Regular expression to validate username

Question

I\'m trying to create a regular expression to validate usernames against these criteria:

1. Only contains alphanumeric characters, underscore an

Answer 1

This one should do the trick:

if (Regex.IsMatch(text, @"
    # Validate username with 5 constraints.
    ^                          # Anchor to start of string.
    # 1- only contains alphanumeric characters , underscore and dot.
    # 2- underscore and dot can't be at the end or start of username,
    # 3- underscore and dot can't come next to each other.
    # 4- each time just one occurrence of underscore or dot is valid.
    (?=[A-Za-z0-9]+(?:[_.][A-Za-z0-9]+)*$)
    # 5- number of characters must be between 8 to 20.
    [A-Za-z0-9_.]{8,20}        # Apply constraint 5.
    $                          # Anchor to end of string.
    ", RegexOptions.IgnorePatternWhitespace))
{
    // Successful match
} else {
    // Match attempt failed
} 

Answer 2

As much as I love regular expressions I think there is a limit to what is readable

So I would suggest

new Regex("^[a-z._]+$", RegexOptions.IgnoreCase).IsMatch(username) &&
!username.StartsWith(".") &&
!username.StartsWith("_") &&
!username.EndsWith(".") &&
!username.EndsWith("_") &&
!username.Contains("..") &&
!username.Contains("__") &&
!username.Contains("._") &&
!username.Contains("_.");

It's longer but it won't need the maintainer to open expresso to understand.

Sure you can comment a long regex but then who ever reads it has to rely on trust.......

Answer 3

private static final Scanner scan = new Scanner(System.in);

public static void main(String[] args) {
    int n = Integer.parseInt(scan.nextLine());
    while (n-- != 0) {
        String userName = scan.nextLine();
        String regularExpression = "^[[A-Z]|[a-z]][[A-Z]|[a-z]|\\d|[_]]{7,29}$";
        if (userName.matches(regularExpression)) {
            System.out.println("Valid");
        } else {
            System.out.println("Invalid");
        }
    }
}

Answer 4

I guess you'd have to use Lookahead expressions here. http://www.regular-expressions.info/lookaround.html

Try

^[a-zA-Z0-9](_(?!(\.|_))|\.(?!(_|\.))|[a-zA-Z0-9]){6,18}[a-zA-Z0-9]$

[a-zA-Z0-9] an alphanumeric THEN (

_(?!\.) a _ not followed by a . OR

\.(?!_) a . not followed by a _ OR

[a-zA-Z0-9] an alphanumeric ) FOR

{6,18} minimum 6 to maximum 18 times THEN

[a-zA-Z0-9] an alphanumeric

(First character is alphanum, then 6 to 18 characters, last character is alphanum, 6+2=8, 18+2=20)

Answer 5

^(?=.{4,20}$)(?:[a-zA-Z\d]+(?:(?:\.|-|_)[a-zA-Z\d])*)+$

You can test the regex here

Answer 6

function isUserName(val){
    let regUser=/^[a-zA-Z0-9](_(?!(\.|_))|\.(?!(_|\.))|[a-zA-Z0-9]){6,18}[a-zA-Z0-9]$/;
    if(!regUser.test(val)){
      return 'Name can only use letters,numbers, minimum length is 8 characters';
    }
  }