本函数类说明：

说是改进，其实核心函数parse()已经完全重写了，而且思路也是不一样的。
不过仍是受原有例子的启发，而且测试的例子还有URLCHECK等几个函数也是沿用的原有程序，目前还没有颜色的功能，但我会加入的。如果在程序上有什么BUG或不便的地方，请给我MAIL。谢谢！
改进功能:
对字符串进行UBB编码,该类目前只支持下列几个简单且实用的编码:
1. URL裢接
http://www.eoncn.com/
http://头可以不需要
如eoncn.com也是可以的。
2. Email裢接
test@163.net
3. 图片裢接
同URL链接一样，前面的http也可以不要。
4. 文字方面
粗体字
斜体字
加下划线
1号标题字
...
6号标题字
[tt][/tt]
[s][/s]
[em][/em]
[strong][/strong]
[samp][/samp]
[kbd][/kbd]
[var][/var]
[dfn][/dfn]
[cite][/cite]
注意以下几点:
1. url,email,img等标签是不分大小写的.
2. 在标签中不允许有TAB键出现，但空格允许。
3. 该类要调用htmlencode,htmlencode4textarea,emailcheck函数和urlcheck类.

4. 修改后支持嵌套，但url,email,img这三个标签不是允许嵌套的。

ubb.php

<? php
include("urlcheck.php");
include("otherfunc.php"); //这两个文件的内容，附在最后。
//ubbcode类
class ubbcode
{
    var $call_time = 0;
    //可处理标签及处理函数对应表
    var $tags = array( //小写的标签 => 对应的处理函数
                    'url' => '$this->url',
                    'email' => '$this->email',
                    'img' => '$this->img',
                    'b' => '$this->simple',
                    'i' => '$this->simple',
                    'u' => '$this->simple',
                    'tt' => '$this->simple',
                    's' => '$this->simple',
                    'strike' => '$this->simple',
                    'h1' => '$this->simple',
                    'h2' => '$this->simple',
                    'h3' => '$this->simple',
                    'h4' => '$this->simple',
                    'h5' => '$this->simple',
                    'h6' => '$this->simple',
                    'sup' => '$this->simple',
                    'sub' => '$this->simple',
                    'em' => '$this->simple',
                    'strong' => '$this->simple',
                    'code' => '$this->simple',
                    'samp' => '$this->simple',
                    'kbd' => '$this->simple',
                    'var' => '$this->simple',
                    'dfn' => '$this->simple',
                    'cite' => '$this->simple',
                    'small' => '$this->simple',
                    'big' => '$this->simple',
                    'blink' => '$this->simple'
                );
    //url裢接属性
    var $attr_url;
    //url合法性检查对象
    var $urlcheck;
    function ubbcode($attr_url)
    {
        $this->attr_url = ''.$attr_url;
        $this->urlcheck = new urlcheck();
    }
    //对$str进行UBB编码解析
    function parse($str)
    {
        $this->call_time++;
        $parse = ''.htmlencode($str);
        $ret = '';
        while(true)
        {
            $eregi_ret = eregi("\{#]{0,1}{:alnum:}{1,7}\]", $parse, $eregi_arr); //查找[xx]
            if(!$eregi_ret)
            {
                $ret . = $parse;
                break; //如果没有，返回
            }
            $pos = @strpos($parse, $eregi_arr[0]);
            $tag_len = strlen($eregi_arr[0]) - 2; //标记长度
            $tag_start = substr($eregi_arr[0], 1, $tag_len);
            $tag = strtolower($tag_start);
            if((($tag == "url") or ($tag == "email") or ($tag == "img")) and ($this->call_time > 1))
            {
                echo $this->call_time."<br>";
                return $parse;//如果不能是不能嵌套的标记，直接返回
            }
            $parse2 = substr($parse, 0, $pos); //标记之前
            $parse = substr($parse, $pos + $tag_len + 2); //标记之后
            if(!isset($this->tags[$tag]))
            {
                echo "$tag_start<br>";
                $ret . = $parse2.'['.$tag_start.']';
                continue;//如果是不支持的标记
            }
            //查找对对应的结束标记
            $eregi_ret = eregi("\[\/".$tag."\]", $parse, $eregi_arr);
            if(!$eregi_ret)
            {
                $ret . = $parse2.'['.$tag_start.']';
                continue;//如果没有对应该的结束标记
            }
            $pos = strpos($parse, $eregi_arr[0]);
            $value = substr($parse, 0, $pos); //这是起止标记之间的内容
            $tag_end = substr($parse, $pos + 2, $tag_len);
            $parse = substr($parse, $pos + $tag_len + 3); //结束标记之后的内容
            if(($tag != "url") and ($tag != "email") and ($tag != "img"))
            {
                $value = $this->parse($value);
            }
            $ret . = $parse2;
            eval('$ret .= '.$this->tags[$tag].'("'.$tag_start.'","'.$tag_end.'","'.$value.'");');
        }
        $this->call_time--;
        return $ret;
    }
    function simple($start, $end, $value)
    {
        return '<'.$start.'>'.$value.'</'.$end.'>';
    }
    function url($start, $end, $value)
    {
        $trim_value = trim($value);
        if (strtolower(substr($trim_value, 0, 7)) != "http://")
            $trim_value = "http://".$trim_value;
        if($this->urlcheck->check($trim_value)) return '<a href="'.$trim_value.'" '.$this->attr_url.'>'.$value.'</a>';
        else return '['.$start.']'.$value.'[/'.$end.']';
    }
    function email($start, $end, $value)
    {
        if(emailcheck($value)) return '<a href="mailto:'.$value.'">'.$value.'</a>';
        else return '['.$start.']'.$value.'[/'.$end.']';
    }
    function img($start, $end, $value)
    {
        $trim_value = trim($value);
        if ((strtolower(substr($trim_value, 0, 7)) != "http://") or ($this->urlcheck->check($trim_value)))
            return '<img src="'.$trim_value.'"></img>';
        else return '['.$start.']'.$value.'[/'.$end.']';
    }
}
//测试
echo '<html>';
echo '<head><title>测试</title></head>';
echo '<body>';
echo '<form action="'.str2url($PATH_INFO).'" method="post">';
echo '<textarea cols="100" rows="10" name="ubb">'.htmlencode4textarea($ubb).'</textarea><br>';
echo '<input type="submit" value="转换">';
echo '</form>';
if(isset($ubb))
{
    $ubbcode = new ubbcode('target="_blank"');
    echo '<hr>'.$ubbcode->parse($ubb);
}
echo '</body>';
echo '</html>';
? >

