Re: [Boost-Users] RegEx: (?= ) and (?! )

From: John Maddock (john_maddock_at_[hidden])
Date: 2003-01-01 06:47:25

> I am using RegEx in Boost v1.29.0.
> (?= ) and (?! ) expressions do not seem to work at a certain
> condition.
>
> In the following example, "(?=a)b" should match "b" in "xyzabcdefg",
> but does not.

Which is correct: (?=a) is a forward lookahead *possitive* assertion, so
(?=a)b can never match anything since the next character can't be both an
'a' and a 'b' at the same time!

> Similarly, "(?!a)b" should not match "b" in "xyzabcdefg" but it does.

It should match: (?!a) will match anything except 'a' so (?!a)b will match
the next 'b' that it finds.

John Maddock
http://ourworld.compuserve.com/homepages/john_maddock/index.htm