String operations--conversion form Alphanumeric and Numeric

i have a matnr which is less than 18 chars

Now if its numeric I have to put leading 0s and if its alphanumeric i have to put trailing spaces and make it 18 chars

The FM CONVERSION_EXIT_ALPHA_INPUT works for only 8 characters

What can be done in this reagrd

I need a exactly same FM but with support for char 18