Android EditText monitor the input string, delete the specified character

alert_ed = (EditText) a.findViewById(R.id.alert_ed_queue_num);

alert_ed.addTextChangedListener(mTextWatcher);

private TextWatcher mTextWatcher = new TextWatcher() {
  
 public void afterTextChanged(Editable s) {
 // 先去掉监听器，否则会出现栈溢出
 alert_ed.removeTextChangedListener(mTextWatcher);
 alert_ed.setText(s.toString().replace("d",""));
 alert_ed.setSelection(alert_ed.getText().length());
 alert_ed.addTextChangedListener(mTextWatcher);
 }
 public void beforeTextChanged(CharSequence s, int start, int count, int after) {
 }
 public void onTextChanged(CharSequence s, int start, int before, int count) {
 }
};