Function validate email SQL informix

I need to do a function in an older version of informix that I need to validate a mail:

so I need a function that check the email pattern [a-zA-z0-9]@[a-zA-Z0-9].[a-zA-Z0-9] Also in the last one that also allow to put like (.net.uk) but that is not finished by '.' and not be '..' Also that don't use special characters like ',' or 'Ü'

The main problem is that I need to do it in a older version of informix so a lot of elements like REGEXP are not compatible

How can I solve this problem?

I try this:

CREATE FUNCTION is_valid_email(email_param VARCHAR(255))
  RETURNING INTEGER;

  DEFINE is_valid INTEGER;

  LET is_valid = 0;

  IF LENGTH(email_param) > 7 
    AND email_param LIKE '%_@_%._%' 
    AND email_param NOT LIKE '%__@__%.__%' 
    AND email_param NOT LIKE '%___@___%.___%'
    AND email_param NOT LIKE '%@%_'
    AND email_param NOT LIKE '%.@%'
    AND email_param NOT LIKE '%..%'
  THEN
    LET is_valid = 1;
  END IF;

  RETURN is_valid;
END FUNCTION;

BUT Is not working