How to check if a String has alphabets & numbers?

How to check if a String has alphabets & numbers?

Hi,

I would like to validate if a string has 4 alphabets and 6 numbers.

For example: alphaNum = "BHGU789412"

I would like to check if the user has entered 4 alphabets and 6 numbers in the above variable alphanum. 
Please let me know.