if you just need to delete 'something-or-other as', something like this may do the trick:

C:\@Work\Perl>perl -wMstrict -e "$_ = shift;
my $item = qr{ \w+ \s+ }xms;  my $group = qr{ \( [^)]* \) \s* }xms;
s{ (?: $item | $group) as }{}xmsgi;  print"
"SELECT f1 as x1,f2,f3,f4 as x2, (f4= f5) as x3 FROM ...."
SELECT  x1,f2,f3, x2,  x3 FROM ....
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if other things may predece 'as', they can be defined and added to the grouped alternation.