In 90% of the cases, "regarding"
e.g. "Regarding your request, I shall answer you tomorrow"
Amazingly, there's a great number of hits in Google. I would actually consider "Regarding to your request" ungrammatical but there's 1-2 hits in British National Corpus, so I might be wrong. However, in both cases the speaker's country of origin is unknown, which probably means they're not native speakers.
Don't confuse "regarding to" with "regard to". This is frequently used, as in this sentence:
"He's crossed the Rubicon with regard to the use of military force as an option."
Perhaps "regarding to your request" is grammatical in some dialect, e.g. AmE?? Some native American speaker could enlighten us..
Student or Learner