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html tags recognition by Regular Expression

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.
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1 Solution
try this code:

// $text  is users input, I'll define something here
$text = "<b> test</b> <textarea>";

$allowtags = "b|||A|||tAble|||tr|||TD|||i|||u";

// split them into array
$tags = explode('|||', $allowtags);

// RegExp itself
$text = preg_replace("/<(\s*(?!\/?(".join("|",$tags).")[>\s])[^>]*)>/i","&lt;\\1&gt;",$text);

// it's ready!
print $text;
quanghocAuthor Commented:
Bit what if:
$text = "< B> test</b> <textarea>";  
 "< B  >" with space and capital. This one should not match.
quanghocAuthor Commented:
Nope, never mind. < B > is not a valid tag. Let's me check for a couple more and the point should be granted to you qucik. Thanks.
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anyway, this < b > case is quite interesting, I'll try to sort this out :) Give me few hours
quanghocAuthor Commented:
No, as I said above, < b> is not even a valid tag. Don't worry about this. As a matter of fact, would you please explain the pattern above? Like what '\/?(' means? and what '[>\s])[^>]*' is for?
Okay, I'll to comment it :)

we are looking for:

< - tag begin
( - everything in this brackets will appear in \\1 value
\s* - zero ore more spaces (space and tab symbols)
(?! - zero-length negative condition
\/  - character "/" (we should use slash here to show that this is just symbol without special meaning)
? - character "/" before this sign may appear 0 or 1 times
(".join("|",$tags).") - after joining this will look like (a|b|td|i) , "|" means logical OR, () need to show borders of expression
[>\s] - one of included symbols must appear. So, after keyword "a", "b", "td" must appear either ">" or \s (space) - we need this to avoid matching "img" as valid tag if "i" is allowed, etc.
) - condition's end
[^>] - everything except > (match tag's body and properties)
* - previous can appear zero or more times (non > symbols)
) - end of grouping and the end of \\1 value
> - tag end

/i in the end shows that expression is case insensitive

Hope this helps!
quanghocAuthor Commented:
Thanks a lot!

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