问题
I have a .net regex which I am testing using Windows Powershell. The output is as follows:
> [System.Text.RegularExpressions.Regex]::Match("aaa aaa bbb", "aaa.*?bbb")
Groups : {aaa aaa bbb}
Success : True
Captures : {aaa aaa bbb}
Index : 0
Length : 11
Value : aaa aaa bbb
My expectation was that using the ? quantifier would cause the match to be aaa bbb, as the second group of a's is sufficient to satisfy the expression. Is my understanding of non-greedy quantifiers flawed, or am I testing incorrectly?
Note: this is plainly not the same problem as Regular Expression nongreedy is greedy
回答1:
This is a common misunderstanding. Lazy quantifiers do not guarantee the shortest possible match. They only make sure that the current quantifier, from the current position, does not match more characters than needed for an overall match.
If you truly want to ensure the shortest possible match, you need to make that explicit. In this case, this means that instead of .*?, you want a subregex that matches anything that is neither aaa nor bbb. The resulting regex will therefore be
aaa(?:(?!aaa|bbb).)*bbb
回答2:
Compare the result for the string aaa aaa bbb bbb:
regex: aaa.*?bbb
result: aaa aaa bbb
regex: aaa.*bbb
result: aaa aaa bbb bbb
The regex engine finds first occurrence of aaa and then skips all characters (.*?) until first occurrence of bbb, but for the greedy operator (.*) it will go on to find a larger result and therefore match the last occurrence of bbb.
回答3:
This is not a greedy/lazy problem. The problem comes to the fact that your string is analysed from left to right. When the first aaa is matched, the regex engine add characters one by one to have the complete pattern.
Note that with a greedy behaviour, in your example, you obtain the same result: the first aaa is matched, the regex engine take all the last characters and backtrack character by character until having the complete match.
回答4:
Well it's really simple, we have the following string
aaa aaa bbb
Let's see we have this regex aaa.*?bbb. The regex engine will start with aaa
aaa aaa bbb
The regex engine has now .*?bbb. It will proceed with the space
aaa space aaa bbb
but we still have some characters until bbb ? So the regex engine will continue it's way and match the second set of a
aaa aaa space bbb
Finally the regex engine will match bbb:
aaa aaa bbb
So let's see, if we only want to match the second aaa we could use the following regex:
(?<!^)aaa.*?bbb, this means to match aaa that is not at the beginning of the sentence.
We may also use aaa(?= bbb).*?bbb, this means to match aaa that is followed by space bbb.
See it working 1 - 2.
Just came to my senses, but why don't you directly use aaa bbb ?
来源：https://stackoverflow.com/questions/16633315/non-greedy-regex-quantifier-gives-greedy-result