Need Regular expression which allows dot between alphabets and dot is not compulsory.

 
Need Regular expression which allows dot between alphabets and dot is not compulsory.
Following scwnario should match:
asdfggfd 
asd.ddd.ddd
Aws.fg
A.
Following Scenario should not match:
A..asd
I have one validation expression but it works only in windows application(desktop) not in asp.net website.In asp.net application it is showing
syntax error.
here is my expression:   ^(?:(?>[a-zA-Z]+)(?:\.(?!\.))?){1,18}$
 
Please provide me the solution
 
.B.C
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