<?php

$text = "Oвес закyпка пoлимoрф прoдажa русский hello world";
$splitExp = "/ /";
$regexp = "/([а-яё]?)([a-z])([а-яё])?/ui";

$parts = preg_split($splitExp, $text);

foreach ($parts as $word) {
	if (preg_match($regexp, $word)) {
	echo "Опечатка в слове {$word}: " . preg_replace($regexp, '$1[$2]$3', $word) . "\n";
	}
}