RegEx in javascript. Allow only letters, comma and punctuation

I try to make a RegEx for validating a form in javascript. The RegEx should only allow letters comma and punctuation. For instance I have this string:

Hi, this is a test of RegEx. Does it work
 

I've tried the following

/^[A-Za-z0-9,. ]{3,50}$/; 
 

But it doesn't seems to work. Solutions?

Thanks!

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