Regex for internal slug:
^[a-z0-9]+(?:-[a-z0-9]+)*$
Regex for external URL:
^(ftp|http|https):\/\/(\w+:{0,1}\w*@)?(\S+)(:[0-9]+)?(\/|\/([\w#!:.?+=&%@!\-/]))?$
Regex for external OR internal slug:
^((ftp|http|https):\/\/(\w+:{0,1}\w*@)?(\S+)(:[0-9]+)?(\/|\/([\w#!:.?+=&%@!\-\/]))?)|(\/[a-z0-9]*(?:-[a-z0-9]+)*)$