Regex to validate username in Javascript

Question

Following is the regex that I tried to validate against the below mentioned criteria, but in some cases its failing. Let me know what I am doing wrong here.

Regex-

/[a-z]|\d|\_{4, 16}$/.test(username)

Criteria -

Allowed characters are:

• lowercase letters
• Numbers
• Underscore
• Length should be between 4 and 16 characters (both included).

Code

 function validateUsr(username) { res = /[az]|\d|\_{4, 16}$/.test(username) return res } console.log(validateUsr('asddsa')); // Correct Output - true console.log(validateUsr('a')); // Correct Output - false console.log(validateUsr('Hass')); // Correct Output - false console.log(validateUsr('Hasd_12assssssasasasasasaasasasasas')); // Correct Output - false console.log(validateUsr('')); // Correct Output - false console.log(validateUsr('____')); // Correct Output - true console.log(validateUsr('012')); // Correct Output - false console.log(validateUsr('p1pp1')); // Correct Output - true console.log(validateUsr('asd43 34')); // Correct Output - false console.log(validateUsr('asd43_34')); // Correct Output - true

Answer 1

You may join the patterns to a single character class and apply the limiting quantifier to the class, not just to the _ pattern. Note the space is meaningful inside a pattern, and {4, 16} matches a {4, 16} string, it is not parsed as a quantifier.

You may use

 var regex = /^[az\d_]{4,16}$/; function validateUsr(username) { return regex.test(username) } console.log(validateUsr('asddsa')); // Correct Output - true console.log(validateUsr('a')); // Correct Output - false console.log(validateUsr('Hass')); // Correct Output - false console.log(validateUsr('Hasd_12assssssasasasasasaasasasasas')); // Correct Output - false console.log(validateUsr('')); // Correct Output - false console.log(validateUsr('____')); // Correct Output - true console.log(validateUsr('012')); // Correct Output - false console.log(validateUsr('p1pp1')); // Correct Output - true console.log(validateUsr('asd43 34')); // Correct Output - false console.log(validateUsr('asd43_34')); // Correct Output - true

The ^[az\d_]{4,16}$ - see its demo - pattern means:

• ^ - start of string
• [ - start of a character class:
  • az - ASCII lowercase letters
  • \d - ASCII digit
  • _ - an underscore
• ]{4,16} - end of the class, repeat four through sixteen times
• $ - end of string.