How to match unicode characters with boost::spirit?

Question

How can I match utf8 unicode characters using boost::spirit?

For example, I want to recognize all characters in this string:

$ echo "На берегу пустынных волн" | ./a.out
Н а б е р е гу п у с т ы н н ы х в о л н

When I try this simple boost::spirit program it will not match the unicode characters correctly:

#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/support_istream_iterator.hpp>
#include <boost/foreach.hpp>
namespace qi = boost::spirit::qi;

int main() {
  std::cin.unsetf(std::ios::skipws);
  boost::spirit::istream_iterator begin(std::cin);
  boost::spirit::istream_iterator end;

  std::vector<char> letters;
  bool result = qi::phrase_parse(
      begin, end,  // input     
      +qi::char_,  // match every character
      qi::space,   // skip whitespace 
      letters);    // result    

  BOOST_FOREACH(char letter, letters) {
    std::cout << letter << " ";
  }
  std::cout << std::endl;
}

It behaves like this:

$ echo "На берегу пустынных волн" | ./a.out | less
<D0> <9D> <D0> <B0> <D0> <B1> <D0> <B5> <D1> <80> <D0> <B5> <D0> <B3> <D1> <83> <D0> <BF> <D1> <83> <D1> <81> <D1> <82> <D1> <8B> <D0> <BD> <D0> <BD> <D1> <8B> <D1> <85> <D0> 
<B2> <D0> <BE> <D0> <BB> <D0> <BD> 

UPDATE:

Okay, I worked on this a bit more, and the following code is sort of working. It first converts the input into an iterator of 32-bit unicode characters (as recommended here):

#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/support_istream_iterator.hpp>
#include <boost/foreach.hpp>
#include <boost/regex/pending/unicode_iterator.hpp>
namespace qi = boost::spirit::qi;

int main() {
  std::string str = "На берегу пустынных волн";
  boost::u8_to_u32_iterator<std::string::const_iterator>
      begin(str.begin()), end(str.end());
  typedef boost::uint32_t uchar; // a unicode code point
  std::vector<uchar> letters;
  bool result = qi::phrase_parse(
      begin, end,             // input
      +qi::standard_wide::char_,  // match every character
      qi::space,              // skip whitespace
      letters);               // result
  BOOST_FOREACH(uchar letter, letters) {
    std::cout << letter << " ";
  }
  std::cout << std::endl;
}

The code prints the Unicode code points:

$ ./a.out 
1053 1072 1073 1077 1088 1077 1075 1091 1087 1091 1089 1090 1099 1085 1085 1099 1093 1074 1086 1083 1085 

which seems to be correct, according to the official Unicode table.

Now, can anyone tell me how to print the actual characters instead, given this vector of Unicode code points?

Answer 1

I haven't got much experience with it, but apparently Spirit (SVN trunk version) supports Unicode.

#define BOOST_SPIRIT_UNICODE // We'll use unicode (UTF8) all throughout

See, e.g. the sexpr parser sample which is in the scheme demo.

BOOST_ROOT/libs/spirit/example/scheme

I believe this is based on the demo from a presentation by Bryce Lelbach¹, which specifically showcases:

• wchar support
• utree attributes (still experimental)
• s-expressions

There is an online article about S-expressions and variant.

---

¹ In case it is indeed, here is the video from that presentation and the slides (pdf) as found here (odp)

Answer 2

In Boost 1.58 I can match any unicode symbols with this:

*boost::spirit::qi::unicode::char_

I don't know how to define a specific range of unicode symbols.

Answer 3

You can't. The problem is not in boost::spirit but that Unicode is complicated. char doesn't mean a character, it means a 'byte'. And even if you work on the codepoint level, still a user perceived character may be represented by more than one codepoint. (e.g. пусты́нных is 9 characters but 10 codepoints. It may be not clear enough in Russian though because it doesn't use diacritics extensively. other languages do.)

To actually iterate over the user perceived character (or grapheme clusters in Unicode terminology), you'll need to use a Unicode specialized library, namely ICU.

However, what is the real-world use of iterating over the characters?