The following application shows how to validate an entry using regular expressions:

import re 
import tkinter as tk 
 
class App(tk.Tk): 
    def __init__(self): 
        super().__init__() 
        self.pattern = re.compile("^w{0,10}$") 
        self.label = tk.Label(self, text="Enter your username") 
        vcmd = (self.register(self.validate_username), "%i", "%P") 
        self.entry = tk.Entry(self, validate="key", 
                              validatecommand=vcmd, 
                              invalidcommand=self.print_error) 
        self.label.pack() 
        self.entry.pack(anchor=tk.W, padx=10, pady=10) 
 
    def validate_username(self, index, username): 
        print("Modification at index " + index) 
        return self.pattern.match(username) is not None 
 
    def print_error(self): 
        print("Invalid username character") 
 
if __name__ == "__main__": 
    app = App() 
    app.mainloop() 

If you run this script and type a non-alphanumeric character in the Entry widget, it will keep the same content and print the error message. This will also happen when you try to type more than 10 valid characters since the regular expression also limits the content's length.