CREATE OR REPLACE FUNCTION is_valid_number(s) AS
    CASE
        WHEN s IS NULL THEN false
        WHEN try_cast(s AS DOUBLE) IS NOT NULL THEN true
        ELSE false
    END;

# Illustration
select s, is_valid_number(s) from
   (select unnest([
      '123.0',
      '123.',
       '-123',
       '3.14159',
       '1.2.3',
       'abc',
       'A440',
       ' 12 ',
       '0A',
       '+1E10',
       '1E-10'
       ]) as s);