如果您不知道日期输入的格式，如何格式化日期字符串？

I have to read through a ton of files based on their file name. They can vary from the format Y-M-D, YMD, M_D_Y, or Y_M_D. There could be others but at the moment this is all I am given to work with.I have to read through a ton of files based on