文件urlcheck.php的内容

<? php
//urlcheck.php
class urlcheck
{
    var $regex = array(//协议名(注意在这里必须写成小写) => 对应的正则表达式
                     'ftp' => '$this->ftpurl',
                     'file' => '$this->fileurl',
                     'http' => '$this->httpurl',
                     'https' => '$this->httpurl',
                     'gopher' => '$this->gopherurl',
                     'news' => '$this->newsurl',
                     'nntp' => '$this->nntpurl',
                     'telnet' => '$this->telneturl',
                     'wais' => '$this->waisurl'
                 );
    var $lowalpha;
    var $hialpha;
    var $alpha;
    var $digit;
    var $safe;
    var $extra;
    var $national;
    var $punctuation;
    var $reserved;
    var $hex;
    var $escape;
    var $unreserved;
    var $uchar;
    var $xchar;
    var $digits;
    var $urlpath;
    var $password;
    var $user;
    var $port;
    var $hostnumber;
    var $alphadigit;
    var $toplabel;
    var $domainlabel;
    var $hostname;
    var $host;
    var $hostport;
    var $login;
    //ftp
    var $ftptype;
    var $fsegment;
    var $fpath;
    var $ftpurl;
    //file
    var $fileurl;
    //http,https
    var $search;
    var $hsegment;
    var $hpath;
    var $httpurl;
    //gopher
    var $gopher_string;
    var $selector;
    var $gtype;
    var $gopherurl;
    //news
    var $article;
    var $group;
    var $grouppart;
    var $newsurl;
    //nntp
    var $nntpurl;
    //telnet
    var $telneturl;
    //wais
    var $wpath;
    var $wtype;
    var $database;
    var $waisdoc;
    var $waisindex;
    var $waisdatabase;
    var $waisurl;
    function check($url)
    {
        $pos = @strpos($url, ':', 1);
        if($pos < 1) return false;
        $prot = substr($url, 0, $pos);
        if(!isset($this->regex[$prot])) return false;
        eval('$regex = '.$this->regex[$prot].';');
        return ereg('^'.$regex.'$', $url);
    }
    function urlcheck()
    {
        $this->lowalpha = '[a-z]';
        $this->hialpha = '[A-Z]';
        $this->alpha = '('.$this->lowalpha.'|'.$this->hialpha.')';
        $this->digit = '[0-9]';
        $this->safe = '[$.+_-]';
        $this->extra = '[*()\'!,]';
        $this->national = '([{}|\^~`]|\\[|\\])';
        $this->punctuation = '[<>#%"]';
        $this->reserved = '[?;/:@&=]';
        $this->hex = '('.$this->digit.'|[a-fA-F])';
        $this->escape = '(%'.$this->hex.'{2})';
        $this->unreserved = '('.$this->alpha.'|'.$this->digit.'|'.$this->safe.'|'.$this->extra.')';
        $this->uchar = '('.$this->unreserved.'|'.$this->escape.')';
        $this->xchar = '('.$this->unreserved.'|'.$this->reserved.'|'.$this->escape.')';
        $this->digits = '('.$this->digit.'+)';
        $this->urlpath = '('.$this->xchar.'*)';
        $this->password = '(('.$this->uchar.'|[?;&=]'.')*)';
        $this->user = '(('.$this->uchar.'|[?;&=]'.')*)';
