Question

I’m using boost regex_match and I have a problem with matching no tab characters.
My test application looks as follows:

#include <iostream>
#include <string>
#include <boost/spirit/include/classic_regex.hpp>

int
main(int args, char** argv)
{
  boost::match_results<std::string::const_iterator> what;

  if(args == 3) {
    std::string text(argv[1]);
    boost::regex expression(argv[2]);

    std::cout << "Text : " << text << std::endl;
    std::cout << "Regex: " << expression << std::endl;

    if(boost::regex_match(text, what, expression, boost::match_default) != 0) {
        int i = 0;

        std::cout << text;

        if(what[0].matched)
          std::cout << " matches with regex pattern!" << std::endl;
        else
          std::cout << " does not match with regex pattern!" << std::endl;

        for(boost::match_results<std::string::const_iterator>::const_iterator     it=what.begin(); it!=what.end(); ++it) {
          std::cout << "[" << (i++) << "] " << it->str() << std::endl;
        }
      } else {
        std::cout << "Expression does not match!" << std::endl;
      }
  } else {
    std::cout << "Usage: $> ./boost-regex <text> <regex>" << std::endl;
  }

  return 0;
}

If I run the program with these arguments, I don’t get the expected result:

$> ./boost-regex "`cat file`" "(?=.*[^\t]).*"
Text : This     text includes    some   tabulators
Regex: (?=.*[^\t]).*
This    text includes    some   tabulators matches with regex pattern!
[0] This        text includes    some   tabulators

In this case I would have expected that what[0].matched is false, but it’s not.

Is there any mistake in my regular expression?
Or do I have to use other format/match flag?

Thank you in advance!

Answer 1

I am not sure what you want to do. My understanding is, you want the regex to fail as soon as there is a tab in the text.

Your positive lookahead assertion (?=.*[^\t]) is true as soon as it finds a non tab, and there are a lot of non tabs in your text.

If you want it to fail, when there is a tab, go the other way round and use a negative lookahead assertion.

(?!.*\t).*

this assertion will fail as soon as it find a tab.