This is incorrect:
l_t_range-sign = 'I'. * is integer
The I indicates that the range is inclusive. If it was E then it would be everything not between low and high.
Have you read the ABAP help on:
RANGES
String operations?
If you are going to be writing routines etc. in ABAP I strongly suggest you buy a book on ABAP programming and work through it (or get your employer to send you on a course). You can hardly be effective if you have to be given guidance on very basic ABAP operations.