An identifier is a sequence of allowed characters

An identifier is a character sequence composed of a combination of the following characters:

The first character may not be a digit. Identifiers are case sensitive. An identifier cannot have the same spelling as a keyword. For example, the word action is a keyword and cannot be used as an identifier. See Lexical Elements for a list of the EPL keywords.

The length of an identifier is limited by available memory. In practice, this means you can make them as long as you want, but very long identifiers are hard to type and harder to read.