If you want at least one letter, then it's trivial:
[a-zA-Z]
Will match any text which contains at least one alphabetic character.
If you want at least one letter-or-number, then that's simple too:
\w
Will match any message containing at least one alphanumeric character.
If that isn't what you are looking for then you need to give us better examples of "good" and "bad" text to check for.
It's also worth getting a Regex tool: I use
Expresso[
^] - it's free, and it examines and generates Regular expressions.