I definitely agree that its not important. I was just looking through the optimizations in this commit and it seemed like all RegExps that contain node names or attributes have been alphabetized, so I thought I'd point it out.
Since we are on the topic of (mostly trivial?) RegExp optimizations within jQuery, I wonder if it may make sense to order the items within groups by most to least common. Here is a rough JSperf test case demonstrating the effects of re-ordering groups: http://jsperf.com/items-in-regex-group-perf; IE8 nets an improvement of ~20% matching the first grouped item over the last.
That makes more sense - although it's starting to get into the realm of speculation. IMO the initial RegExp changes proposed made the most sense as there was tangible differences in how the RegExp engine would operate from the old version to the new one - yielding a very obvious gain. After that the gains kind of peter off. Doing the research on the frequency of specific input types for the reordering of an infrequently used RegExp would probably be better spent towards improving the performance of other aspects of the library. If you're hell-bent on it you can absolutely go to town - just send in a pull request and I'll likely land it.
Is this correct?