Using boost's reg.expression

[giuscri]

Hi everyone! I'm having troubles when using the regular expression tools developed by boost libraries.

My specific problem is: how can I print out January, just adding some lines to the following code?

#include <iostream>
#include <string>

#include <boost/regex.hpp>

int
main()
{

 boost::regex my_regex("\\b(jan)", boost::regex_constants::icase);

 std::string my_month = "It's January!!";

 std::cout << my_month << std::endl;

 if (boost::regex_search(my_month, my_regex,
                        boost::regex_constants::format_perl))
  {
   std::cout << "boost::regex_search() returned 1 \n";
  }

 else
  {
   std::cout << "boost::regex_search() returned 0 \n";
  }

 return 0;
}

Thank you in advance,

Giuseppe

[Cubbi]

You could simply add "cout << "January"" between lines 18 and 20.

If you're looking to capture the entire word that begins with (case-insensitively) "jan", you would have to modify the regex:

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

int main() {
    boost::regex my_regex("\\b(jan\\w*)", boost::regex::icase);
    std::string my_month = "It's January!!";
    std::cout << my_month << '\n';

    boost::smatch m;
    if (boost::regex_search(my_month, m, my_regex))
        std::cout << "boost::regex_search() returned true and matched '"
                  << m[0] << "'\n";
    else 
        std::cout << "boost::regex_search() returned false \n";
}

demo: http://liveworkspace.org/code/28FT1u$0

[giuscri]

Ok, thank you. I've understood something more about this boost's regex. But, I'm far from my real goal, yet. Say, I have some kind of strange string that contains a bithdate. For instance, say that I want to extract the date 21 June 1992 from !!eh?21**?junAn1992.

How can I solve this problem? Reading Boost's documentation it sounds very weard that one wants to extract data from a string and not just knowing if the regex matches.

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

int main()
{

 // Let's start from printing out just the 21.
 // So the regex I've thought is the following one:
 boost::regex my_reg_expression ("[^\\d]*\\d{1,2}",
                                 boost::regex_constants::icase);

 // Here's the strange string:
 std::string my_string = "!!eh21**?junAn1992pl6=";
 
 
 boost::smatch m;
 
 if (boost::regex_search(my_string, m, my_reg_expression,
                     boost::regex_constants::format_perl))
 {                    
  std::cerr << "m.size() returns " << m.size() << std::endl;
  std::cerr << "m.str(0) == " << m.str(0) << std::endl;
 }
 
 else
 {
  std::cerr << "Sorry but regex has not matched. \n";
 }
  
 return 0;
}

Exit code:

m.size() returns 1
m.str(0) == !!eh21

Can someone help me?

Thank you in advance.

[Cubbi]

it's really the same as any other regular expression. Yours matches everything up to the second digit, and that's what you get. You could use round parentheses to make the digits a sub expression, same as in the first example. Could you describe your "real goal" in more detail?