use strict;
use warnings;

while (<DATA>) {
    chomp;
    while (m/(--.*?$)              #that is comment
             |
             ((?:'[^']*'|[^'-]*))  #quated part of sql or ordinal part
            /xg) { #global, extended
        print $2 if $2;
    }
    print "\n";
}

__DATA__
select 'text' from foo        --This is a comment
select '--Not a valid comment' from foo        --But this is
select q from z -- as is this
select '--This is not a valid comment' from foo
select '--Not this' + '--either' from foo
select 'qaws' + make from "a" -- comment with 'a' quote
select 'a' from 'b' with 'c' -- comment with 'a --' comment
-- test comment (add1)
select 'text\'s' from foo        --escaped  ... (add2)
select 'text\'s' from foo        --escaped'  ... (add3)