How to Join two tables with the join being regexp_replace
You can not directly use the alias in ON clause or in WHERE condition

So simply use your expression in ON clause:

SELECT t.indicator,
       regexp_replace(t.indicator,'[^0-9]') as numbers
from temp t
Left join temp2 t2
    on (regexp_replace(t.indicator,'[^0-9]') = t2.&lt;Join column name&gt;);


Cheers!!