        $this->port = $this->digits;
        $this->hostnumber = '('.$this->digits.'.'.$this->digits.'.'.$this->digits.'.'.$this->digits.')';
        $this->alphadigit = '('.$this->alpha.'|'.$this->digit.')';
        $this->toplabel = '('.$this->alpha.'|('.$this->alpha.'('.$this->alphadigit.'|-)*'.$this->alphadigit.'))';
        $this->domainlabel = '('.$this->alphadigit.'|('.$this->alphadigit.'('.$this->alphadigit.'|-)*'.$this->alphadigit.'))';
        $this->hostname = '(('.$this->domainlabel.'\\.)*'.$this->toplabel.')';
        $this->host = '('.$this->hostname.'|'.$this->hostnumber.')';
        $this->hostport = '('.$this->host.'(:'.$this->port.')?)';
        $this->login = '(('.$this->user.'(:'.$this->password.')?@)?'.$this->hostport.')';
        $this->ftptype = '[aidAID]';
        $this->fsegment = '(('.$this->uchar.'|[?:@&=])*)';
        $this->fpath = '('.$this->fsegment.'(/'.$this->fsegment.')*)';
        $this->ftpurl = '([fF][tT][pP]://'.$this->login.'(/'.$this->fpath.'(;[tT][yY][pP][eE]='.$this->ftptype.')?)?)';
        $this->fileurl = '([fF][iI][lL][eE]://('.$this->host.'|[lL][oO][cC][aA][lL][hH][oO][sS][tT])?/'.$this->fpath.')';
        $this->search = '(('.$this->uchar.'|[;:@&=])*)';
        $this->hsegment = '(('.$this->uchar.'|[;:@&=])*)';
        $this->hpath = '('.$this->hsegment.'(/'.$this->hsegment.')*)';
        $this->httpurl = '([hH][tT][tT][pP][sS]?://'.$this->hostport.'(/'.$this->hpath.'([?]'.$this->search.')?)?)';
        $this->gopher_string = '('.$this->xchar.'*)';
        $this->selector = '('.$this->xchar.'*)';
        $this->gtype = $this->xchar;
        $this->gopherurl = '([gG][oO][pP][hH][eE][rR]://'.$this->hostport.'(/('.$this->gtype.'('.$this->selector.'(%09'.$this->search.'(%09'.$this->gopher_string.')?)?)?)?)?)';
        $this->article = '(('.$this->uchar.'|[;/?:&=])+@'.$this->host.')';
        $this->group = '('.$this->alpha.'('.$this->alpha.'|'.$this->digit.'|[-.+_])*)';
        $this->grouppart = '([*]|'.$this->group.'|'.$this->article.')';
        $this->newsurl = '([nN][eE][wW][sS]:'.$this->grouppart.')';
        $this->nntpurl = '([nN][nN][tT][pP]://'.$this->hostport.'/'.$this->group.'(/'.$this->digits.')?)';
        $this->telneturl = '([tT][eE][lL][nN][eE][tT]://'.$this->login.'/?)';
        $this->wpath = '('.$this->uchar.'*)';
        $this->wtype = '('.$this->uchar.'*)';
        $this->database = '('.$this->uchar.'*)';
        $this->waisdoc = '([wW][aA][iI][sS]://'.$this->hostport.'/'.$this->database.'/'.$this->wtype.'/'.$this->wpath.')';
        $this->waisindex = '([wW][aA][iI][sS]://'.$this->hostport.'/'.$this->database.'[?]'.$this->search.')';
        $this->waisdatabase = '([wW][aA][iI][sS]://'.$this->hostport.'/'.$this->database.')';
        $this->waisurl = '('.$this->waisdatabase.'|'.$this->waisindex.'|'.$this->waisdoc.')';
    }
}
? >

