modded userpro_side_validate() from ajax.php of wordpress Userpro plugin

userpro_side_validate()

function userpro_side_validate(){
		global $userpro;
		
		if ( $_POST['action'] != 'userpro_side_validate')
			die();
			
		extract($_POST);
		$output['error'] = '';
		switch($ajaxcheck) {
		
			case 'envato_purchase_code':
				if ( !$userpro->verify_purchase($input_value) ) {
						$output['error'] = __('Invalid purchase code or Envato API is down.','userpro');
				} else {
					$output['error'] = '';
				}
				break;
		
			case 'display_name_exists':
				if ($userpro->display_name_exists($input_value)) {
					$output['error'] = __('The display name is already in use.','userpro');
				}
				break;
			
			case 'username_exists':
				if (username_exists($input_value)){
					$output['error'] = __('Username already taken.','userpro');
				} else if ( !preg_match("/^[A-Za-z0-9_]+$/", $input_value) ) {
					$output['error'] = __('Illegal characters are not allowed in username.','userpro');
				}
				break;
			
			case 'email_exists':
				if (!is_email($input_value)) {
					$output['error'] = __('Please enter a valid email.','userpro');
				} else if (email_exists($input_value)) {
					$output['error'] = __('Email is taken. Is that you? Try to <a href="#" data-template="login">login</a>','userpro');
				}
				break;
			
			case 'validatesecretkey':
				if (strlen($input_value) != 20) {
					$output['error'] = __('The secret key you entered is invalid.','userpro');
				} else {
					$users = get_users(array(
						'meta_key'     => 'userpro_secret_key',
						'meta_value'   => $input_value,
						'meta_compare' => '=',
					));
					if (!$users[0]) {
						$output['error'] = __('The secret key is invalid or expired.','userpro');
					}
				}
				break;
				
		}
		
		//if no check was triggered before
		if ( $output['error'] == '' ){
			$output = apply_filters('custom_ajax_check_callback',$output,$ajaxcheck,$input_value);
		}
		
		$output=json_encode($output);
		if(is_array($output)){ print_r($output); }else{ echo $output; } die;
	}
	

add a custom validation via filter:

// ITALIAN CELL NUMBER
add_filter('custom_ajax_check_callback', 'username_exists_as_cell_phone',10,3);
function username_exists_as_cell_phone($output, $ajaxcheck, $input_value){
    global $userpro;

    if ($ajaxcheck == 'username_exists_as_cell_phone'){
        if ( !preg_match("/^3\d{9}$/", $input_value) ) {
            $output['error'] = __('Is not a valid cell number. Please don\'t use  the international prefix (ex: +39)','userpro');
        } else {
            if (username_exists($input_value)){
                $output['error'] = __('Cell number already used. If it\'s your cell number, try to request a <a href="/cambia-password"><strong>password change</strong></a> to use the account. You will receive a code via SMS to login with this cell number.','userpro');
            }
        }
    }
    return $output;
}
`

This is exactly what I was looking for, only slightly different.

Can you help with the code if I just want to modify the code to do a ajax validation with the phone_number field and prevent registration if phone number already exist?

sorry, i'm reading your comment just now! The code above does exactly these 2 things: validate a phone number via regex and, if it is valid, checks if passed phone number is already in use as username. In that case it outputs an error