Transformation Routine ABAP logic

Hi,

I am load data from a flatfile to BW.One of the field in flat file is Char 30  and free text.

In BW Transformation based conditions on the Free Text I need to populate another field as I am not expert in ABAP what is the logic I should use in the routine

In source field = *INVOICE* or *Invoice* or *invoice* then target field should be ‘I’

Thanks