Regular expression for a string containing one word but not another

This should do it:

^(?!.*details\.cfm).*selector=size.*$

^.*selector=size.*$ should be clear enough. The first bit, (?!.*details.cfm) is a negative look-ahead: before matching the string it checks the string does not contain "details.cfm" (with any number of characters before it).

^(?=.*selector=size)(?:(?!details\.cfm).)+$

If your regex engine supported posessive quantifiers (though I suspect Google Analytics does not), then I guess this will perform better for large input sets:

^[^?]*+(?<!details\.cfm).*?selector=size.*$

regex could be (perl syntax):

`/^[(^(?!.*details\.cfm).*selector=size.*)|(selector=size.*^(?!.*details\.cfm).*)]$/`