I don't think the word 'as' is necessary in the sentence below.
As far as I know, 'as' is a conjunction, which means there must be two clauses if it can be used. However, there's no main clause in the sentence.
Is it correct? Are there any words omitted? I believe that if there are no quotation marks the sentence is acceptable.
As Albert Einstein once said, “Anyone who has never made a mistake has never tried anything new.”
CGEL (p 1146) classes 'as' as a preposition in
27 (i) As you know, we face a difficult year.
and I'd certainly not label this usage that of a conjunction. 'As you know' is an adjunct. It is a pragmatic marker, as incidentally is your 'As far as I know'.
'As you know' is obviously addressee-orientated, possibly used as nothing more than a 'conversational lubricator' / palliative (though it clearly has semantic content).
'As far as I know' addresses modality (it gives an indication of how confident the speaker is of the truth of the statement).
They are both totally idiomatic, often used as sentence-starters.
