Validate user inputted PHP code before passing it to eval()

Question

Before passing a string to eval() I would like to make sure the syntax is correct and allow:

1. Two functions: a() and b()
2. Four operators: /*-+

Answer 1

hakre's answer, using regexes is a nice solution, but is a wee bit complicated. Also handling a whitelist of functions becomes rather messy. And if this does go wrong it could have a very nasty effect on your system.

Is there a reason you don't use the javascript 'eval' instead?