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

its really easy every parentheses will create a variable in the memory so you can use the parentheses value afterward so to not save it in memory just put :? in the parentheses like this (?:) and then fill the rest as you need. that's it and nothing else

Answer 2

It's a non-capture group, which essentially is the same as using (...), but the content isn't retained (not available as a back reference).

If you're doing something like this: (abc)(?:123)(def) You'll get abc in $1 and def in $2, but 123 will only be matched.

Answer 3

From documentation:

(?:...)
A non-capturing version of regular parentheses. Matches whatever regular expression is inside the parentheses, but the substring matched by the group cannot be retrieved after performing a match or referenced later in the pattern.

Answer 4

(?:) 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