正则太菜【继续】

$str = 'aaa.bbb.ccc.ddd $aaa.bbb.ccc.ddd... aaa.bbb.ccc.ddd';

echo preg_replace_callback('/\$([a-z]+)((?:.[a-z]+)+)/i', 'test', $str);

function test($m)
{
    $arr = explode('.',trim($m[0], '$'));
    $join = array();
    foreach($arr as $a) {
        $join[] = $a[0];
    }
    return '$' . implode($join, '.');
}

/**

PHP code

$s = 'aaa.bbb.ccc.ddd $aaa.bbb.ccc.ddd... aaa.bbb.ccc.ddd';

echo preg_replace_callback('/\$[^\s]+/', 'back', $s);

function back($r) {
  return preg_replace('/(\w)+/', '$1', $r[0]);
}