How to filter usernames with “@” in it?

This looks like it might be helpful: https://wordpress.org/plugins/restrict-usernames-emails-characters/

Thank you very much for the replay! But this plugin is an overkill.

I found a more efficient solution.
(We are running BuddyPress.)
I added to functions.php:

<?php

add_action( 'bp_loaded','bpdev_remove_bp_pre_user_login_action') ;
function bpdev_remove_bp_pre_user_login_action(){
   
  remove_action( 'pre_user_login', 'bp_core_strip_username_ats' );
}

add_filter( 'validate_username','bpdev_restrict_at_in_username',10,2) ;
function bpdev_restrict_at_in_username( $valid,$user_name ){

  if ( preg_match('/@/',$user_name ) )
  return false;
  return $valid;
}

It’s working. But one issue is left:

But I’m still looking for a solution to change the error message to the user, because it says “@” is allowed.

• This reply was modified 3 days, 3 hours ago by thomei.
• This reply was modified 3 days, 3 hours ago by thomei.
• This reply was modified 3 days, 3 hours ago by thomei.

SOLVED!

The following code in functions.php does it:

function validate_username_noat (){

    global $bp;

    $username = $bp->signup->username;

    if ($username){

        $tld_index = strrpos($username,'.');
        $tld = substr($username,$tld_index);

        if ($tld != '.edu'){
            $bp->signup->errors['signup_username'] = 'Benutzernamen können nur Buchstaben, Zahlen, "." und "-" enthalten.';
        }

    }

}

add_action( 'bp_loaded','bpdev_remove_bp_pre_user_login_action') ;
function bpdev_remove_bp_pre_user_login_action(){
   
  remove_action( 'pre_user_login', 'bp_core_strip_username_ats' );
}

add_filter( 'validate_username','bpdev_restrict_at_in_username',10,2) ;
function bpdev_restrict_at_in_username( $valid,$user_name ){

  if ( preg_match('/@/',$user_name ) )
  return false;
  return $valid;
}

Or even a better solution:

function username_noat_validation() {
        global $bp;

        if ( !empty( $_POST['signup_username'] ) )
        if ( !valid_noat_username( $_POST['signup_username'] ) ){
            $bp->signup->errors['signup_username'] = __( 'The @-symbol is not possible in the user name.', 'buddypress' );
        }
}
add_action( 'bp_signup_validate', 'username_noat_validation');

function valid_noat_username($ucandidate) {
        $r1='/@/';  //At
        if(preg_match_all($r1,$ucandidate, $o)<1) return TRUE;
        
        return FALSE;
}