请问一下有的论坛里那种[img][/img]贴图功能是如何做的？

Question

function ubb($Text)
{ 
$Text=htmlspecialchars($Text); 
$Text=stripslashes($Text);
$Text=ereg_replace("\r\n","<br/>",$Text); 
$Text=ereg_replace("\r","<br/>",$Text); 
$Text=nl2br($Text); 
$Text=preg_replace("/\\t/is","  ",$Text); 
$Text=ereg_replace(" ","&nbsp;",$Text); $Text=preg_replace("/\[h1\](.+?)\[\/h1\]/is","<h1>\\1</h1>",$Text); 
$Text=preg_replace("/\[h2\](.+?)\[\/h2\]/is","<h2>\\1</h2>",$Text); 
$Text=preg_replace("/\[h3\](.+?)\[\/h3\]/is","<h3>\\1</h3>",$Text); 
$Text=preg_replace("/\[h4\](.+?)\[\/h4\]/is","<h4>\\1</h4>",$Text); 
$Text=preg_replace("/\[h5\](.+?)\[\/h5\]/is","<h5>\\1</h5>",$Text); 
$Text=preg_replace("/\[h6\](.+?)\[\/h6\]/is","<h6>\\1</h6>",$Text); 
$Text=preg_replace("/\[center\](.+?)\[\/center\]/is","<center>\\1</center>",$Text); 
$Text=preg_replace("/\[big\](.+?)\[\/big\]/is","<big>\\1</big>",$Text); 
$Text=preg_replace("/\[small\](.+?)\[\/small\]/is","<small>\\1</small>",$Text); 

$Text=preg_replace("/\[url\](http:\/\/.+?)\[\/url\]/is","<a href=\\1>\\1</a>",$Text); 
$Text=preg_replace("/\[url\](.+?)\[\/url\]/is","<a href=\'http://\\1\'>http://\\1</a>",$Text); 
$Text=preg_replace("/\[url=(http:\/\/.+?)\](.*)\[\/url\]/is","<a href=\\1>\\2</a>",$Text); 
$Text=preg_replace("/\[url=(.+?)\](.*)\[\/url\]/is","<a href=http://\\1>\\2</a>",$Text); 

$Text=preg_replace("/\[img\](.+?)\[\/img\]/is","<img src=\\1 border=0>",$Text); 
$Text=preg_replace("/\[color=(.+?)\](.+?)\[\/color\]/is","<font color=\\1>\\2</font>",$Text); 
$Text=preg_replace("/\[size=(.+?)\](.+?)\[\/size\]/is","<font size=\\1>\\2</font>",$Text); 
$Text=preg_replace("/\[sup\](.+?)\[\/sup\]/is","^\\1",$Text); 
$Text=preg_replace("/\[sub\](.+?)\[\/sub\]/is","_\\1",$Text); 
$Text=preg_replace("/\[pre\](.+?)\[\/pre\]/is","<pre>\\1</pre>",$Text); 
$Text=preg_replace("/\[email\](.+?)\[\/email\]/is","<a href=mailto:\\1>\\1</a>",$Text); 
$Text=preg_replace("/\[i\](.+?)\[\/i\]/is","<i>\\1</i>",$Text); 
$Text=preg_replace("/\[b\](.+?)\[\/b\]/is","<b>\\1</b>",$Text); 
$Text=preg_replace("/\[quote\](.+?)\[\/quote\]/is","<blockquote><font size='1' face='Courier New'>quote:</font><hr>\\1<hr></blockquote>", $Text); 
$Text=preg_replace("/\[code\](.+?)\[\/code\]/is","<blockquote><font size='1' face='Times New Roman'>code:</font><hr color='lightblue'><i>\\1</i><hr color='lightblue'></blockquote>", $Text); 
$Text=preg_replace("/\[sig\](.+?)\[\/sig\]/is","<div style='text-align: left; color: darkgreen; margin-left: 5%'><br/><br/>--------------------------<br/>\\1<br/>--------------------------</div>", $Text); 
$Text=ereg_replace("\[hr\]","<hr>",$Text);
return $Text; 
}
ubb($Text)里的$Text为提交的内容