Introduce commented constants for the segments of an URI regex

The regular expressions used to parse URI's are constructed by
concatenating different segments to a big String. Introduce
String constants for these segements and document them.

Change-Id: If8b9dbaaf57ca333ac0b6c9610c3d3a515c540f9
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>