What does (?: do in a regular expression

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.

(?:) 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