Question

What I really need to know is :

1. What does (?( mean ?
2. What does ?: mean ?

The regex I am trying to figure out is :

(notice the above mentioned symbols in the following regex)

(?(?=and )(and )|(blah))(?:[1][9]|[2][0])[0-9][0-9]

Answer 1

(?(?=and )(and )|(blah)) pattern is used like if-then-else like (?(expression)yes|no)
i.e and would be matched if and is there else blah would be matched

(?:) is a non capturing group.So it would not be included in the group or be used as back-reference \1

So,

(?(?=and )(and )|(blah))(?:[1][9]|[2][0])[0-9][0-9]

would match

and 1900
blah2000
and 2012
blah2013

NOTE(it’s all about the groups)

The samething can be achievend with this regex
(and |blah)(?:[1][9]|[2][0])[0-9][0-9].
The only thing in which these regex differ is the number of groups formed.

So my regex would form 1 group which would contain either and or blah

Your regex would form no groups.It will form a group only if it matches blah..