正则表达式‘(?< =#)[^#] +(?=#)’如何工作? [英] How does the regular expression ‘(?<=#)[^#]+(?=#)’ work?

问题描述

我在C#程序中具有以下正则表达式，但很难理解它:

I have the following regex in a C# program, and have difficulties understanding it:

(?<=#)[^#]+(?=#)

我将其分解为我认为的理解:

I'll break it down to what I think I understood:

(?<=#)    a group, matching a hash. what's `?<=`?
[^#]+     one or more non-hashes (used to achieve non-greediness)
(?=#)     another group, matching a hash. what's the `?=`?

所以我遇到的问题是?<=和?<部分.通过阅读MSDN，?<name>用于命名组，但是在这种情况下，尖括号永远不会关闭.

So the problem I have is the ?<= and ?< part. From reading MSDN, ?<name> is used for naming groups, but in this case the angle bracket is never closed.

我在文档中找不到?=，搜索起来真的很困难，因为搜索引擎通常会忽略那些特殊字符.

I couldn't find ?= in the docs, and searching for it is really difficult, because search engines will mostly ignore those special chars.

推荐答案

它们称为环顾四周；它们允许您断言某个模式是否匹配，而无需实际进行匹配.有4种基本的解决方法:

They are called lookarounds; they allow you to assert if a pattern matches or not, without actually making the match. There are 4 basic lookarounds:

• 积极的解决方法:看看我们是否可以匹配pattern ...
  • (?=pattern)-...到当前位置的右(向前看 )
  • (?<=pattern)-...到当前位置的左(在后面)
  • Positive lookarounds: see if we CAN match the pattern...
    • (?=pattern) - ... to the right of current position (look ahead)
    • (?<=pattern) - ... to the left of current position (look behind)
    • (?!pattern)-...到右
    • (?<!pattern)-...到左
    • (?!pattern) - ... to the right
    • (?<!pattern) - ... to the left

    为方便起见，请环顾四周:

    As an easy reminder, for a lookaround:

    • =是阳性，!是阴性
    • <看起来在后面，否则看起来在前面
    • = is positive, ! is negative
    • < is look behind, otherwise it's look ahead
    • regular-expressions.info/环顾

    有人可能会争辩说不需要在上述模式中进行四处查找，并且#([^#]+)#可以很好地完成工作(提取\1捕获的字符串以获取非#的字符串).

    One might argue that lookarounds in the pattern above aren't necessary, and #([^#]+)# will do the job just fine (extracting the string captured by \1 to get the non-#).

    不完全是.区别在于，由于环顾四周与# 不匹配，因此下次尝试查找匹配项时，它可以再次使用".简单地说，环顾四周允许匹配项"重叠.

    Not quite. The difference is that since a lookaround doesn't match the #, it can be "used" again by the next attempt to find a match. Simplistically speaking, lookarounds allow "matches" to overlap.

    考虑以下输入字符串:

    and #one# and #two# and #three#four#
    

    现在，#([a-z]+)#将给出以下匹配项(如在rubular.com上看到的那样 ) :

    Now, #([a-z]+)# will give the following matches (as seen on rubular.com):

    and #one# and #two# and #three#four#
        \___/     \___/     \_____/
    

    将此与(?<=#)[a-z]+(?=#)进行比较，该匹配将匹配:

    Compare this with (?<=#)[a-z]+(?=#), which matches:

    and #one# and #two# and #three#four#
         \_/       \_/       \___/ \__/
    

    不幸的是，这不能在rubular.com上得到证明，因为它不支持向后看.但是，它确实支持前瞻性，因此我们可以使用#([a-z]+)(?=#)做类似的事情，它匹配(在rubular上看到的.com ):

    Unfortunately this can't be demonstrated on rubular.com, since it doesn't support lookbehind. However, it does support lookahead, so we can do something similar with #([a-z]+)(?=#), which matches (as seen on rubular.com):

    and #one# and #two# and #three#four#
        \__/      \__/      \____/\___/
    

    参考文献

    • regular-expressions.info/Flavor比较

    References

    • regular-expressions.info/Flavor Comparison

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