javascript regex not matching last group; extract time from string

Question

In the match

'meeting room 3 @ 5 am - 6 pm bob'
.match(/(@|at)?\s*?(\d+)\s*?(am|pm|AM|PM)?\s*?-\s*?(\d+)\s*?(am|pm|AM|PM)?/)

Why does the last pm not match?

The result is "["@ 5 am - 6","@","5","am","6",null]"

I expect that null to be pm

Thanks

Answer 1

Make all the \\s*? greedy (especially the last one which is the culprit):

/(@|at)?\s*(\d+)\s*(am|pm|AM|PM)?\s*-\s*(\d+)\s*(am|pm|AM|PM)?/
                                               ^

See the regex demo

The point is that (\\d+)\\s*?(am|pm|AM|PM)? matches and captures 1 or more digits with (\\d+) , then the regex engine tries to match (am|pm|AM|PM)? pattern, not \\s*? , because \\s*? is a lazily quantified atom, and is thus skipped at first. The (am|pm|AM|PM)? pattern can match an empty string, and it does. It matches the empty string right after the digits, and the regex engine calls it a day returning a valid match.