Colon : is simply colon. It means nothing, except special cases like, for example, clustering without capturing (also known as a non-capturing group):
(?:pattern)
Also it can be used in character classes, for example:
[[:upper:]]
However, in your case colon is just a colon.
Special characters used in your regex:
In character class [-+_~.\d\w]:
-means-+means+_means_~means~.means.\dmeans any digit\wmeans any word character
These symbols have this meaning because they are used in a symbol class [].
Without symbol class + and . have special meaning.
Other elements:
=?means=that can occur 0 or 1 times; in other words=that can occur or not, optional=.