1. A computer program product for executing an extended translate and test instruction, the computer program product comprising: a non-transitory computer readable storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method comprising: executing, by a processor, the extended translate and test instruction, the executing comprising: determining whether a value of an argument character has a particular relationship with respect to a predefined size, and based on the value having the particular relationship, using a predefined value for a function code for the argument character, and based on the value not having the particular relationship, using the argument character as an index into a function code data structure to locate a function code corresponding to the argument character; and based on said function code being a first value, storing said function code; or based on said function code being a second value, obtaining a next argument character to be processed.