#! perl
use strict;
use warnings;

my @selects = ("select 'text' from foo        --This is a comment",
               "select '--Not a valid comment' from foo        --But this is",
               "select '--This is not a valid comment' from foo",
               "select '--Not this' + '--either' from foo");

for my $select (@selects)
{
    my ($comment) = $select =~ / ( -- [^']+ ) $ /x;
    print $comment, "\n" if $comment;
}