle of knowledge extends. But would I spend weeks or months or years to truly master regex by heart? No.
One thing that better understanding of the topic does, is that it makes you a better googler. If I have no idea about regex, I can only search for very generic stuff. If I know I want to do backtracking (and I know the term), I can be more efficient in finding the right information.
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