Data Integrity Using Constraints

Hello,

I am fairly new to SQL, so I have a few questions that may sound odd. First of all, I am trying to restrict users to putting in a three character code as an id item, or their team abbreviation. Here is the table definition:

CREATE TABLE TEAMS{
city varchar(20) NOT NULL,
name varchar(20) NOT NULL,
id varchar(3) NOT NULL};

Here is my code for adding the constraint

ALTER TABLE TEAMS
ADD CONSTRAINT Chk_id CHECK (id = `[A-Z][A-Z][A-Z]`);

Here is the error that I get when trying to execute this statement:

Msg 547, Level 16, State 0, Line 1
ALTER TABLE statement conflicted with COLUMN CHECK constraint 'Chk_id'. The conflict occurred in database 'statbookdb', table 'cabateams', column 'id'.

I am using SQL 2005 EM if this makes any difference.

My other question is in regards to once the constraints have been put in. Is there a way to make SQL throw a message out when a user violates a constraint? Right now, I have a numerical constraint in and whenever I violate it, all that happens is a "This page cannot be displayed" error. It doesn't make sense if you can only do this on the front end, as I don't see the point in enforcing it on the backend if there is no way to notify the user.

Thank you all in advance!