The expression function has the following arguments.
Table A–8 external Arguments| Argument | Description | 
|---|---|
| program | The program argument is the file name of an external rewriting program. Because program is executed using the operating system's default shell (/bin/sh on Unix/Linux) or the command interpreter (CMD.EXE on Windows), program should be an absolute path or the name of a program in the operating system's PATH. The server starts the external rewriting program on demand. A given server process never executes more than one instance of the program at a time. Note – The server may start multiple instances of a given external rewriting program when the server is running in multiprocess mode. | 
| value | The value passed to the rewrite program. |