APEX

I need to have an APEX page where my users can upload a fixed length text file (using the file browse item) so I can process the data inside of it (by starting/ending positions).

I know I can use the
select blob_content into v_blob_data from wwv_flow_files
where filename = p_file_name to read the file, but what is the best way to get to the data (by start/end position)?

Thanks