regex - how to search for consecutive repeated characters in a string without using any built-in php functions? -

i have been asked solve question. please suggest me how solve this.

$string = "kkjnmnmnjjjnmn"; 

here need find consecutively repeated string of length 3. example - kkj has occurred once , jnm 2 times , nmn 3 times.

starting first character, going in right side direction, 3 consecutive characters repeated more once should output.

preg_match_all result nmn - 2 times , not 3 times. 

how solve this?

    $string = "kkjnmnmnjjjnmn";     $length = strlen($string);     $pieces = [];     ($i = 0; $i < $length - 2; $i++) {         $piece = substr($string, $i, 3);         if (array_key_exists($piece, $pieces)) {             $pieces[$piece] += 1;         } else {             $pieces[$piece] = 1;         }     } 

// $pieces contain need