We deal with the generation of code selectors in compiler backends. The fundamental concepts are systematically derived from the theory of regular tree grammars and finite tree automata. We use this general approach to construct algorithms that generalize and improve existing methods. 1 Introduction A code generator for a compiler is applied to an intermediate representation (IR) of the input program that has been computed during preceding phases of compilation. This intermediate representation can be viewed as code for an abstract machine. The task of code generation is to translate this code into an efficient sequence of instructions for a concrete machine. Besides register allocation and instruction scheduling (for processors with pipelined architectures), code selection, i.e., the selection of instructions, is one subtask of code generation. It is especially important for CISC (Complex I nstruction Set Computer) architectures where there are usually many possibilities to generat...