looking for a regex code

Hello,
I am looking for a regex code that checks a filed contains at least 1 number, 1 alphabetic letter, 1 special character, and must consist of at least 6 characters.
So far I have this - 
ValidationExpression="(?=^.{6,}$)(?=.*\d)(?=.*\W+)(?![.\n]).*$"
But it doesnt check If an alphabetic letter has been entered. Anyone know what I need to add? I cant get me head around regex code!
Cheers
J
 
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