try this regex @"[0-9]*\.[A-Za-z]*"
example
string ymt4 = @"RP/NYC1S21DD/NYC1S21DD WS/SU 6MAY13/0509Z Y39ZRG
NYC1S21DD/80948W/6MAY13
1.CORTES RIVERA/MARIA(ADT) ";
string ymt5 = @"RT29PVZU
--- TST RLR MSC ---
RP/NYC1S21DD/NYC1S21DD WS/SU 6MAY13/0452Z 29PVZU
NYC1S21DD/9999WS/6MAY13
1.ALVARADO/RITA(ADT) 2.CABELLO/LUIS CARLOS(ADT)
3.CABELLO/CINDY(ADT) 4.GUTIERREZ/MARTHA HILDA(ADT) ";
MatchCollection matches = Regex.Matches(ymt4 , @"[0-9]*\.[A-Za-z]*");
if(matches.Count>1)
MessageBox.Show(matches.Count.ToString());