Non-greedy regex quantifier gives greedy result

Question

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

Answer 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

Answer 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.