[R] Regex question to find a string that contains 5-9 alpha-numeric characters, at least one of which is a number

[Tan, Richard]

Hi, 

This is not exactly an R question but I am trying to use gsub to replace
a string that contains 5-9 alpha-numeric characters, at least one of
which is a number.  Is there a good way to write it in a one line regex?

Thanks,
Richard