\b
is a zero-width assertion: it doesn’t consume any characters, it just asserts that a certain condition holds at a given position. A word boundary asserts that the position is either preceded by a word character and not followed by one, or followed by a word character and not preceded by one. (A “word character” is a letter, a digit, or an underscore.) In your string:
add +
…there’s a word boundary at the beginning because the a
is not preceded by a word character, and there’s one after the second d
because it’s not followed by a word character. The \b
in your regex (/\b\+/
) is trying to match between the space and the +
, which doesn’t work because neither of those is a word character.