It's a poorly kept secret that I've been adding new features to Xpressive on CVS HEAD for a while now. So here are the features already implemented (documentation forthcoming) for Xpressive 2.0: << Semantic Actions >> Specify code to execute when parts of a regex match, a-la Spirit's semantic actions. Eg.: if you want to parse a string of name/value pairs into a std::map, you might: std::map<std::string, int> result; std::string str("aaa=>1 bbb=>23 ccc=>456"); // Like "(\\w+)=>(\\d+)": sregex pair = ( (s1= +_w) >> "=>" >> (s2= +_d) ) [ ref(result)[s1] = as<int>(s2) ]; sregex rx = pair >> *(+_s >> pair); if(regex_match(str, rx)) { assert(result["aaa"] == 1); assert(result["bbb"] == 23); assert(result["ccc"] == 456); } The actions are placed on a queue and executed in order only when the regex match succeeds. << Custom Assertions >> Use the check() function to create a boolean predicate that can participate in the match. Here's a regex that recognizes two integers only if the first is less than the second: sregex rx = ( (s1= +_d) >> ' ' >> (s2= +_d) ) [ check( as<int>(s1) < as<int>(s2) ) ]; Unlike actions, predicates execute immediately. You can also define the predicate out-of-line as a function object. << Dynamic Regex Grammars with Named Regexes >> Using regex_compiler, you can map a name to a regex object, and then refer to that regex from another by name. In this way, you can build grammars from regexes at runtime. sregex_compiler comp; sregex rx = comp.compile("^bar(?$RE)baz$"); comp.compile("(?$RE=)\\d+ \\d+"); There's an alternate syntax for associating a name with a regex that you can use to nest a static regex in a dynamic one. Eg., the last line above could be: comp["RE"] = +_d >> ' ' >> +_d; With these changes, you can now nest static and dynamic regexes within each other freely, giving you lots of flexibility to build grammars and modify them on the fly. << Named Captures >> For dynamic regular expressions, you can create a named capture with (?P<name> ...). You can refer back to the named capture with (?P=name). In substitution strings (for use with regex_replace()), you can refer back to a named capture with \\g<name> when using the format_perl or format_all flags. And more .... I'd like to give props to Dave Jenkins who has been doing m4d things with this stuff already, and who has given me a laundry list of other features he'd like. The new features are only available in CVS HEAD, for now. Feedback is welcome. -- Eric Niebler Boost Consulting www.boost-consulting.com