# Identifiers

Identifiers can refer to the values of message headers and properties, but not to the message body. Identifiers are case-sensitive.

Basic Syntax

An identifier is a sequence of letters and digits, of any length, that begins with a letter. As in Java, the set of letters includes `_` (underscore) and `$` (dollar).

Illegal

Certain names are exceptions, which cannot be used as identifiers. In particular, `NULL`, `TRUE`, `FALSE`, `NOT`, `AND`, `OR`, `BETWEEN`, `LIKE`, `IN`, `IS`, and `ESCAPE` are defined to have special meaning in message selector syntax.

Value

Identifiers refer either to message header names or property names. The type of an identifier in a message selector corresponds to the type of the header or property value. If an identifier refers to a header or property that does not exist in a message, its value is `NULL`.

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