str.replaceAll("\\bmuy\\b", "\\U$0\\E");

private void chechIfExistWord(String line) {
        String[] words = line.split(" ");
        StringBuilder result = new StringBuilder();
        for (String word : words) {
            if(matchWord(word)) {
                result.append(word.toUpperCase());
            } else {
                result.append(word);
            }
        }
    }

    private boolean matchWord(String word) {
        return word.matches("\\p{Punct}*" + wordToModificate + "\\p{Punct}*");
    }