求php大神帮帮忙这段字符串怎么办·

求php大神帮帮忙 这段字符串怎么处理·
$str = "http://www.test.com/imges/1.jpg--1,http://www.test.com/imges/3.jpg--2,http://www.test.com/imges/3.jpg--3";

我想要的结果 不知道怎么写正则

$str1 = "http://www.test.com/imges/1.jpg,http://www.test.com/imges/3.jpg,http://www.test.com/imges/3.jpg";

$str2 = "1,2,3";

求高手帮帮忙

------解决方案--------------------

PHP code

$str = "http://www.test.com/imges/1.jpg--1,http://www.test.com/imges/3.jpg--2,http://www.test.com/imges/3.jpg--3";

$str1= preg_replace('/--(\d+)/','',$str);
preg_match_all('/--(\d+)/',$str,$m);
$str2=join(',',$m[1]);