boost regex iterator returning empty string - c++

Question

I am a beginner to regex in c++ I was wondering why this code:
#include <iostream>
#include <string>
#include <boost/regex.hpp>
int main() {
std::string s = "? 8==2 : true ! false";
boost::regex re("\\?\\s+(.*)\\s*:\\s*(.*)\\s*\\!\\s*(.*)");
boost::sregex_token_iterator p(s.begin(), s.end(), re, -1); // sequence and that reg exp
boost::sregex_token_iterator end; // Create an end-of-reg-exp
// marker
while (p != end)
std::cout << *p++ << '\n';
}
Prints a empty string. I put the regex in regexTester and it matches the string correctly but here when I try to iterate over the matches it returns nothing.

Answer 1

I think the tokenizer is actually meant to split text by some delimiter, and the delimiter is not included. Compare with std::regex_token_iterator:
std::regex_token_iterator is a read-only LegacyForwardIterator that accesses the individual sub-matches of every match of a regular expression within the underlying character sequence. It can also be used to access the parts of the sequence that were not matched by the given regular expression (e.g. as a tokenizer).
Indeed you invoke exactly this mode as per the docs:
if submatch is -1, then enumerates all the text sequences that did not match the expression re (that is to performs field splitting).
(emphasis mine).
So, just fix that:
for (boost::sregex_token_iterator p(s.begin(), s.end(), re), e; p != e;
++p)
{
boost::sub_match<It> const& current = *p;
if (current.matched) {
std::cout << std::quoted(current.str()) << '\n';
} else {
std::cout << "non matching" << '\n';
}
}
Other Observations
All the greedy Kleene-stars are recipe for trouble. You won't ever find a second match, because the first one's .* at the end will by definition gobble up all remaining input.
Instead, make them non-greedy (.*?) and or much more precise (like isolating some character set, or mandating non-space characters?).
boost::regex re(R"(\?\s+(.*?)\s*:\s*(.*?)\s*\!\s*(.*?))");
// Or, if you don't want raw string literals:
boost::regex re("\\?\\s+(.*?)\\s*:\\s*(.*?)\\s*\\!\\s*(.*?)");
Live Demo
#include <boost/regex.hpp>
#include <iomanip>
#include <iostream>
#include <string>
int main() {
using It = std::string::const_iterator;
std::string const s =
"? 8==2 : true ! false;"
"? 9==3 : 'book' ! 'library';";
boost::regex re(R"(\?\s+(.*?)\s*:\s*(.*?)\s*\!\s*(.*?))");
{
std::cout << "=== regex_search:\n";
boost::smatch results;
for (It b = s.begin(); boost::regex_search(b, s.end(), results, re); b = results[0].end()) {
std::cout << results.str() << "\n";
std::cout << "remain: " << std::quoted(std::string(results[0].second, s.end())) << "\n";
}
}
std::cout << "=== token iteration:\n";
for (boost::sregex_token_iterator p(s.begin(), s.end(), re), e; p != e;
++p)
{
boost::sub_match<It> const& current = *p;
if (current.matched) {
std::cout << std::quoted(current.str()) << '\n';
} else {
std::cout << "non matching" << '\n';
}
}
}
Prints
=== regex_search:
? 8==2 : true !
remain: "false;? 9==3 : 'book' ! 'library';"
? 9==3 : 'book' !
remain: "'library';"
=== token iteration:
"? 8==2 : true ! "
"? 9==3 : 'book' ! "
BONUS: Parser Expressions
Instead of abusing regexen to do parsing, you could generate a parser, e.g. using Boost Spirit:
Live On Coliru
#include <boost/spirit/home/x3.hpp>
#include <boost/fusion/adapted.hpp>
#include <iomanip>
#include <iostream>
namespace x3 = boost::spirit::x3;
int main() {
std::string const s =
"? 8==2 : true ! false;"
"? 9==3 : 'book' ! 'library';";
using expression = std::string;
using ternary = std::tuple<expression, expression, expression>;
std::vector<ternary> parsed;
auto expr_ = x3::lexeme [+(x3::graph - ';')];
auto ternary_ = "?" >> expr_ >> ":" >> expr_ >> "!" >> expr_;
std::cout << "=== parser approach:\n";
if (x3::phrase_parse(begin(s), end(s), *x3::seek[ ternary_ ], x3::space, parsed)) {
for (auto [cond, e1, e2] : parsed) {
std::cout
<< " condition " << std::quoted(cond) << "\n"
<< " true expression " << std::quoted(e1) << "\n"
<< " else expression " << std::quoted(e2) << "\n"
<< "\n";
}
} else {
std::cout << "non matching" << '\n';
}
}
Prints
=== parser approach:
condition "8==2"
true expression "true"
else expression "false"
condition "9==3"
true expression "'book'"
else expression "'library'"
This is much more extensible, will easily support recursive grammars and will be able to synthesize a typed representation of your syntax tree, instead of just leaving you with scattered bits of string.