C++: Execute a Markov Algorithm

Create an interpreter for a Markov Algorithm. Rules have the syntax:

<ruleset> ::= ((<comment> | <rule>) <newline>+)*
<comment> ::= # {<any character>}
<rule> ::= <pattern> <whitespace> -> <whitespace> [.] <replacement>
<whitespace> ::= (<tab> | <space>) [<whitespace>]

There is one rule per line. If there is a . present before the <replacement>, then this is a terminating rule in which case the interpreter must halt execution. A ruleset consists of a sequence of rules, with optional comments.

Note: Non-use of iswhite is intentional, since depending on the locale, other chars besides space and tab might be detected by that function.

#include <cstdlib>
#include <iostream>
#include <fstream>
#include <vector>
#include <string>

struct rule
{
	std::string pattern;
	std::string replacement;
	bool terminal;
	rule(std::string pat, std::string rep, bool term):
	pattern(pat),
	replacement(rep),
	terminal(term)
	{
	}
};

std::string const whitespace = " \t";
std::string::size_type const npos = std::string::npos;

bool is_whitespace(char c)
{
	return whitespace.find(c) != npos;
}

std::vector<rule> read_rules(std::ifstream& rulefile)
{
	std::vector<rule> rules;
	std::string line;
	while (std::getline(rulefile, line))
	{
		std::string::size_type pos;

		// remove comments
		pos = line.find('#');
		if (pos != npos)
		line.resize(pos);

		// ignore lines consisting only of whitespace
		if (line.find_first_not_of(whitespace) == npos)
		continue;

		// find "->" surrounded by whitespace
		pos = line.find("->");
		while (pos != npos && (pos == 0 || !is_whitespace(line[pos-1])))
		pos = line.find("->", pos+1);

		if (pos == npos || line.length() < pos+3 || !is_whitespace(line[pos+2]))
		{
			std::cerr << "invalid rule: " << line << "\n";
			std::exit(EXIT_FAILURE);
		}

		std::string pattern = line.substr(0, pos-1);
		std::string replacement = line.substr(pos+3);

		// remove additional separating whitespace
		pattern.erase(pattern.find_last_not_of(whitespace)+1);
		replacement.erase(0, replacement.find_first_not_of(whitespace));

		// test for terminal rule
		bool terminal = !replacement.empty() && replacement[0] == '.';
		if (terminal)
		replacement.erase(0,1);

		rules.push_back(rule(pattern, replacement, terminal));
	}

	return rules;
}

std::string markov(std::vector<rule> rules, std::string input)
{
	std::string& output = input;
	std::vector<rule>::iterator iter = rules.begin();

	// Loop through each rule, transforming our current version
	// with each rule.
	while (iter != rules.end())
	{
		std::string::size_type pos = output.find(iter->pattern);
		if (pos != npos)
		{
			output.replace(pos, iter->pattern.length(), iter->replacement);
			if (iter->terminal)
			break;
			iter = rules.begin();
		}
		else
		++iter;
	}

	return output;
}

int main(int argc, char* argv[])
{
	if (argc != 3)
	{
		std::cout << "usage:\n " << argv[0] << " rulefile text\n";
		return EXIT_FAILURE;
	}

	std::ifstream rulefile(argv[1]);
	std::vector<rule> rules = read_rules(rulefile);

	std::string input(argv[2]);
	std::string output = markov(rules, input);

	std::cout << output << "\n";
}