Prefixes and identifiers SHOULD only consist of the alphabetic US-

ASCII characters A through Z in both uppercase and lowercase, the

numerical digits 0 through 9, and the underscore character, and they

SHOULD NOT begin with an underscore character, numerical digit, or

the characters "xml". The following describes the production of JSON

names in ABNF [RFC5234].

name = ALPHA *( ALPHA / DIGIT / "_" )

Figure 1: ABNF for JSON Names

This restriction is a union of the Ruby programming language

identifier syntax and the XML element name syntax and has two

purposes. First, client implementers using modern programming

languages such as Ruby or Java can use libraries that automatically

promote JSON names to first-order object attributes or members.

Second, a clean mapping between JSON and XML is easy to accomplish

using these rules.