Insert a 'NULL' string in a table in PostgreSQL
'NULL' is not the same as NULL, the first one is an string just like any other string, so your constraint (NOT NULL) won't work.

I think you are working with a programming language (like PHP or Java) that you put your value inside a string with a variable like 

INSERT INTO originators(originator, id, regexp) 
VALUES ('$variable', $id, $regexp); 


so in this case you have to correct your code to prevent the quotes, or if you can't do that you constraint should be CHECK(upper(originator) != 'NULL').