Lexers may be written by hand. This is practical if the list of tokens is small, but lexers generated by automated tooling as part of a compiler-compiler toolchain are more practical for a larger amount of potential tokens. These tools generally accept regular expressions that describe the tokens allowed in the input stream. Each regular expression is associated with a production rule in the lexical grammar of the programming language that evaluates the lexemes matching the regular expression. These tools may generate source code that can be compiled and executed or construct a state transition table for a finite-state machine (which is plugged into template code for compiling and executing).