A traditional usage rule draws a distinction between comparisons using as . . . as
and so . . . as.
The rule states the so . . . as
construction is required in negative sentences (as in Shakespeare's "'tis not so deep as a well"), in questions (as in Is it so bad as she says?), and in certain if
-clauses (as in If it is so bad as you say, you ought to leave). But this so . . . as
construction is becoming increasingly rare in American English, and the use of as . . . as
is now entirely acceptable in all contexts.
As … as is standard in both positive and negative constructions: The fleet was as widely scattered then as it had been at the start of the conflict. Foreign service is not as attractive as it once was.
So … as is sometimes used in negative constructions (… not so attractive as it once was
) and in questions (“What is so rare as a day in June?”
). as - Definitions from Dictionary.com