Permalinks for tags throws error

I have added permalinks to the tag base like “?tag=”.
Please find that in screenshot https://ibb.co/nHEyod

On navigating to the post page I get compilation failed error in “class-wp.php”
Error is “Warning: preg_match(): Compilation failed: nothing to repeat at offset 1 in /var/www/html/wp-includes/class-wp.php on line 218

Warning: preg_match(): Compilation failed: nothing to repeat at offset 1 in /var/www/html/wp-includes/class-wp.php on line 219”

Please find the screenshot.
https://ibb.co/cU8r8d