Spirit Parser Framework

<h2 id="operators">Operators</h2>
<p>Because of limitations of the C++ language, the syntax of Spirit has been designed around the operator precedences of C++, while bearing resemblance to both <a href="/facts/Extended_Backus%25E2%2580%2593Naur_form/89KI2QTe">EBNF</a> and <a href="/facts/Regular_expressions/KFL3veHX">regular expressions</a>.
</p>
<table><tbody><tr><th>syntax</th><th>explanation</th></tr><tr><td>x >> y</td><td>Match x followed by y.</td></tr><tr><td>x > y</td><td>After matching x, expect y.</td></tr><tr><td>*x</td><td>Match x repeated zero or more times. This represents the <a href="/facts/Kleene_star/I6SX0f2Y">Kleene star</a>; C++ lacks an unary postfix <a href="/facts/Operator_(programming)/7TnU5LKu">operator</a> *.</td></tr><tr><td>x | y</td><td>Match x.  If x does not match, try to match y.</td></tr><tr><td>+x</td><td>Match a series of one or more occurrences of x.</td></tr><tr><td>-x</td><td>Match x zero or one time.</td></tr><tr><td>x & y</td><td>Match x and y.</td></tr><tr><td>x - y</td><td>Match x but not y.</td></tr><tr><td>x ^ y</td><td>Match x, or y, or both, in any order.</td></tr><tr><td>x || y</td><td>Match x, or y, or x followed by y.</td></tr><tr><td>x [ <i>function_expression</i> ]</td><td>Execute the function/functor returned by function_expression, if x matched.</td></tr><tr><td>( x )</td><td>Match x (can be used for priority grouping)</td></tr><tr><td>x % y</td><td>Match one or more occurrences of x, separated by occurrences of y.</td></tr><tr><td>~x</td><td>Match anything but x (only with character classes such as ch_p or alnum_p)</td></tr></tbody></table>
<h2 id="example">Example</h2>
<p>This example shows how to use an inline parser expression with a semantic action.
</p>
#include <string>
#include <iostream>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
 
int main()
{
  namespace qi = boost::spirit::qi;

std::string input;
 
  std::cout << "Input a line: \n";
  getline(std::cin, input);
  std::cout << "Got '" << input << "'.\n";
 
  unsigned count = 0;
  /*  
      Next, parse the input (input.c_str()),
      using a parser constructed with the following semantics:
 
      Zero or more occurrences of (
          literal string "cat" (when matched, increment the counter "count")
      or  any character (which will be skipped)
      )

The parser is constructed by the compiler using operator overloading and
     template matching, so the actual work is done within qi::parse(), and the
     expression starting with * only initializes the rule object that the parse
     function uses.

*/
  auto rule = *(qi::lit("cat") [ ++qi::_val ] | qi::omit[qi::char_]);
  qi::parse(input.begin(), input.end(), rule, count);

// Finally, show results.
  std::cout << "The input contained " << count << " occurrences of 'cat'\n";
}

<h2 id="external-links">External links</h2>
<ul><li><a href="https://github.com/boostorg/spirit/">Spirit parser framework github page</a></li>
<li><a href="https://boost-spirit.com/home/">Spirit parser framework</a></li>
<li><a href="http://www.boost.org/libs/spirit/index.html">Documentation in the Boost project</a></li>
<li><a href="http://www.ddj.com/article/printableArticle.jhtml?articleID=184401692&dept_url=/cpp/">Article on Spirit by designer Joel de Guzman in Dr. Dobb's Journal</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Guzman, Joel de; Kaiser, Hartmut. "Spirit 2.59 - Introduction". Boost.org. Boost. Retrieved 22 March 2025. <a href="https://www.boost.org/doc/libs/1_87_0/libs/spirit/doc/html/spirit/introduction.html" target="_blank">https://www.boost.org/doc/libs/1_87_0/libs/spirit/doc/html/spirit/introduction.html</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
</ol>

Spirit Parser Framework open-in-new

Spirit Parser Framework