Just encountered a regex which I have never seen before because I'd have never thought to try it:
((hello)\2)
I would have assumed this is an error. The back reference is referencing the group that it's actually in. This seems valid though and will match:
hellohello
Can someone explain the logic please? I presume this falls under the category of a nested back reference (or maybe even a forward reference) but the logic employed is unclear to me.
This case seems similar to "Nested References" here http://www.regular-expressions.info/backref2.html but it's not the same as far as I can see.
FYI I'm already 99.9% sure I'd never use this in a production environment. I don't think I'd wish that upon any colleague!
答案 0 :(得分：2)
((hello)\2)
^====1st group
^===2nd group
2nd群组包含hello，\2与之匹配。因此True获得hellohello。