Try to simplify the username regular expression, if possible

[brylie]

Context

Pull request #1574 made changes to the username regular expression.

Before

// Username must be 3-15 alphanumeric string with hyphens allowed.
const UsernameRegEx = /^[a-z0-9A-Z_\-]{3,15}$/;

After

// Username must be 3-15 alphanumeric string with hyphens allowed.
const UsernameRegEx = /^(?!\d)(?!.*-.*-)(?!.*-$)(?!-)[a-zA-Z0-9-]{3,15}$/;

^ - Match at the beginning
(?!\d) - Do not match if the string starts with a digit
(?!.*-.*-) - Do not match if the string has 2 hyphens anywhere
(?!.*-$) - Do not match if the string ends with a hyphen
(?!-) - Do not match if the string starts with a hyphen
[a-zA-Z0-9-_]{3,15} - Match 3 to 15 characters from the range specified (username can have 1 hyphen and under_score with chars and numbers in it)
$ - Assert the end of string.

Issue

We have recently modified the reqular expression, and had trouble understanding its purpose. The comment does not describe the regular expression.

Task

• Determine how the regular expression works as currently written.
• Add comments describing why the regular expression is written as it is.
• If possible, simplify the regular expression.

[brylie]

@NNN & @jykae, why did we change the username RegEx in pull request #1574? What does the current regex do, in plain language?

/cc @bajiat

[brylie]

It seems like the regular expression was designed to match the rules of Github usernames:

Since we are not syncing usernames upstream to Github, I am not sure that we need as strict of username rules.

[brylie]

Also, Github usernames can start with numbers, so we don't need to prevent our usernames from starting with numbers, in any case:

// We can probably remove this from our regex
/^(?!\d)

[brylie]

Also, Github usernames can have multiple hyphens, just not together. So, the following can be removed or modified:

// Github usernames can have multiple hyphens, just not together
(?!.*-.*-)