SELECT tbl_register.register_username AS register_username, tbl_comment.comment_heading AS comment_heading, tbl_comment.comment_content AS comment_content, tbl_comment.comment_mreference AS comment_mreference FROM tbl_comment

INNER JOIN tbl_register

ON tbl_register.register_username = CASE

WHEN tbl_comment.comment_mreference REGEXP '^[0-9]{1,5}$' THEN tbl_comment.comment_mreference
ELSE 0
END