use strict;
use warnings;
while(<DATA>){
  print /(?<!XL5 )(?<![#_])ALPHA/ ? "OK\n" : "NOT OK\n";
}
__DATA__
I AM ALPHA AND GOOD ALPHA
I AM ALPHA AND BETA
I AM ALPHA AND #ALPHA
I AM ALPHA AND _ALPHA
I AM ALPHA AND XL5 ALPHA
I AM #ALPHA
I AM _ALPHA
I AM XL5 ALPHA

__OUTPUT__
OK
OK
OK
OK
OK
NOT OK
NOT OK
NOT OK