Regular Expression help for html tags

[Anon]

I program a forum on my own. I have a problem when people post html tags. So I have a variable for allowable tags:
$allowtags = "b|||A|||tAble|||tr|||TD|||i|||u";
Any other tags not in the above list will be converted "<" to "&lt;" and ">" to "&gt;". Now, I need help to build a pattern for this. The pattern must work for multi-line string. And if there is a tag like "< sCripT >", it must also match (there is space/captial letters). Please help. I'm not good at Regular Expression. Thanks.

[Anon]

It was kinda mess up. This is the right one:

I program a forum on my own. I have a problem when people post html tags. So I have a variable for allowable tags:
$allowtags = "b|||A|||tAble|||tr|||TD|||i|||u";
Any other tags not in the above list will be converted "<" to "&amp;lt;" and ">" to "&amp;gt;". Now, I need help to build a pattern for this. The pattern must work for multi-line string. And if there is a tag like "< sCripT >", it must also match (there is space/captial letters). Please help. I'm not good at Regular Expression. Thanks.