仅当没有第二组（包括变体）时才提取第一组+第二组或第一组的正则表达式

Question

最好的解释方式是确切显示我要实现的目标：

• 案例1： "search for fenway park in boston"
  摘录：第1组-> "fenway park" ，第2组-> "boston"

• 案例2： "search for fenway park"
  摘录：第1组-> "fenway park"

请注意，在两种情况下，我都希望能够满足"search" （ "look for" ， "find"等）和"in" （ "at" ， "around"等）的变化。 ..）。

我尝试了许多不同的变体，但要么以在第1组中提取"fenway park in boston"为最终结果，但在第2组中没有任何结果，或者如果我对情况1正确，则情况2将不起作用。

Answer 1

这应该为你工作

^(?:search for|look for|find)\s*(.*?)(?:\s*(?:in|around|at)\s*(.*))?$

您可以通过将moer 或子句添加到非捕获组来添加诸如look for/in/at的更多子句。

说明：

@"
^                   # Assert position at the beginning of a line (at beginning of the string or after a line break character)
(?:                 # Match the regular expression below
                       # Match either the regular expression below (attempting the next alternative only if this one fails)
      search\ for         # Match the characters “search for” literally
   |                   # Or match regular expression number 2 below (attempting the next alternative only if this one fails)
      look\ for           # Match the characters “look for” literally
   |                   # Or match regular expression number 3 below (the entire group fails if this one fails to match)
      find                # Match the characters “find” literally
)
\s                  # Match a single character that is a “whitespace character” (spaces, tabs, and line breaks)
   *                   # Between zero and unlimited times, as many times as possible, giving back as needed (greedy)
(                   # Match the regular expression below and capture its match into backreference number 1
   .                   # Match any single character that is not a line break character
      *?                  # Between zero and unlimited times, as few times as possible, expanding as needed (lazy)
)
(?:                 # Match the regular expression below
   \s                  # Match a single character that is a “whitespace character” (spaces, tabs, and line breaks)
      *                   # Between zero and unlimited times, as many times as possible, giving back as needed (greedy)
   (?:                 # Match the regular expression below
                          # Match either the regular expression below (attempting the next alternative only if this one fails)
         in                  # Match the characters “in” literally
      |                   # Or match regular expression number 2 below (attempting the next alternative only if this one fails)
         around              # Match the characters “around” literally
      |                   # Or match regular expression number 3 below (the entire group fails if this one fails to match)
         at                  # Match the characters “at” literally
   )
   \s                  # Match a single character that is a “whitespace character” (spaces, tabs, and line breaks)
      *                   # Between zero and unlimited times, as many times as possible, giving back as needed (greedy)
   (                   # Match the regular expression below and capture its match into backreference number 2
      .                   # Match any single character that is not a line break character
         *                   # Between zero and unlimited times, as many times as possible, giving back as needed (greedy)
   )
)?                  # Between zero and one times, as many times as possible, giving back as needed (greedy)
$                   # Assert position at the end of a line (at the end of the string or before a line break character)
"