正则表达式中的双加号是什么？

Question

我在一些PHP脚本中看到了这一点：

[a-zA-Z0-9_]++

双加意味着什么？

Answer 1

那是Possessive Quantifier。

它基本上意味着如果正则表达式引擎稍后失败匹配，它将不会返回并尝试撤消它在此处所做的匹配。在大多数情况下，它允许引擎fail much faster，并且可以在您需要的地方为您提供控制 - 这在大多数情况下非常罕见。

Answer 2

给你一个非常简单的例子：

假设您有一个字符串"123"。在以下示例中，匹配的字符下面带有^。

1. 正则表达式：\d+?. 部分匹配！

   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：\d+. 完全匹配！

   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：\d++. 没有匹配项！

   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.