regex - 贪婪 vs. 不情愿 vs. 占有 限定词

Question

我找到了这个tutorial关于正则表达式，虽然我直观地理解“贪婪”、“不情愿”和“占有”限定符的作用，但我的理解似乎存在严重漏洞。

具体来说，在以下示例中:

Enter your regex: .*foo // Greedy qualifier
Enter input string to search: xfooxxxxxxfoo
I found the text "xfooxxxxxxfoo" starting at index 0 and ending at index 13.

Enter your regex: .*?foo // Reluctant qualifier
Enter input string to search: xfooxxxxxxfoo
I found the text "xfoo" starting at index 0 and ending at index 4.
I found the text "xxxxxxfoo" starting at index 4 and ending at index 13.

Enter your regex: .*+foo // Possessive qualifier
Enter input string to search: xfooxxxxxxfoo
No match found.

解释提到吃整个输入字符串，字母被消耗，匹配器后退，最右边出现的“foo”已经<强>反流等

不幸的是，尽管有很好的比喻，我仍然不明白谁吃什么......你知道另一个教程可以(简洁地)解释正则表达式引擎如何工作吗？

或者，如果有人可以用稍微不同的措辞解释以下段落，我们将不胜感激:

The first example uses the greedy quantifier .* to find "anything", zero or more times, followed by the letters "f", "o", "o". Because the quantifier is greedy, the .* portion of the expression first eats the entire input string. At this point, the overall expression cannot succeed, because the last three letters ("f", "o", "o") have already been consumed [by whom?]. So the matcher slowly backs off [from right-to-left?] one letter at a time until the rightmost occurrence of "foo" has been regurgitated [what does this mean?], at which point the match succeeds and the search ends.

The second example, however, is reluctant, so it starts by first consuming [by whom?] "nothing". Because "foo" doesn't appear at the beginning of the string, it's forced to swallow [who swallows?] the first letter (an "x"), which triggers the first match at 0 and 4. Our test harness continues the process until the input string is exhausted. It finds another match at 4 and 13.

The third example fails to find a match because the quantifier is possessive. In this case, the entire input string is consumed by .*+ [how?], leaving nothing left over to satisfy the "foo" at the end of the expression. Use a possessive quantifier for situations where you want to seize all of something without ever backing off [what does back off mean?]; it will outperform the equivalent greedy quantifier in cases where the match is not immediately found.

Answer 1

我会尝试一下。

贪婪量词首先尽可能多地匹配。因此 .* 匹配整个字符串。然后匹配器尝试匹配后面的 f，但没有剩余字符。因此它“回溯”，使贪婪量词少匹配一个字符(使字符串末尾的“o”不匹配)。这仍然与正则表达式中的 f 不匹配，因此它又回溯了一步，使贪婪量词再次少匹配一个字符(使字符串末尾的“oo”不匹配)。 still 与正则表达式中的 f 不匹配，因此它又回溯了一步(使字符串末尾的“foo”保持不匹配)。现在，匹配器最终匹配正则表达式中的 f，并且 o 和下一个 o 也匹配。成功!

不情愿或“非贪婪”量词首先匹配尽可能少的内容。因此 .* 首先不匹配任何内容，从而使整个字符串不匹配。然后匹配器尝试匹配后面的 f，但字符串中不匹配的部分以“x”开头，因此不起作用。因此，匹配器回溯，使非贪婪量词再匹配一个字符(现在它匹配“x”，使“fooxxxxxxfoo”不匹配)。然后它尝试匹配 f，结果成功，并且正则表达式中的 o 和下一个 o 也匹配。成功!

在您的示例中，它然后使用字符串的剩余不匹配部分“xxxxxxfoo”重新启动该过程，遵循相同的过程。

占有量词就像贪婪量词一样，但它不会回溯。因此它以 .* 开始匹配整个字符串，不留下任何不匹配的内容。那么它就没有任何东西可以与正则表达式中的 f 匹配。由于所有格量词不会回溯，因此匹配失败。