How to set a limit of a character frequency or disable the onclick button on some conditions?

If you want to check if a string ends in a digit, you can use this

String currentTxt = display.getText().toString();

if( currentTxt.matches("^.*?\\d$") ) { // check that it ends with a number
    // allow adding a + symbol at the end
    display.setText(currentTxt + "+");
}

The $ in the regex matches the end of the string, and the ^ matches the front of the string. So this matches a full string with zero or more characters (.*) followed by an ending digit (?\\d). You can always test Regex patterns online.

In this case, a regex isn't strictly necessary though, you could also just get the last character and call isDigit

String currentTxt = display.getText().toString();

if( !currentTxt.isEmpty() && Character.isDigit(currentTxt.charAt(currentTxt.length()-1)) ) { // check that it ends with a number
    // allow adding a + symbol at the end
    display.setText(currentTxt + "+")
}