MySQL error in file: /engine/modules/show.short.php at line 83

Error Number: 3685

The Error returned was:
Illegal argument to a regular expression.

SQL query:

SELECT p.id, p.autor, p.date, p.short_story, CHAR_LENGTH(p.full_story) as full_story, p.xfields, p.title, p.category, p.alt_name, p.comm_num, p.allow_comm, p.fixed, p.tags, e.news_read, e.allow_rate, e.rating, e.vote_num, e.votes, e.view_edit, e.editdate, e.editor, e.reason FROM mobi_post p LEFT JOIN mobi_post_extras e ON (p.id=e.news_id) WHERE category regexp '[[:<:]](21)[[:>:]]' AND approve=1 ORDER BY date DESC LIMIT 0,15