文件otherfunc.php的内容

<? php
//otherfunc.php
function htmlencode($str)
{
    $str = (string)$str;
    $ret = '';
    $len = strlen($str);
    $nl = false;
    for($i = 0; $i < $len; $i++)
    {
        $chr = $str[$i];
        switch($chr)
        {
        case '<':
            $ret . = '<';
            $nl = false;
            break;
        case '>':
            $ret . = '>';
            $nl = false;
            break;
        case '"':
            $ret . = '"';
            $nl = false;
            break;
        case '&':
            $ret . = '&';
            $nl = false;
            break;
            /*
            case ' ':
            $ret .= ' ';
            $nl = false;
            break;
            */
        case chr(9):
            $ret . = ' ';
            $nl = false;
            break;
        case chr(10):
            if($nl) $nl = false;
            else
            {
                $ret . = '<br>';
                $nl = true;
            }
            break;
        case chr(13):
            if($nl) $nl = false;
            else
            {
                $ret . = '<br>';
                $nl = true;
            }
            break;
        default:
            $ret . = $chr;
            $nl = false;
            break;
        }
    }
    return $ret;
}
function htmlencode4textarea($str)
{
    $str = (string)$str;
    $ret = '';
    $len = strlen($str);
    for($i = 0; $i < $len; $i++)
    {
        $chr = $str[$i];
        switch($chr)
        {
        case '<':
            $ret . = '<';
            break;
        case '>':
            $ret . = '>';
            break;
        case '"':
            $ret . = '"';
            break;
        case '&':
            $ret . = '&';
            break;
        case ' ':
            $ret . = ' ';
            break;
        case chr(9):
            $ret . = ' ';
            break;
        default:
            $ret . = $chr;
            break;
        }
    }
    return $ret;
}
function emailcheck($email)
{
    $ret = false;
    if(strstr($email, '@') && strstr($email, '.'))
    {
        if(eregi("^([_a-z0-9]+([\\._a-z0-9-]+)*)@([a-z0-9]{2,}(\\.[a-z0-9-]{2,})*\\.[a-z]{2,3})$", $email))
        {
            $ret = true;
        }
    }
    return $ret;
}
function str2url($path)
{
    return eregi_replace("%2f", "/", urlencode($path));
}
?>