Boost.Regex parsing

Question

Let's say I want to parse a simple programming language with boost regular expressions.

import a import b class c

Now I want to have a layout like this:

#include <string> #include <boost/filesystem.hpp> #include <exception> using namespace std; using namespace boost; using namespace boost::filesystem; class parser { string _source; unsigned int pos; public: parser(const string& source) : _source(source), pos(0) {} string expect(const regex& expr) { string s = next(expr) if (s != "") return s; else { --pos; throw exception("Expected another expression."); } } string next(const regex& expr) { // Removing all whitespace before first non-whitespace character // Check if characters 0 till x matches regex expr // Return matched string of "" if not matched. } bool peek(const regex& expr); parse() { regex identifier("\a*"); if (peek("import")) string package = expect(identifier); else if (peek("class")) string classname = expect(identifier); } };

Now I need your help to define the function parser::next(const regex&). It's not clear to me how to iterate with boost regular expression trough a std::string.

I hope some one can help me!

Dietmar Kühl · Accepted Answer · 2012-11-18 16:44:30Z

Assuming that Boost regexp uses a similar approach as the regular expressions from the standard C++ library do (I realize that those in the standard are based on a proposal coming from Boost but other components are not entirely identical), you'd use the information obtained in a std::match_results<...> object to determine information associated with the match.

Tim · Accepted Answer · 2012-11-21 10:33:24Z

For people who are interested. I solved it this way in the current implementation:

Note that crucial parts may be missing, but this answers the question

int submatch(const std::string& input, const regex& e) { boost::match_results<std::string::const_iterator> what; if(0 == boost::regex_match(input, what, e, boost::match_default | boost::match_partial)) { // the input so far could not possibly be valid so reject it: return 0; } // OK so far so good, but have we finished? if(what[0].matched) { // excellent, we have a result: return 2; } // what we have so far is only a partial match... return 1; } void skip_ws() { // Skip all whitespaces regex ws("\\s"); while ((pos < (source.length() - 1)) && boost::regex_match(source.substr(pos++, 1), ws)) { } pos -= 1; } string lex(const token& t) { skip_ws(); string sub; unsigned int subpos = pos; bool matched = false; while (subpos < (source.length() - 1)) { sub.push_back(source[subpos++]); int result = submatch(sub, t.expr); if (result == 1) // Partial { continue; } else if (result == 2) { matched = true; continue; } else if (result == 0) // No match { if (matched) { sub.erase(sub.end()-1); subpos -= 1; break; } else { return ""; } } } return sub; } string expect(const token& t) { cout << " string expect(\"" << t.expr << "\")"; string s = lex(t); pos += s.length(); if (s != "") { cout << endl; return s; } else { --pos; cout << "-> False" << endl; throw string("Expected another expression."); } }

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