regular expression pattern for name field

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C#

Javascript

C#4.0

ASP.NET4

I need regex with space between words without digits and without any special characters except dot if client enters their short name and Min,Max value{3,20} -

Expected value:--
----------------------
S.K THAKUR-----OK(dots allowed)
Saurav --------OK(MORE than 3 char,Not holding digits/spl char without dot)
Saurav kumar Thakur---OK(space allowed between worrds

Un-Expected value:--
----------------------

Saurav32@---Not OK (hold digits and spl char)
Sauravkumar@---Not OK (hold spl char)
3Saurav-----Not OK (hold digits)
sa_urav----Not OK (includes spl char underscore)