regex - Positive lookahead not working as expected

Question

I have the following regex with a positive lookahead:

/black(?=hand)[ s]/

I want it to match blackhands or blackhand. However, it doesn't match anything. I am testing on Regex101.

What am I doing wrong?

Answer 1

Lookahead does not consume the string being searched. That means that the [ s] is trying to match a space or s immediately following black. However, your lookahead says that hand must follow black, so the regular expression can never match anything.

To match either blackhands or blackhand while using lookahead, move [ s] within the lookahead: black(?=hand[ s]). Alternatively, don't use lookahead at all: blackhand[ s].