"As far as I know" is the idiomatic expression I'm familiar with.
as far asas far as conj. To the degree or extent that: They returned at nine, as far as we knowThey returned at nine, as far as we know. Usage Note: As far as the Usage Panel is concerned, as far asas far as had better be followed by both a subject and a form of gogo or be concernedbe concerned. As far asAs far as is sometimes used as a preposition meaning "as for" or "regarding," especially in speech, but a large majority of the Panel frowns upon this usage. Eighty percent find the as far asas far as construction in this sentence unacceptable: As far as something to do on the weekend, we didn't even have miniature golf.As far as something to do on the weekend, we didn't even have miniature golf. Eighty-four percent reject the sentence The Yankees are still very much alive, as far as the divisional race.The Yankees are still very much alive, as far as the divisional race. Further, 89 percent object to as far asas far as when followed by a noun clause, as in As far as how Koresh got shot, we don't know yet.As far as how Koresh got shot, we don't know yet.
[The American Heritage® Dictionary of the English Language, Fourth Edition copyright ©2000 by Houghton Mifflin Company. Updated in 2009]
According to the same source "so far as" is a variation of "insofar as" :
so far asso far as conj. Insofar as: So far as I am concerned, the project is over.So far as I am concerned, the project is over.
