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

From: Yutaka Emura (yutaka_at_[hidden])
Date: 2003-01-02 11:47:14

Hi John,
It was my mistake.
Thank you for clarification!

--------------
Yutaka Emura
Emurasoft, Inc.
http://www.emurasoft.com/
E-mail: yutaka_at_[hidden]

-----Original Message-----
From: John Maddock [mailto:john_maddock_at_[hidden]] 
Sent: Wednesday, January 01, 2003 3:47 AM
To: Boost-Users_at_[hidden]
Subject: Re: [Boost-Users] RegEx: (?= ) and (?! )

> 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

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