RegExp for matching usernames: min 3 chars, max 20 chars, optional underscore in between chars

Question

I'm attempting to match roblox usernames (which follow these guidelines):

• Minimum of 3 characters

• Maximum of 20 characters

• Maximum of 1 underscore

• Underscore may not be at the beginning or end of the username

I am running on node.js version 10.12.0.

My current RegExp is: /^([a-z0-9])(\w)+([a-z0-9])$/i, but this does not account for the limit of 1 underscore.

Answer 1

You could use

^(?=^[^_]+_?[^_]+$)\w{3,20}$

See a demo on regex101.com (there are newline characters for demo purposes)

---

Broken down this is

^         # start of the string
(?=
    ^     # start of the string
    [^_]+ # not an underscore, at least once
    _?    # an underscore
    [^_]+ # not an underscore, at least once
    $     # end of the string
 )
\w{3,20}  # 3-20 alphanumerical characters
$         # end

---

The question has received quite some attention so I felt to add a non-regex version as well:

let usernames = ['gt_c', 'gt', 'g_t_c', 'gtc_', 'OnlyTwentyCharacters', 'poppy_harlow'];

let alphanumeric = new Set(['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '_']);

function isValidUsername(user) {
    /* non-regex version */
    // length
    if (user.length < 3 || user.length > 20)
        return false;

    // not allowed to start/end with underscore
    if (user.startsWith('_') || user.endsWith('_'))
        return false;
        
    // max one underscore
    var underscores = 0;
    for (var c of user) {
        if (c == '_') underscores++;
        if (!alphanumeric.has(c))
            return false;
    }

    if (underscores > 1)
        return false;
        
    // if none of these returned false, it's probably ok
    return true;
}

function isValidUsernameRegex(user) {
    /* regex version */
    if (user.match(/^(?=^[^_]+_?[^_]+$)\w{3,20}$/))
        return true;
    return false;
}

usernames.forEach(function(username) {
    console.log(username + " = " + isValidUsername(username));
});

I personally think the regex version is shorter and cleaner but it's up to you to decide. Especially the alphanumeric part requires either some comparisons or a regex. With the latter in mind, you could use a regex version altogether.