替换大小写

类似：
http://img.qq.com/news/2010-9/201091922363125079_C.jpg
替换为
http://img.qq.com/news/2010-9/201091922363125079_c.jpg
http://img.qq.com/news/2010-9/201091922361111071_C.jpg
替换为
http://img.qq.com/news/2010-9/201091922361111071_c.jpg
http://img.qq.com/news/2010-9/201091922293193192_A.jpg
替换为
http://img.qq.com/news/2010-9/201091922293193192_a.jpg
http://img.qq.com/news/2010-9/201091922293103194_A.jpg
替换为
http://img.qq.com/news/2010-9/201091922293103194_a.jpg
http://img.qq.com/news/2010-9/201091919175936284_B.jpg
替换为
http://img.qq.com/news/2010-9/201091919175936284_b.jpg
http://img.qq.com/news/2010-9/201091923175936285_B.jpg
替换为
http://img.qq.com/news/2010-9/201091923175936285_b.jpg

本文转自 liang3391 51CTO博客，原文链接:http://blog.51cto.com/liang3391/623584