or download this
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 ....