We make no guarantees about ''which'' elements get passed to a custom pseudo function, only that we will honor its return value for excluding elements from the final matched set: http://jsfiddle.net/hCzUd/1/

But for the record, there is currently a performance penalty if your custom pseudos is prefixed with a standard positional pseudo (even|odd|eq|gt|lt|nth|first|last), the effects of which you are observing here.

Please make a note of this in the documentation here:

https://github.com/jquery/sizzle/wiki/Sizzle-Documentation

The return result from this function should be a boolean (true if the element matches the selector, false if not).

What would you have us change?

I'd like to see the documentation to include:

"Note: Custom pseudo functions prefixed with standard position pseudo (even|odd|eq|gt|lt|nth|first|last) may incur a performance penalty. In general, there are no guarantees on which elements are passed to the custom pseudo function (ie. elements passed to function may not match the given selector)."

Can you explain why this is the case? Is it an optimization in jQuery or just on oversight?

It is axiomatic that elements passed to the function may not match... the ''purpose'' of the function is to help find out.

As for the reason why custom pseudos prefixed with a positional selector might hurt performance more than those that aren't: positional selectors need special processing, and we check for their presence with a regular expression before tokenization to skip it if possible (or at least start with a small seed set for non-positional matching). Anything that ''looks'' positional doesn't get the benefits.

Honestly, I'd recommend using the .filter( fn )method instead of defining a custom pseudo, if that would work for your purposes.