前面见到有人发表
但是速度比较慢，也非常长
这里给出一种

function ubb($str) {  
$color=Array('red','blue','green');
$str=eregi_replace('/[url/]([a-zA-Z0-9@:%_.~#-/?&]+)/[//url/]','<a href=http:////1>//1</a>',$str);//url
$str=eregi_replace('/[url=http://([a-zA-Z0-9@:%_.~#-/?&]+)/](.+)/[//url/]','<a href=//1 target=_blank>//2</a>',$str);
$str=eregi_replace('/[url=([a-zA-Z0-9@:%_.~#-/?&]+)/](.+)/[//url/]','<a href=http:////1 target=_blank>//2</a>',$str);
$str=eregi_replace('/[img/]([a-zA-Z0-9@:%_.~#-/?&]+)/[//img/]','<img src=http:////1>//1</img>',$str);//img
$str=eregi_replace('/[h([1-6])/](.+)/[//h[1-6]/]','<h//1>//2</h//1>',$str);//h1-6
$str=eregi_replace('/[email/]([_/.0-9a-z-]+@([0-9a-z][0-9a-z-]+/.)+[a-z]{2,3})/[//email/]','<a href=mailto://1>//1</a>',$str);//email
$str=eregi_replace('/[email=([_/.0-9a-z-]+@([0-9a-z][0-9a-z-]+/.)+[a-z]{2,3})/](.+)/[//email/]','<a href=mailto://1>//2</a>',$str);
$str=eregi_replace('/[b/](.+)/[//b/]','<b>//1</b>',$str);
$str=eregi_replace('/[i/](.+)/[//i/]','<i>//1</i>',$str);
$str=eregi_replace('/[size=(.+)/](.+)/[//size/]','<font size=//1>//2</font>',$str);
$str=eregi_replace('/[color=(.+)/](.+)/[//color/]','<font color=//1>//2</font>',$str);
$str=eregi_replace('/[sub/](.+)/[//sub/]','<sub>//1</sup>',$str);//下裱
$str=eregi_replace('/[sup/](.+)/[//sup/]','^//1',$str);//上标
for($i=0;$i<=count($color);$i++)$str=eregi_replace('/['.$color[$i].'/](.+)/[//'.$color[$i].'/]','<font color='.$color[$i].'>//1</font>',$str);
$str=preg_replace("//[quote/](.+?)/[//quote/]/is","<blockquote><font size='1' face='Courier New'>quote:</font><hr>//1<hr></blockquote>", $str);
$str=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>", $str);
$str=preg_replace("//[sig/](.+?)/[//sig/]/is","<div style='text-align: left;color: darkgreen;margin-left: 5%'><br><br>--------------------------<br>//1<br>--------------------------</div>", $str);
return $str;
}