What does this regular expression mean - '(?

Question

I found this regular expression in Raisin code:

my ($a, $b, $r) = ("(?<$token>", ')', undef);

In this case $token is some word (param name). This code used in functon for creating regular expression. Sample result for /users/:id is:

/users/(?<id>[^/]+?)(?:\.[^.]+?)?

I dont understand what does its mean: (?<id> ?

I know positive and negative look-behind - (?<= and (?<!. But i don`t know this.