6.3 字面字符

如果愿意，可以在语法规则里面使用单个的字面字符，例如：

def p_binary_operators(p):
    """expression : expression "+" term
                  | expression "-" term
       term       : term "*" factor
                  | term "/" factor"""
    if p[2] == "+":
        p[0] = p[1] + p[3]
    elif p[2] == "-":
        p[0] = p[1] - p[3]
    elif p[2] == "*":
        p[0] = p[1] * p[3]
    elif p[2] == "/":
        p[0] = p[1] / p[3]

字符必须像’+’那样使用单引号。除此之外，需要将用到的字符定义单独定义在lex文件的literals列表里：

# Literals.  Should be placed in module given to lex()
literals = ["+","-","*","/" ]

字面的字符只能是单个字符。因此，像’<=’或者’==’都是不合法的，只能使用一般的词法规则（例如t_EQ = r’==’)。