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

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 _