To define a regular expression to validate username with JavaScript, we can add variuous patterns to the regex.
For instance, we write
const re =
/^[a-zA-Z0-9](_(?!(\.|_))|\.(?!(_|\.))|[a-zA-Z0-9]){6,18}[a-zA-Z0-9]$/;
to define the re regex with various patterns.
[a-zA-Z0-9] an matches alphanumeric characters.
_(?!\.) matches a _ not followed by a .
\.(?!_) mathes a . not followed by a _
{6,18} matches minimum 6 to maximum 18 times.
