Don't @#$@!$% just yet, we're not going to use any of that fancy-schmancy stuff. We're going to start with something simple, like finding all the words that contain the word "cat" in them.
/cat/ is the magic that will find all the cats in the house. No, wait, that's not it. That's just a cat in a hat. We're looking for a pattern that will find all the cat-titude, all the cat-astrophies, and all the cat-alysts.
/cat[a-z0-9]/ is better, but still not quite there. We're not just looking for any old cat, we're looking for the cat's pajamas.
/cat[^a-zA-Z0-9]/ is getting closer, but it's still too restrictive. We want to find all the cats, even if they're in a word like "cat's eye" or "catnip".
After some careful consideration, we've come up with the perfect pattern: /cat[^a-zA-Z0-9]*/. This will find all the cat-related words, from cat's eye to cat-astrophe.
Now that we have our pattern, we can replace all the cat-related words with the cat-alyst. But wait, what if we want to replace all the cat-titude words with just "cat"? That's not as simple as it sounds. We'll need to use a more complex pattern: /cat[^a-zA-Z0-9]*/g. The "g" at the end tells regex to replace all matches, not just the first one.
But what if we want to replace just the first occurrence of "cat" in each word? That's even more complicated. We'll need to use the "i" flag: /cat[^a-zA-Z0-9]*/g/i. The "i" flag tells regex to be case-insensitive, which is just what we need to avoid replacing words like "Cat" and "cat's eye".
But what if... you know where this is going. We'll never finish explaining all the intricacies of regex, because there are just too many cats.