What does (?: do in a regular expression

Question

I have come across a regular expression that I don\'t fully understand - can somebody help me in deciphering it:

^home(?:\\/|\\/index\\.asp)?(?:\\?.+)?$
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Answer 1

(?:) creates a non-capturing group. It groups things together without creating a backreference.

A backreference is a part you can refer to in the expression or a possible replacement (usually by saying \1 or $1 etc - depending on flavor). You can also usually extract them from a match afterwards when using regex in a programming language. The only reason for using (?:) is to avoid creating a new backreference, which avoids incrementing the group number, and saves (a usually negligible amount of) memory