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 =  /[a-z]|\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

Wiktor Stribiżew · Accepted Answer · 2020-04-25 21:52:36Z

3

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 = /^[a-z\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 ^[a-z\d_]{4,16}$ - see its demo - pattern means:

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

edited Apr 25, 2020 at 21:52

answered Apr 25, 2020 at 21:40

Wiktor Stribiżew

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2 Comments

Nesh Over a year ago

What is the meaning of ^ in /^[a-z\d_]{4,16}$/ ?

Wiktor Stribiżew Over a year ago

@Nesh I added details.

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