解密正则表达式

Question

请有人帮我理解这个用于匹配 HTML 中 img 标记的 src 属性的正则表达式吗？

src=(?:(['""])(?<src>(?:(?!\1).)*)\1|(?<src>[^\s>]+))


src=                               this is easy
(?:(['""])(?<src>(?:(?!\1).)*)     ?: is unknown (['""]) matches either single or double quotes, followed by a named group "src" that matches unknown strings
\1                                 unknown
|                                  "or"
(?<src>[^\s>]+))                   named group "src" matches one or more of line start or whitespace

简而言之，?: 是什么意思？

所以(?:...) 是常规括号的非捕获版本。匹配括号内的任何正则表达式，但组匹配的子字符串在执行匹配后无法检索或稍后在模式中引用。

谢谢@mbratch

\1 是什么意思？

最后，感叹号在这里有什么特殊意义吗？ （否定？）

Answer 1

这可能有助于您理解正则表达式。

(?:(['""])((?:(?!\1).)*)\1|([^\s>]+))

Edit live on Debuggex

Answer 2

例如，将src="img.jpg" 视为我们正在解析的文本

在正则表达式中，\1 指的是第一个捕获组。在这种特殊情况下，第一个捕获组是(['""])。在我们的示例中，(?:(['""])(?<src>(?:(?!\1).)*) 部分是匹配 "img.jpg 的非捕获组。特别是，(['""]) 匹配任何引号字符。那么(?!\1) 是第一个组中匹配的引号字符的负前瞻，因此(?:(?!\1).) 匹配不是第一个组匹配的引号字符的任何字符，(?<src>(?:(?!\1).)*) 匹配命名捕获组中的一个序列结束引号字符之前的字符数。然后下面的\1 匹配结束引号字符。

Answer 3

src=      # matches literal "src="
(?:       # the ?: suppresses capturing. generally a good practice if capturing
          # is not explicitly necessary
  (['"])  # matches either ' or ", and captures what was matched in group 1
          # (because this is the first set of parentheses where capturing is not
          # suppressed)
  (?<src> # start another (named) capturing group with the name "src"
    (?:   # start non-capturing group
      (?!\1)
          # a negative lookahead, if its contents match, the lookahead causes the
          # pattern to fail
          # the \1 is a backreference and matches what was matched in capturing
          # group no. 1
    .)*   # match any character, end of non-capturing group, repeat
          # summary of this non-capturing group: for each character, check that
          # it is not the kind of quote we matched at the start. if it's not,
          # then consume it. repeat as long as possible.

  )       # end of capturing group "src"
  \1      # again a backreference to what was matched inside capturing group 1
          # i.e. match the same kind of quote that started the attribute value
|         # or
  (?<src> # again a capturing group with the name "src"
    [^\s>]+
          # match as many non-space, non-> character as possible (at least one)
  )       # end of capturing group. this case treats unquoted attribute values.
)         # end of non-capturing group (which was used to group the alternation)

为您进一步阅读：

• Grouping, Capturing and Backreferences
• Lookarounds

如果您想稍微更新一下您的正则表达式知识，我建议您通读整个教程。绝对值得您花时间。

更多资源可帮助您理解复杂的表达式：

• Regex 101 从正则表达式生成解释。但是，它使用 PHP 的 PCRE 引擎，因此它会阻塞某些 .NET 功能，例如重复命名的捕获组（在您的情况下为 src）。
• Debuggex 可让您逐步执行正则表达式并生成流程图。到目前为止，它的正则表达式风格更加有限（对于 JavaScript 的 ECMAScript 风格）
• Regexper 专注于流程图。不过，到目前为止，它还仅限于 JavaScript 正则表达式。

Answer 4

我使用 RegexBuddy 得到这个输出：

Match the characters “src=” literally «src=»
Match the regular expression below «(?:(['""])(?<src>(?:(?!\1).)*)\1|(?<src>[^\s>]+))»
   Match either the regular expression below (attempting the next alternative only if this one fails) «(['""])(?<src>(?:(?!\1).)*)\1»
      Match the regular expression below and capture its match into backreference number 1 «(['""])»
         Match a single character present in the list “'"” «['""]»
      Match the regular expression below and capture its match into backreference with name “src” «(?<src>(?:(?!\1).)*)»
         Match the regular expression below «(?:(?!\1).)*»
            Between zero and unlimited times, as many times as possible, giving back as needed (greedy) «*»
            Assert that it is impossible to match the regex below starting at this position (negative lookahead) «(?!\1)»
               Match the same text as most recently matched by capturing group number 1 «\1»
            Match any single character that is not a line break character «.»
      Match the same text as most recently matched by capturing group number 1 «\1»
   Or match regular expression number 2 below (the entire group fails if this one fails to match) «(?<src>[^\s>]+)»
      Match the regular expression below and capture its match into backreference with name “src” «(?<src>[^\s>]+)»
         Match a single character NOT present in the list below «[^\s>]+»
            Between one and unlimited times, as many times as possible, giving back as needed (greedy) «+»
            A whitespace character (spaces, tabs, line breaks, etc.) «\s»
            The character “>” «>»

这个正则表达式对于你所描述的非常糟糕。 src=" 是一个有效的输入。

Answer 5

1>它首先捕获组 1 中的任意一个 ['""]，即 (['""])

2>然后它将 0 匹配到多个不是第 1 组中捕获的字符，即(?:(?!\1).)*

3>它执行第 2 步，直到它与第 1 组中捕获的匹配，即\1

以上3个步骤与(['""])[^\1]*\1类似

或

1>匹配所有非空格，>src= 之后的字符，即[^\s>]+

---

注意 我会使用src=(['""]).*?\1

.* 是贪心的，它尽可能匹配..

.*? 是懒惰的，它匹配的越少越好..

例如，考虑这个字符串hello hi world

对于正则表达式^h.*l，输出将是hello hi worl

对于正则表达式^h.*?l，输出将是hel