The regex matches a backslash (\\) followed by any number of non-backslash characters ([^\\]*) followed by end-of-string ($). This forces the matched backslash to be the last backslash in the string, so the matched non-backslash characters are the filename ("foo.exe"). The matched characters ("\foo.exe") are replaced by the empty string, which gives the directory containing the script file ("E:\programs\foo").

The OP accomplished this less efficiently by splitting the string on backslashes (("E:","programs","foo","foo.exe")), popping the last element of the array ("foo.exe", leaving ("E:","programs","foo")), and rejoining the array with backslashes ("E:\programs\foo").