単純なものを試す。選択

#!/usr/bin/env python

import ply.lex as lex

tokens = (
  'A', 'B', 'C',
  'D', 'E', 'F',
)

t_A = r'a'
t_B = r'b'
t_C = r'c'
t_D = r'd'
t_E = r'e'
t_F = r'f'
t_ignore = " \t"

def t_error(t):
    print "Illegal character '%s'" % t.value[0]
    t.lexer.skip(1)

lex.lex()


import ply.yacc as yacc

def p_input_1(p):
    'input : A B C'
    print "*abc*"

def p_input_2(p):
    'input : D E F'
    print "*def*"

def p_error(p):
    print "Syntax error at '%s'" % p.value

yacc.yacc()


while 1:
    try:
        s = raw_input('input > ')
    except EOFError:
        break
    if not s: continue
    yacc.parse(s)

で、

input > a b c
*abc*
input > d e f
*def*

ちょっと違う書きかた。

#!/usr/bin/env python

import ply.lex as lex

tokens = (
  'A', 'B', 'C',
  'D', 'E', 'F',
)

t_A = r'a'
t_B = r'b'
t_C = r'c'
t_D = r'd'
t_E = r'e'
t_F = r'f'
t_ignore = " \t"

def t_error(t):
    print "Illegal character '%s'" % t.value[0]
    t.lexer.skip(1)

lex.lex()


import ply.yacc as yacc

def p_input(p):
    '''input : A B C
             | D E F'''
    if p[1] == 'a':
        print "*abc*"
    else:
        print "*def*"

def p_error(p):
    print "Syntax error at '%s'" % p.value

yacc.yacc()


while 1:
    try:
        s = raw_input('input > ')
    except EOFError:
        break
    if not s: continue
    yacc.parse(s)

で、

input > a b c
*abc*
input > d e f
*def*

分岐の正しい仕方は、まだ不明