PHP regex non-capture non-match group

Question

I\'m making a date matching regex, and it\'s all going pretty well, I\'ve got this so far:

\"/(?:[0-3])?[0-9]-(?:[0-1])?[0-9]-(?:20)[0-1][0-9]/\"

Answer 1

1. It is a non capturing group. You can not back reference it. Usually used to declutter backreferences and/or increase performance.
2. It means the previous capturing group is optional.