>perl -wMstrict -le
"sub substr_offsets {
   my ($str, $substr, $subst) = @_;
   $subst = qq{\0} unless defined $subst and length $subst;
   die qq{substitution string '$subst' found in '$str'}
     if $str =~ m{\Q$subst\E}xms;
   die 'substring same as substitution string'
     if $substr eq $subst;
   $str =~ s{ \Q$substr\E }{$subst}xmsg;
   my @offsets;
   $str =~ s{ \Q$subst\E }{ push @offsets, pos $str }xmsge;
   return @offsets;
   }
 for (@ARGV) {
   my @offsets = substr_offsets($_, 'n[115]');
   print qq{string: '$_';  offsets: (@offsets)};
   }
"
""  a  ab  n[115]  An[115]  n[115]A
n[115]n[115]  n[115]n[115]n[115]
An[115]n[115]n[115]  n[115]n[115]n[115]A
01234n[115]012345n[115]n[115]
string: '';  offsets: ()
string: 'a';  offsets: ()
string: 'ab';  offsets: ()
string: 'n[115]';  offsets: (0)
string: 'An[115]';  offsets: (1)
string: 'n[115]A';  offsets: (0)
string: 'n[115]n[115]';  offsets: (0 1)
string: 'n[115]n[115]n[115]';  offsets: (0 1 2)
string: 'An[115]n[115]n[115]';  offsets: (1 2 3)
string: 'n[115]n[115]n[115]A';  offsets: (0 1 2)
string: '01234n[115]012345n[115]n[115]';  offsets: (5 12 13)