How to apply regex to a field in lookup file?

I have a lookup file query as follows | inputlookup ABCD.csv which displays the results as follows **Host** efgh ijkl.mno.com pqrs.tuv.net wxyz Now how can i add a regex to display only the hostname and avoid the extra string which ever after the dot(.). I just want to apply the regex to display the result as follows **Host** efgh ijkl pqrs wxyz