Use a textwatcher with your EditText , it has three methods
afterTextChanged(Editable s)
beforeTextChanged(CharSequence s, int start, int count, int after)
onTextChanged(CharSequence s, int start, int before, int count)
Use beforeTextChanged and check if the special character( = or whatever) has been inserted , keep old text
http://developer.android.com/reference/android/text/TextWatcher.html