I find regional differences in the nicknames for
Rebecca to be interesting.
Rebecca was quite popular in my age group (at least in the schools I attended) when I was growing up. All of them were Bec/Beck or Bex/Becs/Becks...they maybe got
Becky occasionally from their mother or something as a cutesy informal nickname but I never knew a
Rebecca that went by
Becca or
Becky until the first time I lived in the US in 2008. I prefer
Becca to Bec personally but I'd never come across it among all the Rebeccas that I have known in Australia.