Validate user input using regular expressions

Question

I would like to be able to validate that the user has entered a valid name within my program. If they have not entered a valid name i want the program to continue to prompt them to enter their name again. If the name entered is valid i would like the program to greet the user.

So far i have:

import re

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)

How would i loop this if their name is not valid??

If you don't want to allow an empty input, I would recommend using ^[A-Za-z]+$ instead of ^[A-Za-z]*$. — Proghero
– Proghero, Commented Nov 23, 2015 at 21:53
Limiting your input to ASCII letters does users around the world a disservice. If there are characters you truly can't handle, like spaces, blacklist them, but let your users choose names that are meaningful to them. — dimo414
– dimo414, Commented Nov 24, 2015 at 17:24
What are the rules for names that you are trying to enforce, can you enumerate them? — yourcomputergenius
– yourcomputergenius, Commented May 20, 2021 at 2:54

Mike Müller · Accepted Answer · 2015-11-23 21:49:03Z

5

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

answered Nov 23, 2015 at 21:49

Mike Müller

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Comments

Tom · Accepted Answer · 2021-05-20 02:47:08Z

2

user_name = '1'  #something that doesn't validate
while not re.match("^[A-Za-z]*$", user_name):
    user_name = input("Please enter your name: ")
    print ("Error! Make sure you only use letters in your name")
else:
    print("Hello! "+ user_name)

edited May 20, 2021 at 2:47

answered Nov 23, 2015 at 21:51

Tom

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Comments

R Nar · Accepted Answer · 2015-11-24 17:15:47Z

0

put everything in a while loop and have the condition your matching statement:

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

Edit:

using str.isalpha()

while not user_name.isalpha():

edited Nov 24, 2015 at 17:15

answered Nov 23, 2015 at 21:49

R Nar

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2 Comments

Miss Jones Over a year ago

Fantastic thank you! I was thinking along those lines but was not sure if i could use not with while. Are their other methods that can be used to validate users only enter letters?

R Nar Over a year ago

you could use str.isalpha()

David A. Wheeler · Accepted Answer · 2024-04-09 19:17:01Z

0

WARNING: In Python, use "\Z" not "$" to mean "end of string".

In Python the "$" is permissive - it allows at least an extra newline.

Use this pattern instead: "^[A-Za-z]*\Z"

answered Apr 9, 2024 at 19:17

David A. Wheeler

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