将特定的字符串模式替换为html

For my stock-market chat I want to replace every specific string pattern to html code. For example if I type "b $goog 780" i want this string to be replaced with:

Buy <a href="/stocks/goog">$goog</a> at 780 

How can I do this specific task with preg_replace?

$cmd='b $goog 780';

if(preg_match('/^([bs])\s+?\$(\w+?)\s+?(.+)$/i',$cmd,$res))
{
   switch($res[1])
   {
     case 'b': $cmd='buy';break;
     case 's': $cmd='sell';break;
   }
   $link=$cmd.' <a href="/stocks/'.$res[2].'">'.$res[2].'</a> at '.$res[3];
   echo $link;
}

$stocks = array('$goog' => '<a href="/stocks/goog">$goog</a>',
                '$apple' => '<a href="/stocks/apple">$apple</a>');

// get the keys.
$keys = array_keys($stocks);

// get the values.
$values = array_values($stocks);

// replace
foreach($keys as &$key) {
        $key = '/\b'.preg_quote($key).'\b/';
}

// input string.    
$str = 'b $goog 780';

// do the replacement using preg_replace                
$str = preg_replace($keys,$values,$str);

Does it have to be preg_replace? Using preg_match you can extract the components of the string and recombine them to form your link:

<?php
$string = 'b $goog 780';
$pattern = '/(b \$([^\s]+) (\d+))/';
$matches = array();
preg_match($pattern, $string, $matches);
echo 'Buy <a href="/stocks/' . $matches[2] . '">$' . $matches[2] . '</a> at ' . $matches[3] ; // Buy <a href="/stocks/goog">$goog</a> at 780 

What the pattern is looking for, is the letter 'b', followed by a dollar symbol (\$ - we escape the dollar as this is a special character in regex), then any and every character until it gets to a space ([^\s]+), then a space, and finally any number of numbers (\d+).