wordpress元框\自定义字段验证

I am trying to validate custom meta box fields. On the server side I tried to use "return false" on the "save_post" action, but WP disregards it (apparently the save_post is called after the post is already inserted to the DB). I then resorted to JQuery on the client-side, but using "return false" froze the page, and it stopped responding, as if I was using an infinite loop.I am trying to validate custom meta box fields.