View source for Spirit Parser Framework

The '''Spirit Parser Framework''' is an [[object oriented]] [[Recursive descent parser|recursive descent]] [[parser generator]] framework implemented using template [[metaprogramming]] techniques. [[Expression templates]] allow users to approximate the syntax of [[extended Backus–Naur form]] (EBNF) completely in [[C++]]. <ref>{{cite web |last1=Guzman |first1=Joel de |last2=Kaiser |first2=Hartmut |title=Spirit 2.59 - Introduction |url=https://www.boost.org/doc/libs/1_87_0/libs/spirit/doc/html/spirit/introduction.html |website=Boost.org |publisher=Boost |access-date=22 March 2025}}</ref>Parser objects are composed through [[operator overloading]] and the result is a backtracking [[LL parser|LL(∞)]] parser that is capable of parsing rather [[ambiguous grammar|ambiguous]] grammars.

Spirit can be used for both [[lexing]] and parsing, together or separately.

This framework is part of the [[Boost C++ Libraries | Boost libraries]].

== Operators ==

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 [[extended Backus–Naur form|EBNF]] and [[regular expressions]].

{|class="wikitable"
! syntax
! explanation
|-
| <code>x >> y</code>
| Match x followed by y.
|-
| <code>x > y</code>
| After matching x, expect y.
|-
| <code>*x</code>
| Match x repeated zero or more times. This represents the [[Kleene star]]; C++ lacks an unary postfix [[Operator (programming)|operator]] *.
|-
| <code>x &#124; y</code>
| Match x.  If x does not match, try to match y.
|-
| <code>+x</code>
| Match a series of one or more occurrences of x.
|-
| <code>-x</code>
| Match x zero or one time.
|-
| <code>x & y</code>
| Match x and y.
|-
| <code>x - y</code>
| Match x but not y.
|-
| <code>x ^ y</code>
| Match x, or y, or both, in any order.
|-
| <code>x &#124;&#124; y</code>
| Match x, or y, or x followed by y.
|-
| <code>x [ ''function_expression'' ]</code>
| Execute the function/functor returned by function_expression, if x matched.
|-
| <code>( x )</code>
| Match x (can be used for priority grouping)
|-
| <code>x % y</code>
| Match one or more occurrences of x, separated by occurrences of y.
|-
| <code>~x</code>
| Match anything but x (only with character classes such as ch_p or alnum_p)
|}

== Example ==
This example shows how to use an inline parser expression with a semantic action.

<syntaxhighlight lang="cpp">
#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";
}
</syntaxhighlight>

==References==
{{reflist}}

==External links==
*[https://github.com/boostorg/spirit/ Spirit parser framework github page]
*[https://boost-spirit.com/home/ Spirit parser framework]
*[http://www.boost.org/libs/spirit/index.html Documentation in the Boost project]
*[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]

[[Category:Parser generators]]


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