search for: xxx184_yyy_zzz

Hi, I need to select a substring from the filename of a file in a list. I can find all the filenames in a directory by list.files(fn_dir) Each filename has the format (the following is an example because field lengths are variable) XXX184_YYY_ZZZ.dat XXX: 2 or 3 alphabetic characters 184: 3 numeric characters ... I'd like to select the substring XXX which has variable length (from each filename). I saw regular expressions help but still I can't figure out how to do. Thanks, Arnaud Chozo [[alternative HTML version deleted]]