Flat file mapping problem.

we were doing Flat file to IDOC.

we were getting a field of length 12 from which we need to convert take first 10 values from left to right.

we used the sub string function till now...

functional team says...they might also send 5 digits number...so in that case...i need to take the 5 digits number along with the 5 spaces...how will i do that.