如何限制用户在EditText框中只能输入一定范围的值？

InputFilter filter = new InputFilter() {
    public CharSequence filter(CharSequence source, int start, int end,
            Spanned dest, int dstart, int dend) {
        for (int i = start; i < end; i++) {
            if (!Character.isDigit(source.charAt(i))) {
                return "";
            }
        }
        return null;
    }
};

editText.setFilters(new InputFilter[] { filter });

editText.addTextChangedListener(new TextWatcher() {
    public void afterTextChanged(Editable s) {
        if (s.length() != 6) {
            editText.setError("请输入长度为6的字符串");
        }
    }

    public void beforeTextChanged(CharSequence s, int start, int count, int after) {
    }

    public void onTextChanged(CharSequence s, int start, int before, int count) {
    }
});

String pattern = "^[a-zA-Z0-9]+$";
String input = editText.getText().toString();
if (!input.matches(pattern)) {
    editText.setText("");
    editText.setError("只能输入字母和数字");
}