I've got some Source data with 3 fields:

...and a lookup file with 3 corresponding fields that I want to match to bring in the 2 TEST_FLAG fields:

From the two I can easily create a key. The challenge is that whereas in the Source data all 3 fields are populated with real values, in the lookup we have wildcards * that represent any value.
For example in the lookup TAX_KEY value 431_*_* should join to Record 2 in source i.e. value 431_12345_123 or indeed any record that starts 431_
In Looksups the first part of the key will always have a real value, but the second and final parts may either be wild or real.