(?:) as a whole represents a non-capturing group.
Regular-expressions.info mentions this syntax :
The question mark and the colon after the opening round bracket are the special syntax that you can use to tell the regex engine that this pair of brackets should not create a backreference. Note the question mark [...] is the regex operator that makes the previous token optional. This operator cannot appear after an opening round bracket, because an opening bracket by itself is not a valid regex token. Therefore, there is no confusion between the question mark as an operator to make a token optional, and the question mark as a character to change the properties of a pair of round brackets. The colon indicates that the change we want to make is to turn off capturing the backreference.