コンパイラつく〜ろ〜♪　３番BNF記法 - pokotyamuさんのメモ帳 http://pokotyamu.hatenablog.com/entry/2015/05/15/005033

my_numeric.y

# coding: utf-8
# 
# <数値> ::= <数字列> | <符号><数字列> | <数字列>.<数字列> | <符号><数字列>.<数字列>
# <数字列> ::=  <数字> | <数字列> <数字>
# <数字> ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 
# <符号> ::= + | - | ± # ±は省略

class MyNumeric
rule
  target       : numeric_num { result = 'ok' }
  numeric_num  : digit_string
               | SIGN digit_string
               | digit_string '.' digit_string
               | SIGN digit_string '.' digit_string # 追加する必要があるのでは？
  digit_string : NUMBER 
               | digit_string NUMBER
end

---- header
require 'strscan'
---- inner
def parse(str)
  scanner = StringScanner.new(str)
  @q = []
  until scanner.eos?
    scanner.scan(/\A\s+/)  ? nil :
    scanner.scan(/\A\d/)   ? @q << [:NUMBER, scanner.matched] : 
    scanner.scan(/\A[+-]/) ? @q << [:SIGN, scanner.matched] :
    scanner.scan(/\A\./)   ? @q << [scanner.matched, scanner.matched] :
    nil
  end
  pp @q
  do_parse
end

def next_token
  @q.shift
end

$ racc -g -o numeric_parser.rb my_numeric.y; ruby numeric_parser.rb

123
[[:NUMBER, "1"], [:NUMBER, "2"], [:NUMBER, "3"]]
ok

+123
[[:SIGN, "+"], [:NUMBER, "1"], [:NUMBER, "2"], [:NUMBER, "3"]]
ok

-123
[[:SIGN, "-"], [:NUMBER, "1"], [:NUMBER, "2"], [:NUMBER, "3"]]
ok

1.23
[[:NUMBER, "1"], [".", "."], [:NUMBER, "2"], [:NUMBER, "3"]]
ok

-1.23
[[:SIGN, "-"], [:NUMBER, "1"], [".", "."], [:NUMBER, "2"], [:NUMBER, "3"]]

parse error on value "." (".")