描述：URL地址中的指定位置添加字符串
查找：(.*)/(.*)
替换：
\0
\1/b_\2

实例：
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http://www.example.com/html/photo/0/0/13/11722.html
http://www.example.com/html/photo/0/0/56079.html

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http://www.example.com/html/photo/0/0/13/11722.html
http://www.example.com/html/photo/0/0/13/b_11722.html
http://www.example.com/html/photo/0/0/56079.html
http://www.example.com/html/photo/0/0/b_56079.html

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