I'm not sure about that. The "any" processing is an "or" operation, ie. the end result is true if the result for any of the words is true. '|' would be an ideal candidate (referring to the C/C++ operator and also supported by Google), but as Dotpitch mentions, that looks a lot like a few other characters. Perhaps there is another single character used for "or" in other programming/scripting languages?
Another idea would be to take it a step further, and allow the "any" processing to be grouped. For example, "a|b c|d" would mean that "a" or "b" needs to be present, and "c" or "d" needs to be present (assuming '|' is the any/or operator).