Kivy Python: Detect backspace in Text Input

I am trying to create a simple TextInput for dates that limits input to numbers and auto-populates the the forward slashes for a (mm/dd/yy) format. I was successful in creating a filter that does this by redefining insert_text(), except when the user backspaces, I want to automatically remove the slashes as well. But I don't know how to detect when the user backspaces in the text input so that I can fire an event to erase the slashes if necessary.

Here's a snippet that explains what I want to do, but TextInput does not have an "on_key_up" attribute. Is there a way to add one? Or a better way to go about this?

# .kv file

<DateInput>
    on_key_up: self.check_for_backspace(keycode) # not a true attribute

# .py file

class DateInput(TextInput):

    # checks if last character is a slash and removes it after backspace keystroke.  Not sure this would work.
    def check_for_backspace(self, keycode):
        if keycode[1] == 'backspace' and self.text[-1:] == '/':
            self.text = self.text[:-1]

    #filter for date formatting which works well aside from backspacing
    pat = re.compile('[^0-9]')
    def insert_text(self, substring, from_undo=False):
        pat = self.pat
        if len(substring) > 1:
            substring = re.sub(pat, '', (self.text + substring))
            self.text = ''
            slen = len(substring)
            if slen == 2:
                s = substring[:2] + '/'
            elif slen == 3:
                s = substring[:2] + '/' + substring[2:]
            elif slen == 4:
                s = substring[:2] + '/' + substring[2:] + '/'
            else:
                s = substring[:2] + '/' + substring[2:4] + '/' + substring[4:8]
        elif len(self.text) > 9:
            s = ''
        elif len(self.text) == 1:
            s = re.sub(pat, '', substring)
            if s != '':
                s = s + '/'
        elif len(self.text) == 4:
            s = re.sub(pat, '', substring)
            if s != '':
                s = s + '/'
        else:
            s = re.sub(pat, '', substring)
        return super(DateInput, self).insert_text(s, from_undo=from_undo)

Since version 1.9.0 TextInput is inherent to FocusBehavior so if you want to detect when you press backspace you must use the keyboard_on_key_down() method or keyboard_on_key_up() method:

from kivy.app import App
from kivy.uix.textinput import TextInput

class DateInput(TextInput):
    def keyboard_on_key_down(self, window, keycode, text, modifiers):
        if keycode[1] == "backspace":
            print("print backspace down", keycode)
        TextInput.keyboard_on_key_down(self, window, keycode, text, modifiers)

    def keyboard_on_key_up(self, window, keycode, text, modifiers):
        if keycode[1] == "backspace":
            print("print backspace up", keycode)
        TextInput.keyboard_on_key_down(self, window, keycode, text, modifiers)


class MyApp(App):
    def build(self):
        return DateInput()

if __name__ == '__main__':
    MyApp().run()