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.