替换_和.jpg之间的url字符串中的字符

I have this url in a string.

$string = 'http://www.mysite.com/category/subcategory/date/12345678901234b31af3215702298964_100x100.jpg';

I need to replace characters between _ and .jpg and put example instead.

You could do:

$string = preg_replace('/_(.*)\.jpg/','_example.jpg',$string);