Best way to alphanumeric check in JavaScript

Question

What is the best way to perform an alphanumeric check on an INPUT field in JSP? I have attached my current code

Answer 1

Here are some notes: The real alphanumeric string is like "0a0a0a0b0c0d" and not like "000000" or "qwertyuio".

All the answers I read here, returned true in both cases. This is not right.

If I want to check if my "00000" string is alphanumeric, my intuition is unquestionably FALSE.

Why? Simple. I cannot find any letter char. So, is a simple numeric string [0-9].

On the other hand, if I wanted to check my "abcdefg" string, my intuition is still FALSE. I don't see numbers, so it's not alphanumeric. Just alpha [a-zA-Z].

The Michael Martin-Smucker's answer has been illuminating.

However he was aimed at achieving better performance instead of regex. This is true, using a low level way there's a better perfomance. But results it's the same. The strings "0123456789" (only numeric), "qwertyuiop" (only alpha) and "0a1b2c3d4f4g" (alphanumeric) returns TRUE as alphanumeric. Same regex /^[a-z0-9]+$/i way. The reason why the regex does not work is as simple as obvious. The syntax [] indicates or, not and. So, if is it only numeric or if is it only letters, regex returns true.

But, the Michael Martin-Smucker's answer was nevertheless illuminating. For me. It allowed me to think at "low level", to create a real function that unambiguously processes an alphanumeric string. I called it like PHP relative function ctype_alnum (edit 2020-02-18: Where, however, this checks OR and not AND).

Here's the code:

function ctype_alnum(str) {
  var code, i, len;
  var isNumeric = false, isAlpha = false; // I assume that it is all non-alphanumeric

  for (i = 0, len = str.length; i < len; i++) {
    code = str.charCodeAt(i);

    switch (true) {
      case code > 47 && code < 58: // check if 0-9
        isNumeric = true;
        break;

      case (code > 64 && code < 91) || (code > 96 && code < 123): // check if A-Z or a-z
        isAlpha = true;
        break;

      default:
        // not 0-9, not A-Z or a-z
        return false; // stop function with false result, no more checks
    }
  }

  return isNumeric && isAlpha; // return the loop results, if both are true, the string is certainly alphanumeric
}

And here is a demo:

function ctype_alnum(str) {
  var code, i, len;
    var isNumeric = false, isAlpha = false; //I assume that it is all non-alphanumeric

    
loop1:
  for (i = 0, len = str.length; i < len; i++) {
    code = str.charCodeAt(i);
        
        
        switch (true){
            case code > 47 && code < 58: // check if 0-9
                isNumeric = true;
                break;
            case (code > 64 && code < 91) || (code > 96 && code < 123): //check if A-Z or a-z
                isAlpha = true;
                break;
            default: // not 0-9, not A-Z or a-z
                return false; //stop function with false result, no more checks
                
        }

  }
    
  return isNumeric && isAlpha; //return the loop results, if both are true, the string is certainly alphanumeric
};

$("#input").on("keyup", function(){

if ($(this).val().length === 0) {$("#results").html(""); return false};
var isAlphaNumeric = ctype_alnum ($(this).val());
    $("#results").html(
        (isAlphaNumeric) ? 'Yes' : 'No'
        )
        
})

 is Alphanumeric? 

This is an implementation of Michael Martin-Smucker's method in JavaScript.