use Regexp::English;

my $re = Regexp::English
        -> start_of_line
        -> literal('mail xmlfeedback -s')
        -> remember
            -> literal('"feed invalid for agent ')
            
            -> multiple
                -> digit
            -> end
            
            -> literal('"')
        -> end
        
        -> zero_or_more
            -> whitespace_char
        -> end
        
        -> end_of_line;
print $re;
__END__
(?^:^mail\ xmlfeedback\ \-s(\"feed\ invalid\ for\ agent\ (?:\d+)\")(?:\s*)$)