[boost] [regex] empty expression issue

30 May 2007

      Hi All,

I've just started to use boost::regex I met with the following
incompatibility with Perl regular expressions:

It is impossible to use empty expressions,
neither alone nor inside of another regex statement.

Sample code:

#include <iostream>
#include <boost/regex.hpp>

void check_regexp(const char * text)
{
    try 
    {
        boost::regex e("(\\d+)\\s*(kbit|)");
        boost::cmatch what;
        if (boost::regex_match(text, what, e))
        {
            std::string item1(what[1].first, what[1].second);
            std::string item2(what[2].first, what[2].second);
            std::cout 
                << "matched. " << std::endl
                << "Value : " << item1 << std::endl
                << "Units : " << item2 << std::endl
                << std::endl;
        }
    }
    catch (boost::regex_error & e)
    {
        std::cout << "ERROR: " << e.what() << std::endl;
    }
}

int main(int argc, char** argv)
{
    check_regexp("96 kbit");
}

Program output:

ERROR: Empty expression

Such regular expressions are quite legal in Perl (e.g. /(Ann|Bob|)/)

One possible (quick-n-dirty) workaround for this is to replace
  boost::regex e("(\\d+)\\s*(kbit|)");
with
  boost::regex e("(\\d+)\\s*(kbit|.{0})");

Program output would be:

matched.
Value : 96
Units : kbit

Would it be considered as an issue for further improvement?

Thank you.

-- 
 impulse9