\w means 'any word character' (which includes the underscore as per John's comment) where as \d means 'any digit'.

So, using \w{6,9} would allow you to enter any word characters as long as its length is between 6-9.

As John's comment, if you need to ensure the format (e.g. first 3 characters must be alphabet), you will need to use more 'complex' regex expression.

FYI, you can use regex101.com to test your regex expression