use strict;
use warnings;

my $sel = 'SELECT f1 AS x1,f2,f3,f4 as x2, (f4= f5) as x3 FROM';

$sel =~ /SELECT\s+(.*?)FROM/i or die "Select clause not found in: $sel";

my @parts = map {/(\w+)\s*$/; $1} split /\s*,\s*/, $1;

print "SELECT ", join (', ', @parts), " FROM\n";

##</code><code>##

SELECT x1, f2, f3, x2, x3 FROM