JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id="ITSWUsers" cols="50" rows="10">
John.smith
Peter.jones1
</textarea>
<button onclick="testFunc()">Click Me</button>

JavaScript

function testFunc()
{
    var usernamecheck = /^([A-Za-z0-9]+\.[A-Za-z0-9]+(\r)?(\n)?)+$/;
    
    if(!$.trim(document.all.ITSWUsers.value).match(usernamecheck))
    {
        alert ("Please write the usernames in the correct format (with a full stop between first and last name).");
        return false;
    }
}