The field MATNR carry a CONVERSION-EXIT, so external format (what you see/input) differs from internal value (field value in program, database)
So check domain MATNR to get correct conversion-exit (it is not always ALPHA, and there is at least one available customer exit available...) and use the associated FM to get external value (e.g. CONVERSION_EXIT_MATN1_OUTPUT) or a WRITE INTO statement to get back external value.
Why do you require this information, curiosity ?
Regards,
Raymond