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From
Java Language Specification, 3.8 Identifiers...
An identifier is an unlimited-length sequence of Java letters and Java digits, the first of which must be a Java letter...
The Java letters include uppercase and lowercase ASCII Latin letters A-Z (\u0041-\u005a), and a-z (\u0061-\u007a), and, for historical reasons, the ASCII underscore (_, or \u005f) and dollar sign ($, or \u0024). The $ character should be used only in mechanically generated source code or, rarely, to access preexisting names on legacy systems.