html - 使用 Lua 转义数字 XML 实体

Question

什么是取消转义数字 HTML/XML 实体的好实现，例如 并将它们替换为等效的 ASCII？

表示为单元测试:

local orig = "It's the "end" &ok;
"
local fixd = unescape(orig) -- Implement this
assert( fixd == "It's the \"end\" &ok;\n" )

Answer 1

下面是一个简单的实现，它也处理核心的命名 XML 实体:

function unescape(str)
  str = string.gsub( str, '&lt;', '<' )
  str = string.gsub( str, '&gt;', '>' )
  str = string.gsub( str, '&quot;', '"' )
  str = string.gsub( str, '&apos;', "'" )
  str = string.gsub( str, '&#(%d+);', function(n) return string.char(n) end )
  str = string.gsub( str, '&#x(%d+);', function(n) return string.char(tonumber(n,16)) end )
  str = string.gsub( str, '&amp;', '&' ) -- Be sure to do this after all others
  return str
end

print(unescape("&#34;Hello&quot; &apos;World&#39;")) --> "Hello" 'World'

但是，请注意，对于一种病态情况，这会失败:数字和符号实体后跟文本 amp;:

print(unescape("Ampersand entity is &")) --> Ampersand entity is &
-- The result should actually be                     Ampersand entity is &

我们可以通过一次处理所有实体来修复这种边缘情况，但代码会变得有点丑陋:

function unescape(str)
  local map={ ["lt"]="<", ["gt"]=">", ["amp"]="&", ["quot"]='"', ["apos"]="'" }
  str = string.gsub( str, '(&(#?x?)([%d%a]+);)', function(orig,n,s)
    return (n=='' and map[s])
           or (n=="#x" and tonumber(s,16)) and string.char(tonumber(s,16))
           or (n=="#" and tonumber(s)) and string.char(s)
           or orig
  end )
  return str
end

print(unescape("Ampersand entity is &#38;amp;")) --> Ampersand entity is &amp;

最后，我们可以打开它以提高一点速度:

local gsub, char = string.gsub, string.char
local entityMap  = {["lt"]="<",["gt"]=">",["amp"]="&",["quot"]='"',["apos"]="'"}
local entitySwap = function(orig,n,s)
  return (n=='' and entityMap[s])
         or (n=="#" and tonumber(s)) and string.char(s)
         or (n=="#x" and tonumber(s,16)) and string.char(tonumber(s,16))
         or orig
end
function unescape(str)
  return (gsub( str, '(&(#?x?)([%d%a]+);)', entitySwap ))
end