检测格式错误的 Accept-Language HTTP 标头

Question

我想解析 Accept-Language 标头。我找到的所有答案都涉及解析字符串，但不处理输入格式错误的情况。

例如，如果用户发送此标头Accept-Language: en,es;q=0.5;*;q=0.5，由于第二个; 而格式错误，该怎么办。是否有任何包可以提供简单的解析和适当的异常引发？

Answer 1

首先您应该了解Accept-Language 标头的正确格式：https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.4

可以看到Accept-Language头域的定义是：

Accept-Language = "Accept-Language" ":"
                  1#( language-range [ ";" "q" "=" qvalue ] )
       language-range  = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )

格式正确的标题示例是：Accept-Language: da, en-gb;q=0.8, en;q=0.7。可以看到每个逗号,分隔语言元组，其中每个语言元组是language-range和quality weight（可选）。

既然您知道Accept-Language 标头是如何定义的，那么唯一的问题就是如何解析它。

你可以根据语言以多种方式实现这一点，但我会写一个伪代码：

function parseAcceptLanguageHeader(headerValue):
    parsedLanguages = []
    languageStrings = headerValue.split(",")
    foreach languageStrings as S do
        parsedLanguages.add(parse(S))
    return parsedLanguages


// Here we define parse(S)
function parse(S): // expecting format of S to be like: 'language-range [";q=<number>"]'
    vals = S.trim().split(";") // remove leading and trailing spaces and split by ;
    if vals.length == 1: // means 'q=qvalue' part is missing
      return vals[0].trim(), 1.0 // default q is 1.0; you can additionally verify that vals[0] is one of the languages that you support
    else if vals.length == 2:
      return vals[0].trim(), parseQuality(vals[1])
    else raise an error ("Expected two tokens but, got: " + S)


// Implement parse quality
function parseQuality(S):
  // We expect to see 'q=<number>'
  vals = q.split("=")
  if (vals.length != 2):
    raise an error ("Expected exactly two tokens for quality, but got: " + S)
  else if (vals[0] != 'q'):
    raise an error ("Expected quality (q) but got: " + S)
  else 
    return parseInt(vals[1].trim()) // This can also throw an error, but I am not going to write implementation for that function

注意，根据不同的语言处理错误是不同的。