hi all trying to ask for username and only letters or numbers or _ can be used. if anything else is provided i need to ask again. this is what i have so far but when i try to run it just keeps going instead of completing. so after the first question, if i enter username like "jack" it should just print username: jack, but instead it just keeps asking the second question in the code 'Please enter a username(only letters and numbers) how do i fix this?

Username = input ("What is your username?")

while Username != "^[A-Za-z0-9]*$":
    
    Username = input('Please enter a username(only letters and numbers)')
    
print("Username: ", Username)

CodePudding user response：

There can be two ways.

1. To cycle through the string and check if each character belongs to the range of letters or numbers. Though this is not advised as it is time and memory consuming.
2. Using str.isalnum(). This is easy short and better than the previous.

Code:

Username = input ("What is your username?")

while not Username.replace('_', '').isalnum(): #Edit: Removed _ as an invalid character
    
    Username = input('Please enter a username(only letters and numbers)')
    
print("Username: ", Username)

CodePudding user response：

A small modification to this https://stackoverflow.com/a/71506891/18248018 answer would be to substitute the line while not Username.replace('_', '').isalnum(): leaving everything else the same.

This would allow the username to still contain underscores, if this is your intent.

.replace(old, new) returns a new string (leaving the original one unchanged) with all instances of the substring old replaced with string new. This means that the modified line of code above calls .isalnum() on a string equivalent to the Username input stripped of underscores, and checks whether all the remaining characters are alphanumeric.

Otherwise, .isalnum() will reject input containing underscores.