mepr-validate-username.php

<?php

function mepr_validate_username($errors) {
  $username = isset($_POST['user_login']) ? trim($_POST['user_login']) : '';
  $first_name = isset($_POST['user_first_name']) ? trim($_POST['user_first_name']) : '';
  $last_name = isset($_POST['user_last_name']) ? trim($_POST['user_last_name']) : '';

  // If username is empty, don't run any validations.
  if(empty($username)) {
    return $errors;
  }

  // Make sure username doesn't include an @ symbol.
  if(strpos(strtolower($username), '@') !== false) {
    $errors[] = 'Your username can\'t include an @.';
  }

  // Make sure first name or last name isn't included in the username.
  if(!empty($first_name) && strpos(strtolower($username), strtolower($first_name)) !== false) {
    $errors[] = 'Your username can\'t include your first name.';
  }

  if(!empty($last_name) && strpos(strtolower($username), strtolower($last_name)) !== false) {
    $errors[] = 'Your username can\'t include your last name.';
  }

  return $errors;
}
add_filter('mepr-validate-signup', 'mepr_validate_username');