If not condition in TIME_PREFIX

I am working in the FIX log messages and have two fields that contain timestamps. I need to check for one field and if that is not present check for other field. I'm facing problem if both of the fields is present. Ex: I have 50=timestamp | 70=XYZ | 60=timestamp. I am trying to extract timestamp from 60 if that not present then 52 but not able to write the regex if both are present because 50 is coming before 60 everytime so it's taking 50 field as timestamp. TIME_PREFIX=(50=|60=) Any idea how can I do that if both are present.