I remember testing this way back when the filter var was first released, and found that FILTER_VAR did not appear to adhere to RFC standard (not w3, I don't know if / why they'd have an email specification since this has nothing to do with w3). It is however close enough.
Writing a regex to adhere to RFC is also quite complex. Some dirty examples from wiki to give you an idea:
Are all valid examples.
My suggestion? Use the filter_var. The only time you may have problems is likely the same problems you'd have with regex.