ruby 正则表达式不一致

Question

我正在尝试使用带反向引用的 gsub 将其转换为罗马数字，但我发现了一个奇怪的不一致之处。

$ ruby -v 
ruby 2.2.5p319 (2016-04-26 revision 54774) [x86_64-linux]

$ irb
irb(main):001:0>   BASES = {
irb(main):002:1*     1000 => 'M',
irb(main):003:1*     500 => 'D',
irb(main):004:1*     100 => 'C',
irb(main):005:1*     50 => 'L',
irb(main):006:1*     10 => 'X',
irb(main):007:1*     5 => 'V',
irb(main):008:1*     1 => 'I'
irb(main):009:1>   }
=> {1000=>"M", 500=>"D", 100=>"C", 50=>"L", 10=>"X", 5=>"V", 1=>"I"}
irb(main):010:0>   BASE_KEYS = BASES.keys
=> [1000, 500, 100, 50, 10, 5, 1]
irb(main):011:0> rom = 'IIII'
=> "IIII"

上面是设置

下面我试图识别任何重复 4 次的字符，并将其替换为该字符的 1 个和下一个 BASE 字符。

例如:III => IV

irb(main):012:0>     rom.gsub(/((.)\2{3})/,
irb(main):013:1*       "#{
irb(main):014:0>         BASES[BASE_KEYS.select.with_index{ |bk, i|
irb(main):015:2>           BASES[BASE_KEYS[i]] == $2
irb(main):016:2>         }.first]
irb(main):017:0>        }
irb(main):018:1"        #{BASE_KEYS.select.with_index{ |bk, i|
irb(main):019:1>           BASES[BASE_KEYS[i]] == $2
irb(main):020:1>         }.first}
irb(main):021:1"        #{
irb(main):022:0>         BASES[BASE_KEYS.select.with_index{|bk, i|
irb(main):023:2>           BASES[BASE_KEYS[i+1]] == $2
irb(main):024:2>         }.first]
irb(main):025:0>        }
irb(main):026:1"        #{BASE_KEYS.select.with_index{ |bk, i|
irb(main):027:1>           BASES[BASE_KEYS[i+1]] == $2
irb(main):028:1>         }.first}
irb(main):029:1"       "
irb(main):030:1>     )
=> "\n       \n       I\n       1\n      "

所以我得到了错误的答案..(带有调试信息以获得更多见解)

irb(main):031:0>     rom.gsub(/((.)\2{3})/,
irb(main):032:1*       "#{
irb(main):033:0>         BASES[BASE_KEYS.select.with_index{ |bk, i|
irb(main):034:2>           BASES[BASE_KEYS[i]] == $2
irb(main):035:2>         }.first]
irb(main):036:0>        }
irb(main):037:1"        #{BASE_KEYS.select.with_index{ |bk, i|
irb(main):038:1>           BASES[BASE_KEYS[i]] == $2
irb(main):039:1>         }.first}
irb(main):040:1"        #{
irb(main):041:0>         BASES[BASE_KEYS.select.with_index{|bk, i|
irb(main):042:2>           BASES[BASE_KEYS[i+1]] == $2
irb(main):043:2>         }.first]
irb(main):044:0>        }
irb(main):045:1"        #{BASE_KEYS.select.with_index{ |bk, i|
irb(main):046:1>           BASES[BASE_KEYS[i+1]] == $2
irb(main):047:1>         }.first}
irb(main):048:1"       "
irb(main):049:1>     )
=> "I\n       1\n       V\n       5\n      "
irb(main):050:0> 

诚然，我的正则表达式代码很难理解，但为什么我在第二次调用相同代码时得到不同的结果？

irb(main):050:0> rom
=> "IIII"

注意 rom 没有改变...

Answer 1

您的代码在计算正则表达式之前使用了 $2。第一次运行它后 $2 被设置并且代码按预期工作。考虑使用 block 而不是字符串，因为您的字符串是在匹配发生之前插入的。

In the block form, the current match string is passed in as a parameter, and variables such as $1, $2, $`, $&, and $' will be set appropriately. The value returned by the block will be substituted for the match on each call.

这是一致的:

rom.gsub(/((.)\2{3})/) { |s|
  "#{
     BASES[BASE_KEYS.select.with_index{ |bk, i|
       BASES[BASE_KEYS[i]] == $2
     }.first]
    }
    #{BASE_KEYS.select.with_index{ |bk, i|
       BASES[BASE_KEYS[i]] == $2
     }.first}
    #{
     BASES[BASE_KEYS.select.with_index{|bk, i|
       BASES[BASE_KEYS[i+1]] == $2
     }.first]
    }
    #{BASE_KEYS.select.with_index{ |bk, i|
       BASES[BASE_KEYS[i+1]] == $2
     }.first}
   "
}
# => "I\n    1\n    V\n    5\n   "