Read [abc], [^0-9], and \d \w \s and predict the single character each accepts, expand every shorthand into the explicit set it stands for, and tell when a class is tighter than a literal.

You wrote [0-9] to match a digit and it worked, so you wrote [cat] to match the word 'cat', and it matched a single letter instead. Brackets count characters differently than you expect.

Pin a match to the input's edges with ^ and $, isolate a whole word with \b, branch with |, and predict which substring the eager engine commits to when the bar's reach and the anchors collide.

You wrote cat to find the animal and your search lit up category, scatter, and bobcat too. The pattern matched a run of letters anywhere it could sit, because nothing told it where to stand.

Trace how a quantifier repeats just the token on its left, watch greedy take the maximum and hand characters back while lazy +? grows from the minimum, and tell which span each numbered, non-capturing, or named group captures.

.+ reads like "grab a few characters." Aim it at <EM>first</EM> and it swallows everything to the line's end, then hands characters back one by one until the closing > finally has something to match.