Notice how we type "check" in this example:

F:\fb3dev\fbbuild\firebird30\temp\Win32\Debug\firebird\bin>isql
Use CONNECT or CREATE DATABASE to specify a database
SQL> create database 'surprise.fdb';
SQL> create domain d int chEck (value > 0);
SQL> ^Z

Now, let's extract metadata:

F:\fb3dev\fbbuild\firebird30\temp\Win32\Debug\firebird\bin>isql -x SURPRISE.FDB
Database: SURPRISE.FDB

SET SQL DIALECT 3;

/* CREATE DATABASE 'SURPRISE.FDB' PAGE_SIZE 4096 DEFAULT CHARACTER SET NONE */


/* Domain definitions */
CREATE DOMAIN D AS INTEGER
         ( chEck (value > 0));

If you submit this script to the server, the extra pair of parentheses is rejected in the syntax. This extra pair comes from isql assuming that the constraint is a non-SQL one (from very old times before IB4) that needs such artifact and in turn, isql naively expects that the user types "check" or "CHECK", but not mixed case.