PHP正则替换文件，有详细代码

Question

$str  ="<div class='asasas'><a href='www.baidu.com'/>点击</a><p>asssa</p><img src='attachment/201404/15/1_139755736546p8.jpg'></div>";
$mtstr = preg_replace('/src=/','src=http://test.disuzs.logo.com/',$str);

print_r($mtstr);  //结果http://test.uchome.disuzs.lodo.com/'attachment/201404/15/1_139755736546p8.jpg'  

//这里img标签多了一对'' 如何破？？