In 4.2.1 build 98164 I'm using the Interactive Field Extractor and the Generated pattern automatically generates an expression similar to (?:[^:\n]*:){6}//(?P<FIELDNAME2>[^\.]+).
When attempting to save the expression a validation error pops: "Each field must have a name. 1 field(s) do not have names."
Manually Editing the expression and removing the number in the parameter <FIELDNAME> i.e. (?:[^:\n]*:){6}//(?P<FIELDNAME>[^\.]+) will allow you to save the expression.
Regenerating the regex pattern changes the <FIELDNAME> parameter back to <FIELDNAME2>.
