Trouble understanding IGOR open command, s_filename, and returning strings

Function ShortenFileName(long_filename)
    //This function takes a long_filename and shortens it to the yyyymmdd_## format
    //that is used for TNMR files
    //long_filename is obtained as part of the NMR STRUCT  
    String long_filename
    //short_filename is the new name, keeps wave names a manageable legnth
    //FileID is essentially the format of short_filename, and a traditional way of keeping track of experiments
    String short_filename, FileID="([[:digit:]]+_[[:digit:]]+)"
    //Looks for just that format in long_filename, dumps it into short_filename
    SplitString/E=(FileID) long_filename, short_filename
    //print for troubleshooting
    print short_filename
//  return short_filename
End

Function/S DoOpenFileDialog()
Variable refNum
String message = "Select a file"
String outputPath
String fileFilters = "Data Files (*.txt,*.dat,*.csv):.txt,.dat,.csv;"
fileFilters += "All Files:.*;"
Open /D /R /F=fileFilters /M=message refNum
outputPath = S_fileName
return outputPath // Will be empty if user canceled
End