database - Microsoft access validation -

i need allow only alphanumeric characters entered field. data type 'short text'.

what 'validation rule' using expression builder?

you can find lots of useful examples validation rules here:
http://allenbrowne.com/validationrule.html

in case -

digits , letters only

is null or not "*[!((a-z) or (0-9))]*" 

accepts z , 0 9, no punctuation or other characters.