# `sep` can be a single character or a string but it will be interpreted
# as part of a regexp within square brackets, so be careful if characters
# such as `-` are used.
create or replace function remove_comment(s, sep) as (
  regexp_replace(s, '\s*[' || sep || '].*', '')
);

# Examples

select s,
       remove_comment(s, '#') as decommented,
       regexp_replace(decommented, ' ', '_','g') as visible
from unnest(
   ['apples ; pears # and bananas',
    'apples # pears ; and bananas']
  ) _s(s);