In a jsp form the user enter the report file URL i.e., https://s3.amazonaws.com/cdn.gs.com/live/reports/MVR_Q3_2009.pdf or cdn.gs.com/live/reports/MVR_Q3_2009.pdf
But when the form is submitted, a java class(struts2 action) has to checks whether its a valid url by taking the cdn.gs.com as bucket name and /reports/MVR_Q3_2009.pdf as file path and then searches in the AWS s3 bucket.
For now we are getting the file_path from the url, but the cdn.gs.com is entered as a static variable before checking into S3.
Ofcourse you can do that. But to know how, you need to get clear for yourself what rules exactly you want to apply to find that part of the string. For example you could split the string by the '/' character and find the right element in the resulting array of strings. Once you have clear for yourself what rules you'd need to apply it's easy to do it with methods from class String.