正则有关问题 将图片宽度替换为100%

正则问题 将图片宽度替换为100%
宽度不定的图片

$str = '<p><br></p><img style="width: 513px;" src="
http://xxxx/uploads/details/142163431741415.jpg"><img style="width: 513px;" src="
http://xxxx/uploads/details/14216343172321.jpg"><p><br></p>';

宽度替换为100%

$str = '<p><br></p><img style="width: 100%;" src="
http://xxxx/uploads/details/142163431741415.jpg"><img style="width: 100%;" src="
http://xxxx/uploads/details/14216343172321.jpg"><p><br></p>';

------解决思路----------------------

$str = '<p><br></p><img style="width: 513px;" src="
http://xxxx/uploads/details/142163431741415.jpg"><img style="width: 613px;" src="
http://xxxx/uploads/details/14216343172321.jpg"><p><br></p>';

echo preg_replace('/(?<=style="width:).+?px;/i', '100%', $str);

<p><br></p><img style="width:100%" src="
http://xxxx/uploads/details/142163431741415.jpg"><img style="width:100%" src="
http://xxxx/uploads/details/14216343172321.jpg"><p><br></p>

------解决思路----------------------
这样呢

echo  preg_replace('/(?<=<img)(.+?)style="width\: .+?"/s', '$1style="width: 100%;"', $str);