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

Remove ? from all \s*?.

Srdjan M.
– Srdjan M.

2018-02-22 22:44:46 +00:00
Commented Feb 22, 2018 at 22:44 — Srdjan M.
– Srdjan M., Commented Feb 22, 2018 at 22:44

Wiktor Stribiżew · Accepted Answer · 2018-02-22 22:44:08Z

2

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.

answered Feb 22, 2018 at 22:44

Wiktor Stribiżew

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1 Comment

Senica Gonzalez Over a year ago

Thanks. I did not realize the regex engine skips lazy matches at first. Makes sense, though.

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