How to find a character in a string in VI

Hi there VI fans;

I have a field that contains an Invoice Number that I am trying to import into AP Invoice Header. Unfortunately the string also contains a lot of stuff that is not invoice numbers. For example, here are 3 of the values in that field:
INVOICE #8859 - SEWER SERVICES
INVOICE#149211 - 1933-04/XYZ
INVOICE #8659 - REPAIR SERVICES

I was thinking I could use POS("#"=TEMP001$) to find the position of the # sign and then from there use the next x number of characters until there is a space. However I can't even get the POS() function to work, so I am obviously not smart enough to understand how that works.

Anyone feel like they can shed some light on this? Version is 5.0 (testing on Standard, will implement on Premium).

Thanks