How do I match a character only once per word using RegEx?

I want to match a character only once per word(of a length defined by me using a variable) and I am using this RegEx "\b\w{3,8}\b" for limiting words outside of a given range which is (3 to 8 words) here from being picked and Can anybody send me a Regex for doing this,Here are are a few Examples-

Aam-Not Picked

als-picked("als")

//As it does not contain any repeating character and its length is within 3 to 8 words//

aamm-Not picked

//As it contains two repeating letters "a" and "m" respectively//

a-Not picked'

//As its a's length is less than 3 words//

Thanks-Picked

//As its length is within the range of 3 to 8 letters and this word does not have any repeating letters//