OK, say you want need an alphanumeric field for something like a product code, grab yourself a string datatype and add a regular expression similar to;In this particular example,

1. [A-Z0-9] allows only UPPERCASE letters from A-Z OR numeric values between 0 and 9
2. {2,10} length is restricted to a minimum of 2 and maximum of 10

Valid

1. 12345ABCDE
2. M12
3. SHOES115
4. F1DEF

Invalid

1. 12345ABCDEF
2. M
3. 9
4. f1def