仅当负向前瞻和后向查找均失败时才匹配

Question

我正在尝试创建一个正则表达式，当且仅当该单词未用单引号引起来时检查句子中是否存在单词。

我尝试过不同的正则表达式，例如：

(?<!' )(?i:THEN)|(?i:THEN)(?! ')

匹配 'then ' | 'then'，它不应该匹配。

(?<!' )(?i:THEN)(?! ') or (?<!(' ))(?i:THEN)|(?i:THEN)(?!( '))

匹配不应匹配的“then”

我真的很困惑，因为我不知道哪个正则表达式有效。我也尝试过其他正则表达式，但它无法匹配：

' then I jumped.
He said then 'Wow'.

一些意见将不胜感激！

谢谢你！

Answer 1

描述

then该正则表达式将匹配未用引号引起来的单词。

\bTHEN\b(?<!'\s*THEN(?=\s*'))

有些语言不允许交替，例如\s?或\s*内部回顾。因此，如果您使用其中一种语言，那么您需要深入了解空格测试。

\bTHEN\b(?<!'\s*THEN(?=\s*'))

例子

现场演示

https://regex101.com/r/gS4zU8/1

\bTHEN\b(?<!'\sTHEN(?=\s'))(?<!'THEN(?='))

解释

NODE                     EXPLANATION
----------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
----------------------------------------------------------------------
  THEN                     'THEN'
----------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
----------------------------------------------------------------------
  (?<!                     look behind to see if there is not:
----------------------------------------------------------------------
    '                        '\''
----------------------------------------------------------------------
    \s                       whitespace (\n, \r, \t, \f, and " ")
----------------------------------------------------------------------
    THEN                     'THEN'
----------------------------------------------------------------------
    (?=                      look ahead to see if there is:
----------------------------------------------------------------------
      \s                       whitespace (\n, \r, \t, \f, and " ")
----------------------------------------------------------------------
      '                        '\''
----------------------------------------------------------------------
    )                        end of look-ahead
----------------------------------------------------------------------
  )                        end of look-behind
----------------------------------------------------------------------
  (?<!                     look behind to see if there is not:
----------------------------------------------------------------------
    'THEN                    '\'THEN'
----------------------------------------------------------------------
    (?=                      look ahead to see if there is:
----------------------------------------------------------------------
      '                        '\''
----------------------------------------------------------------------
    )                        end of look-ahead
----------------------------------------------------------------------
  )                        end of look-behind
----------------------------------------------------------------------