Lines, and the word boundary

The m flag moves ^ and $ from the ends of the input to the ends of every line. Nothing else about the pattern changes — . still refuses to cross a line break, because that is the s flag's job and not this one's.

• /^\w+$/gNo lines, so no match.
• /^\w+$/gm

\b is a position where a word character meets something that is not one — where \w and \W sit next to each other, or where one of them sits against the end of the input. \B is every other position.

So \b inherits \w's definition, including the underscore. cat_alogue has no word boundary after cat, and cat-like does.

• /\bcat\b/gOne of these surprises people.
• /\Bcat\B/g\B is every position \b is not, which makes it the way to find a word inside another one.