Chameleon has a matching rule for every message definition defined in the VMD file. It relies on checking the values of the fields within the segments of the message. These rules are defined through each message definition's Identity Window. Each matching rule is applied to a segment, a field and a valid regular expression.

Chameleon follows a simple yet powerful algorithm to match each incoming message:

• The incoming flat wire message is parsed into a logical segment tree;
• The engine then sequentially checks the messages in the Message Definition Matching Order window to see if the incoming message matches;
• The incoming message is assigned to the first matching message in the Message Definition Matching Order window; and
• If no message matches a given message definition, the specified default message in the Message Definition Matching Order window is invoked if the Make last message definition match any message option is checked. If this option is not checked, an error will be given. For more information, see Specifying Default Messages.

A message that is not recognized by your application should not be ignored. The default message allows you to make your HL7 interfaces gracefully handle unexpected messages. Your interface can ACKnowledge the unexpected message, then log it for later investigation by an administrator. For more information, see Auto ACKnowledgments.

	A common mistake with message identities is to have one configured to use a zero character, '0', when in fact the letter 'O' is being used.