Snippet to add nicename field to WordPress user profile

add-user-nicename.php

<?php

add_action( 'personal_options', 'user_edit_ob_start' ); 
add_action( 'show_user_profile', 'insert_nicename_input' );
add_action( 'edit_user_profile', 'insert_nicename_input' );
add_action( 'user_profile_update_errors', 'profile_update', 10, 3 );

function user_edit_ob_start() {
    ob_start();
}

function insert_nicename_input( $user ) {
    $content = ob_get_clean();
    $regex = '/<tr(.*)class="(.*)\buser-user-login-wrap\b(.*)"(.*)>([\s\S]*?)<\/tr>/';
    $nicename_row = sprintf(
        '<tr class="user-user-nicename-wrap"><th><label for="user_nicename">%1$s</label></th><td><input type="text" name="user_nicename" id="user_nicename" value="%2$s" class="regular-text" />' . "\n" . '<span class="description">%3$s</span></td></tr>',
        esc_html__( 'Nicename' ),
        esc_attr( $user->user_nicename ),
        esc_html__( 'Must be unique.' )
    );
    echo preg_replace( $regex, '\0' . $nicename_row, $content );
}

function profile_update( $errors, $update, $user ) {
    if ( !$update ) return;
    if ( empty( $_POST['user_nicename'] ) ) {
        $errors->add(
            'empty_nicename',
            sprintf(
                '<strong>%1$s</strong>: %2$s',
                esc_html__( 'Error' ),
                esc_html__( 'Please enter a Nicename.' )
            ),
            array( 'form-field' => 'user_nicename' )
        );
    } else {
        $user->user_nicename = $_POST['user_nicename'];
    }
}

?>