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Are these sentences the same?

As far as I know, he's going to Chicago.

So far as I know, he's going to Chicago.

In so far as I know, he's going to Chicago.

I think that they are the same, but the 1st one is formal, the 2nd one is informal and the 3rd one is very formal.

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So far as I know is slightly marked, as formal, in US English. Therefore less common in speech. Insofar as (with variable spacing) is a technical legal term. Don't use it if you aren't a lawyer. – John Lawler Jan 29 '13 at 19:47
@JohnLawler why is insofar as specifically legal? It is archaic I grant you, but exclusively legal? Could you provide some references for that? – terdon Jan 29 '13 at 19:52
As far as popular registers*, archaic = legal; any distinction would be important only to a lawyer or an historian. – John Lawler Jan 29 '13 at 21:23
* as far as they go, that is. – John Lawler Jan 29 '13 at 21:24

As far as I know, he’s going to Chicago.

So far as I know, he’s going to Chicago.

These are identical in meaning and register. They differ only by dialect or individual speaker preference. According to Google Ngrams, they are approximately equal in popularity:

Google Ngram chart showing relationship between *as far as*, *so far as*, and *in so far as*.

In so far as I know, he’s going to Chicago.

This is a less popular variant, which to my ear sounds slightly more formal simply because it’s wordier. In American English the spelling is insofar, meaning “to such an extent”.

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The first one is the most common usage in casual and even semi-formal speech/writing.

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