^Lexer:
IDENTIFIER_OR_KEYWORD :
      [a-z A-Z] [a-z A-Z 0-9 _]^*
   | _ [a-z A-Z 0-9 _]⁺

IDENTIFIER :
IDENTIFIER_OR_KEYWORD _{Except a strict or reserved keyword}

An identifier is any nonempty ASCII string of the following form:

Either

• The first character is a letter.
• The remaining characters are alphanumeric or _.

Or

• The first character is _.
• The identifier is more than one character. _ alone is not an identifier.
• The remaining characters are alphanumeric or _.