$ perl -le '
> $str = q{YYSFTVMETDPVN[115]HMVGVISVEGRPGLFWFN[115]ISGGDKN[115]};
> $posn = -1;
> while ( ( $posn = index $str, q{N[115]}, $posn ) > -1 )
> {
>     print +( $posn ++ );
>     substr $str, $posn, 5, q{};
> }'
12
30
37
$