我试图找出一个使用 (?(ID/NAME).....) 的正则表达式

Question

>>> p = re.compile(r'(((x)|y)(?(2)y|x))+')
>>> p.search('yxxyyx')
<_sre.SRE_Match object; span=(2, 4), match='xy'>

My questions: why it matches only 'xy'? and does \\2 always refer to "x" within the group (.......)+??

edited: Thank you for all your answers! I found a helpful website to make regex visualize.

Answer 1

Questions:

1. Why does it matches only 'xy'?
2. Does \\2 always refer to "x" within the group (.......)+??

Answer Question 2 first:

In this regex, you are using:

(?(id/name)yes-pattern|no-pattern)

Will try to match with yes-pattern if the group with given id or name exists, and with no-pattern if it doesn't. no-pattern is optional and can be omitted. For example, (<)?(\\w+@\\w+(?:.\\w+)+)(?(1)>) is a poor email matching pattern, which will match with '' as well as 'user@host.com', but not with '

In the regex given ((x)|y) , there is only one group before the question (?(2)y|x))+ that asks whether the second group has matched. Since there is only one group the conditional will always be false, and thus will always try to match x

Answer Question 1:

Yes, since there is only one group, the conditional will always match x , and the entire regex only match xyx

( DOCS )

Answer 2

我发现这个网站是学习和测试不同正则表达式模式的好资源。