The underscore character _ (U+005F) is considered a letter.
下划线字符(U+005F)被认为是一个字母
letter = unicode_letter | "_" . decimal_digit = "0" … "9" . octal_digit = "0" … "7" . hex_digit = "0" … "9" | "A" … "F" | "a" … "f" .