RegEx: (?= ) and (?! )

From: Yutaka Emura yutaka_at_[hidden])
Date: 2002-12-24 00:51:08

Hi,
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.  

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

Can please let me know what is wrong? Thank you for your reply in 
advance.

Yutaka Emura

---------

boost::regex expression;
boost::match_results<std::string::const_iterator> subexp;

expression.assign("(?=a)b", boost::regbase::perl);
if (boost::regex_search((std::string::const_iterator)"xyzabcdefg", 
(std::string::const_iterator)"xyzabcdefg" + 10, subexp, expression, 
boost::match_default)) {
{
        // should go here
}
else {
        // goes here
}

expression.assign("(!=a)b", boost::regbase::perl);
if (boost::regex_search((std::string::const_iterator)"xyzabcdefg", 
(std::string::const_iterator)"xyzabcdefg" + 10, subexp, expression, 
boost::match_default)) {
{
        // goes here
}
else {
        // should go here
}