Search and replace

I am looking to replace these column software version numbers with an ammended value by looking for the full stop someohow

OLD VALUE
11.2.4.5
10.3.0.0
10.0.0.1
3
2
unknown
null

NEW VALUE
11.X
10.X
10.X
3.X
2.X
Version unknown
Version unknown

So
IF it finds the full stop minus 1 character then

11.X
IF its just a number EG 2 then

2.X

else
version unknown