Something like

lcDigits = "0123456789"
lcLetters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
lcAllowedChars = lcDigits + lcLetters 
lcValue = "7CM4991"
* Is it valid?
? LEN(CHRTRAN(lcValue, lcAllowedChars, "")) = 0 ;
		AND LEN(CHRTRAN(lcValue, lcDigits, "")) > 0 ;
		AND LEN(CHRTRAN(lcValue, lcLetters, "")) > 0

>I have a field
>
>Suppose use enter data to the field, as would like the behaivor as below:
>
>"ABDF"
>= messagebox ("Field must contain numbers and letters")
>
>"8887"
>= messagebox ("Field must contain numbers and letters")
>
>7CM4991
>= messagebox ("OK")