Boost Xpressive sregex assignment and capture group issue

Question

I notice strange behavior in the boost xpressive sregex assignments. See the code below. The first code snippet that does not work has sregex has object preliminary assignments and then being used in the main expression later. The second code snippet that does work well has no prior sregex assignments (except the final main one). Please let me know if I am using the boost xpressive api incorrectly.

Code that does not work

  mark_tag Value1(1), Value2(2), Value3(3), Value4(4), Value5(5), Value6(6), Value7(7);
  boost::xpressive::sregex name,multicast,rtsp;

  name = ( (Value1 =  (+boost::xpressive::set[_w|_d|'-'|'_'|as_xpr(' ')]) )  >> ',' );

  name1 =
       ( (Value2 = icase(as_xpr("mark1:") ) )
    >> (Value3 =  (+boost::xpressive::set[_d|'.']) )
    >> ':'
    >> (Value4 =  (+boost::xpressive::set[_d]) ) >> optional(as_xpr(",")) );

  name2 =
       ( (Value5 = icase(as_xpr("mark2:") ) )
    >> (Value6 =  (+boost::xpressive::set[_d|'.']) )
    >> ':'
    >> (Value7 =  (+boost::xpressive::set[_d]) ) >> optional(as_xpr(",")) ) ;

   boost::xpressive::sregex pt = bos
    >> (
    name
    >> repeat<0,2>(
    name1
    |
    name2)
    )
    >> eos;


    boost::trim(string_to_parse);
    smatch what;
    if ( !regex_search(string_to_parse, what, pt)) {
        std::stringstream ss;
        ss << "Unable to parse: " << string_to_parse;
        throw parse::MyException(ss.str());
    }

    std::string Value1_str = what[Value1]; // print them later
    std::string Value2_str = what[Value2]; // print them later
    std::string Value3_str = what[Value3]; // print them later
    std::string Value4_str = what[Value4]; // print them later
    std::string Value5_str = what[Value5]; // print them later
    std::string Value6_str = what[Value6]; // print them later
    std::string Value7_str = what[Value7]; // print them later

string_to_parse = NameX,mark1:192.168.1.100:5555,mark2:192.168.1.101:5556; (fails parsing) Meaning what[<>] does not contain any value.

Code that works

   mark_tag Value1(1), Value2(2), Value3(3), Value4(4), Value5(5), Value6(6), Value7(7);
   sregex pt = bos
    >> (
    ( (Value1 =  (+boost::xpressive::set[_w|_d|'-'|'_'|as_xpr(' ')]) ) >> ',' )
    >> repeat<0,2>(
    ( (Value2 = icase(as_xpr("mark1:") ) ) >> (Value3 =  (+boost::xpressive::set[_d|'.']) ) >> ':' >> (Value4 =  (+boost::xpressive::set[_d]) ) >> optional(as_xpr(",")) )
    |
    ( (Value5 = icase(as_xpr("mark2:") ) ) >> (Value6 =  (+boost::xpressive::set[_d|'.']) ) >> ':' >> (Value7 =  (+boost::xpressive::set[_d]) ) >> optional(as_xpr(",")) ) )
    )
    >> eos;

    boost::trim(string_to_parse);
    smatch what;
    if ( !regex_search(string_to_parse, what, pt)) {
        std::stringstream ss;
        ss << "Unable to parse: " << string_to_parse;
        throw parse::MyException(ss.str());
    }

    std::string Value1_str = what[Value1]; // print them later
    std::string Value2_str = what[Value2]; // print them later
    std::string Value3_str = what[Value3]; // print them later
    std::string Value4_str = what[Value4]; // print them later
    std::string Value5_str = what[Value5]; // print them later
    std::string Value6_str = what[Value6]; // print them later
    std::string Value7_str = what[Value7]; // print them later

string_to_parse = NameX,mark1:192.168.1.100:5555,mark2:192.168.1.101:5556; (passes parsing)

score 2 · Accepted Answer · answered May 10 '13 at 14:46

2

When you match a pattern with nested regexes, you get nested match results. This explains it all.

answered May 10 '13 at 14:46

Eric Niebler

5,927
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It states "The author has no way of knowing whether the inner regex will stomp the sub-match vector or not." So what is the solution? Using regex_match() or regex_search(), the match_results<>??? – enthusiasticgeek May 11 '13 at 18:12
1

Read the docs! It's all there. Whether you use regex_match or regex_search, all the backreferences are saved, but you have to look in the right place, which in this case will be in the collection returned by match_results::nested_results. But seriously. Read the docs. – Eric Niebler May 12 '13 at 09:39

Boost Xpressive sregex assignment and capture group issue

1 Answers1