解读正则表达式

Question

有人可以帮我理解这个用于匹配HTML中src标记的img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

简要说明?:是什么意思？

所以(?:...)是常规括号的非捕获版本。匹配括号内的正则表达式，但在执行匹配后或在模式中稍后引用时，无法检索组匹配的子字符串。

谢谢@mbratch

\ 1是什么意思？

最后，感叹号在这里有什么特别的意义吗？ （否定？）

Answer 1

这可以帮助您理解正则表达式。

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

Edit live on Debuggex

Answer 2

举个例子，考虑src="img.jpg"作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(['""])。部分(?:(['""])(?<src>(?:(?!\1).)*)是一个非捕获组，在我们的示例中与"img.jpg匹配。特别是，(['""])匹配任何引号字符。然后(?!\1)是第一组中匹配的引号字符的负前瞻，因此(?:(?!\1).)匹配任何不是第一组匹配的引号字符且(?<src>(?:(?!\1).)*)匹配的字符，命名捕获组，在结束引号字符之前的一系列字符。然后，以下\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)

进一步阅读：

• Grouping, Capturing and Backreferences
• Lookarounds

如果您想稍微刷新一下您的正则表达式知识，我建议您阅读整个教程。这绝对物有所值。

需要更多资源来帮助理解复杂的表达式：

• Regex 101从正则表达式生成解释。但是，它使用PHP的PCRE引擎，因此它会阻塞一些.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，它的正则表达风格更加有限（对于JavaScript的ECMAScript风格）
• Regexper侧重于流程图tihng。截至目前，它也仅限于JavaScript正则表达式。

Answer 4

1＆gt;它首先捕获组1中的['""]中的任何1个，即(['""])

2＆gt;然后它匹配0到多个字符，这不是第1组中捕获的字符，即(?:(?!\1).)*

3＆gt;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＆gt;它匹配所有非空格，＆gt; src=之后的字符，即[^\s>]+

---

注意 我会使用src=(['""]).*?\1

.*贪婪，尽可能匹配..

.*?是懒惰的，它尽可能地匹配..

例如，请考虑此字符串hello hi world

表示正则表达式^h.*l输出为hello hi worl

表示正则表达式^h.*?l输出为hel

Answer 5

我使用RegexBuddy获取此输出：

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 “>” «>»

这个正则表达式对你所描述的非常糟糕。 src="是有效的输入。