I think BrowserUk's solution is going to be faster.

That would not be my guess, unless it's the /i that's killing the performance. /a/ will not use the regexp engine, the optimizer will do it. If speed is an issue, and you want to be case sensitive, my bet would go to:

if ((/a/ || /A/) && (/e/ || /E/) && (/i/ || /I/) && (/o/ || /O/) && (/
+u/ || /U/)) { ... }
[download]

but I'm too lazy to come up with a good benchmark (which should test for both match and non-match). And if the query set would be English words, I'd order the vowels from least frequently occurring to most frequently (probably u-i-o-a-e, but I'd have to look that up), in order to fail faster.

Of course, it's also very likely speed doesn't matter at all.