Using a while True loop only break out with break if you got a right answer:

while True:
    user_name = input("Please enter your name: ")
    if not re.match("^[A-Za-z]*$", user_name):
        print ("Error! Make sure you only use letters in your name")
    else:
        print("Hello "+ user_name)
        break

Similar to Java. Use re.error exception:

import re

try:
    re.compile('[')
    is_valid = True
except re.error:
    is_valid = False

exception re.error

Exception raised when a string passed to one of the functions here is not a valid regular expression (for example, it might contain unmatched parentheses) or when some other error occurs during compilation or matching. It is never an error if a string contains no match for a pattern.

Here is the Regex for at least one digit, one uppercase letter, at least one lowercase letter, at least one special character

import re
password = input("Enter string to test: ")
# Add any special characters as your wish I used only #@$
if re.match(r"^(?=.*[\d])(?=.*[A-Z])(?=.*[a-z])(?=.*[@#$])[\w\d@#$]{6,12}$", password):
    print ("match")
else:
    print ("Not Match")

Hope this will Help You...

import re
pattern = re.compile("^([A-Z][0-9]+)+$")
pattern.match(string)