Cannot search keyword

I don’t know since when I can no longer use the search form.

At the front-end, I can search for english words but if I enter CHinese words, it will give this error:

WordPress database error: [Got error ’empty (sub)expression’ from regexp]
SELECT posts.*, CASE WHEN posts.post_title REGEXP ” THEN 1 ELSE 0 END AS keyword_in_title, MATCH ( posts.post_title, posts.post_content ) AGAINST ( ” ) AS mysql_score FROM wp_posts as posts WHERE posts.post_date_gmt <= ‘2007-04-20 14:01:00’ AND ( posts.post_password = ” ) AND ( posts.post_status = ‘static’ OR posts.post_status = ‘publish’ ) AND ( posts.post_title REGEXP ” OR posts.post_content REGEXP ” ) ORDER BY keyword_in_title DESC, mysql_score DESC, posts.post_date DESC LIMIT 0, 5

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And in Admin -> Manage Post, I can’t even search anything even for english words. It will give the same error.

Can someone help? Thanks!!