Mapping in SAP PI

Hi,

     I am working on File to IDoc scenario. I am getting a string with 480 characters from the scource side.

I need to search for a value 'LN' in the string and populate the next character after LN into the POSEX field in IDoc. The index where LN appears always differ.

The next character of LN consists of the Item Number.

Can i achieve this using standard functions in sap PI?

If yes, how do i map this  else how do i achieve this using user defined function.

Regards,

Deepthi Premnath.