regex - Regular expression: why a?? pattern matches empty string "" from any string including "a" itself?

Question

From this tutorial I learned about "Regular Expressions - Quantifiers", and based on this test code used in this tutorial.

Enter your regex: a??
Enter input string to search: a
I found the text "" starting at index 0 and ending at index 0.
I found the text "" starting at index 1 and ending at index 1.

And

Enter your regex: a??
Enter input string to search: aaa
I found the text "" starting at index 0 and ending at index 0.
I found the text "" starting at index 1 and ending at index 1.
I found the text "" starting at index 2 and ending at index 2.
I found the text "" starting at index 3 and ending at index 3.

Also

Enter your regex: a??
Enter input string to search: cab
I found the text "" starting at index 0 and ending at index 0.
I found the text "" starting at index 1 and ending at index 1.
I found the text "" starting at index 2 and ending at index 2.
I found the text "" starting at index 3 and ending at index 3.

Why?

Answer 1

??是一个量词，所以它表示量化项应该匹配多少次，0 或 1 次，最好是 0。??它本身不匹配任何东西，它只是装饰另一个表达式，说明该表达式在测试字符串中要匹配多少次。

如果表达式的其余部分与惰性 0 times部分匹配不匹配，它将再次尝试1 time部分匹配。

确实，如果整个正则表达式仅包含某个术语的惰性可选匹配，它将始终匹配测试字符串中的空位置。所以这种量词只有在它周围有其他术语时才有用。例如，表达式ba??d将首先尝试匹配bd，然后bad.

不过，为什么正则表达式匹配字符串中的每个字符一次（最后加一个）？好吧，一个空匹配是一个有效的正则表达式。例如，搜索^or $（字符串的开始，分别结束）将产生一个匹配，尽管它是空的。对于这个“无用”的表达式也是如此，测试字符串中的每个位置都是表达式的有效匹配，不会对匹配施加任何约束。

Answer 2

这是因为您正在使用?LAZY 操作符，?所以它会尽可能少地匹配。

您正在尝试匹配a0 或 1 次，但您告诉正则表达式引擎尽可能少地匹配，以便匹配 0 次并且匹配字符串中的尽可能多的字符 (+1)。