Compilation of boost regex v4 (from the CVS) fails with the following error under VC6: f:\projects\cpptool\cppparser_dependencies\cppparser\include\boost\regex\v4\ basic_regex.hpp(221) : error C2143: syntax error : missing ',' before '<' f:\projects\cpptool\cppparser_dependencies\cppparser\include\boost\regex\v4\ basic_regex.hpp(298) : see reference to class template instantiation 'boost::reg_expression<charT,traits,Allocator>' being compiled Looking at the sources (inside template reg_expression definition): bool BOOST_REGEX_CALL operator==(const reg_expression& e)const { return compare(e) == 0; } bool operator != (const basic_regex<charT, traits, Allocator>& e) // <<< *** error here *** { return compare(e) != 0; } basic_regex is defined later in the source, hence the compilation error. Also, the fact that operator == & != have different prototype is very suspicious... Using reg_expression instead or basic_regex would fix the error. Baptiste.