Allow hyphenated usernames in Wordpress Multisite

gistfile1.php

<?php

/**
 * Allow hypenated usernames
 *
 * @wp-hook wpmu_validate_user_signup
 * @param   array $result
 * @return  array
 */
function wpse_59760_allow_hyphenated_usernames( $result ) {
  $error_name = $result[ 'errors' ]->get_error_message( 'user_name' );

  if ( ! empty ( $error_name ) 
      && $error_name == __( 'Usernames can only contain lowercase letters (a-z) and numbers.' ) 
      && $result['user_name'] == $result['orig_username'] 
      && ! preg_match( '/[^-a-z0-9]/', $result['user_name'] ) 
  ) {
    unset ( $result[ 'errors' ]->errors[ 'user_name' ] );
    return $result;
  } else {
    return $result;
  }
}
add_filter( 'wpmu_validate_user_signup', 'wpse_59760_allow_hyphenated_usernames' );

For those reading this on 2021.

I'm creating a multisite on WordPress 5.6 and I could allow hyphenated and dashed usernames by changing the following on the original wpmu_validate_user_signup function in wp-includes/ms-functions.php:

function wpmu_validate_user_signup( $user_name, $user_email ) {
    ...

    if ( $user_name != $orig_username || preg_match( '/[^-_a-z0-9]/', $user_name ) ) {
    $errors->add( 'user_name', __( 'Usernames can only contain lowercase letters (a-z), -, _ and numbers.' ) );
    $user_name = $orig_username;
    }

    ...
}

Just change the regular expression to be: /[^-_a-z0-9]/