什么是正则表达式中的双加号？

这是一个所有格量词。

这基本上意味着，如果正则表达式引擎稍后无法匹配，它将不会返回并尝试撤消在此处进行的匹配。在大多数情况下，它允许引擎更快地失效，并且可以在您需要的地方为您提供一些控制 - 这对于大多数用途来说非常罕见。

举一个非常简单的例子：

假设您有一个字符串 。在以下示例中，匹配的字符在下面有一个。"123"^

1. 正则表达式：部分匹配！\d+?.

   123  # The \d+? eats only 1 because he's lazy (on a diet) and leaves the 2 to the .(dot).
   ^^   # This means \d+? eats as little as possible.
   

2. 正则表达式：完全匹配！\d+.

   123  # The \d+ eats 12 and leaves the 3 to the .(dot).
   ^^^  # This means \d+ is greedy but can still share some of his potential food to his neighbour friends.
   

3. 正则表达式：没有匹配！\d++.

   123  # The \d++ eats 123. He would even eat more if there were more numbers following. 
        # This means \d++ is possessive. There is nothing left over for the .(dot), so the pattern can't be matched.