encrypt some chars of a string

emoji.java

public static String encryptString(String str, String encryptStr) {
    String resultStr;

    int startIndex = indexOfNormalText(str, false);
    int endIndex = indexOfNormalText(str, true);

    if (startIndex == NOT_FOUND) {
        resultStr = str + encryptStr;
    } else {
        if (str.length() == 1) {
            resultStr = str + encryptStr;
        } else {
            String firstChar, lastChar;

            // first char
            if (startIndex == 0) {
                firstChar = str.substring(0, 1);
            } else {
                firstChar = str.substring(0, startIndex);
            }

            // last char
            if (endIndex == str.length() - 1) {
                lastChar = str.substring(endIndex, str.length());
            } else {
                lastChar = str.substring(endIndex + 1, str.length());
            }

            resultStr = firstChar + encryptStr + lastChar;
        }
    }

    return resultStr;
}

private static final int NOT_FOUND = Integer.MAX_VALUE;
public static int indexOfNormalText(String originalString, boolean reverse) {
    int index;
    boolean found = false;

    if (!reverse) {
        for (index = 0; index < originalString.length(); index++) {
            String subString = String.valueOf(originalString.charAt(index));
            if (!subString.matches("[\ud000-\ue000]")) {
                found = true;
                break;
            }
        }

        if (found) {
            return index;
        } else {
            return NOT_FOUND;
        }
    } else {
        for (index = originalString.length() - 1; index >= 0; index--) {
            String subString = String.valueOf(originalString.charAt(index));
            if (!subString.matches("[\ud000-\ue000]")) {
                found = true;
                break;
            }
        }

        if (found) {
            return index;
        } else {
            return NOT_FOUND;
        }
    }
}