The Tcl/Tk documentation suggests not mixing the validatecommand and the textvariable options since setting an invalid value to the Tk variable will turn off validation. The same occurs if the validatecommand function do not return a Boolean value.

In case you are not familiar with the re module, you can check out the detailed introduction to regular expressions in the official Python documentation at https://docs.python.org/3.6/howto/regex.html.