1. A computer program product for executing an instruction, the computer program product comprising: a computer readable storage medium readable by a processing circuit and storing instructions for performing a method comprising: executing an extended translate and test reversed 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 one value, decrementing an argument character pointer by a size of the argument character and obtaining, using the argument character pointer, a next argument character to be processed.