How do I check for special characters in a String[Java]?

Question

I am creating a console game.. and I want the players to be able to choose their own usernames. Having usernames like .-|\12b-}|can be really weird. I want to be able to check if the username has "special characters" in it. If the player has a special character in his/her username, I want the system to print out a message(something like: Please change your username) . I don't want the code to just replace the letters.

Here is an example(follow this if you have an answer):

import java.util.Scanner; 
class StackOverflowExample {
  public static void main(String[] args) {
    System.out.println("Welcome New Player! What will your name be?");
    Scanner userInteraction = new Scanner(System.in);
    String userInput = userInteraction.nextLine();
    System.out.println("Welcome " + userInput + "!");
  }
}

Output :

You can see why this would be super weird..

I want to be able to scan:

String userInput = userInteraction.nextLine();

for any weird characters. How would I go about scanning the String?

Answer 1

Edited to have correct java code

You can check what each letter in a string is by using a int-asted charAt call to return the # the character is in ASCII [See here] ( https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/charCodeAt )

public static boolean isAlphaNumeric(String str) {
  int code, i, len;
  len = str.length();
  for (i = 0; i < len; i++) {
    code = (int) str.charAt(i);
    if (!(code > 47 && code < 58) && // numeric (0-9)
        !(code > 64 && code < 91) && // upper alpha (A-Z)
        !(code > 96 && code < 123)) { // lower alpha (a-z)
      return false;
    }
  }
  return true;
};
public static void main(String[] args) {
   String userInput = userInteraction.nextLine();
   if(isAlphaNumeric(userInput )){
       System.out.println("Welcome New Player! What will your name be?");
       Scanner userInteraction = new Scanner(System.in);
       String userInput = userInteraction.nextLine();
       System.out.println("Welcome " + userInput + "!");
   }
   else{
      System.out.println("Change your name from" + userInput + "!");
   }}

Answer 2

Try this. It continues to prompt for a username if anything other than letters are provided. Usernames like Battle101 or ~Hello~ are not allowed.

Scanner userInteraction = new Scanner(System.in);
String userInput = "";
while (true) {
    System.out.println("Welcome New Player! What will your name be?");
    userInput = userInteraction.nextLine();
    if (userInput.matches("[a-zA-Z]+")) { // check that the input only contains letters
       // okay so exit loop
        break;
    }
    // else explain the problem and reprompt.
    System.out.println("Only alphabetic characters are permitted.\nPlease try again.");
}
System.out.println("Welcome " + userInput + "!");