<img class="friends_photo_img" src="http://xxx.com/d_f922a00c.jpg" />

var s='<img class="friends_photo_img" src="http://xxx.com/d_f922a00c.jpg" />'
s=s+'<img src="http://1.jpg" />'
s=s+'<img class="friends_photo_img" src="http://xxx.com/d_f922a00c.jpg" />'
s=s+'<div>test</div>'
s=s+'<img class="friends_photo_img" src="http://xxx.com/d_f922a00c.jpg" />'
var s1='<img class="friends_photo_img" src="'
var urn='new_urn'
var r=/<img class="friends_photo_img" src="[^">]*/gi
alert(s.replace(r,s1+urn))

var r=/<img class="friends_photo_img" src="[^"]*/gi

/<img[^>]+src=([\'"])?((?(1).+?|[^\s>]+))(?(1)\1)/