can't pull products from DB with REGEXP?

I have table with products. In column categories are listed numbers of categories a product belongs, like: 1. 21, 2. 35, 8, 72, 1, 4, 3. 23, 11, 48, 4. 65, 5. 11, 6. 23, 7. ... (somebody else created the table this way and I have to keep it for the moment) Now, I have to list all products that belongs to category with cat. number, e.g. 11. If I use query: select * from products where categories like '11, ' product no. 5 will be listed but not product no. 3 select * from products where categories like '%11,% ' will list products 3 and 5 but select * from products where categories like '%1,% ' will list products 2, 3 and 5 too - and, 3 & 5 are wrong. I got from friend this solution: select * from products where categories REGEXP '(^".$cat_num."|, ".$cat_num.")' it works just fine if I use 2 digit cat number. e.g. 11 or 26 or 62. But if I select for cat. num. 2 it will list all products from categories 2 and everything between 20 and 29. 32 or 62 will not be listed. Confusing, ha? Thanks for any help! -afan