diff --git a/toml/combinator.hpp b/toml/combinator.hpp
new file mode 100644
index 0000000..a81eeed
--- /dev/null
+++ b/toml/combinator.hpp
@@ -0,0 +1,354 @@
+#ifndef TOML11_COMBINATOR_HPP
+#define TOML11_COMBINATOR_HPP
+#include "traits.hpp"
+#include "result.hpp"
+#include "utility.hpp"
+#include "region.hpp"
+#include <type_traits>
+#include <iterator>
+#include <limits>
+#include <iomanip>
+#include <cctype>
+
+// they scans characters and returns region if it matches to the condition.
+// when they fail, it does not change the location.
+// in lexer.hpp, these are used.
+
+namespace toml
+{
+namespace detail
+{
+
+// to output character as an error message.
+inline std::string show_char(const char c)
+{
+    if(std::isgraph(c))
+    {
+        return std::string(1, c);
+    }
+    else
+    {
+        std::ostringstream oss;
+        oss << std::hex << std::setfill('0') << std::setw(2) << "0x"
+            << static_cast<int>(c);
+        return oss.str();
+    }
+}
+
+template<char C>
+struct character
+{
+    static constexpr char target = C;
+
+    template<typename Cont>
+    static result<region<Cont>, std::string> invoke(location<Cont>& loc)
+    {
+        static_assert(std::is_same<char, typename Cont::value_type>::value,
+                      "internal error: container::value_type should be `char`.");
+
+        if(loc.iter() == loc.end()) {return err("empty input");}
+        const auto first = loc.iter();
+
+        const char c = *(loc.iter());
+        if(c != target)
+        {
+            return err(concat_to_string("expected '", show_char(target),
+                                        "' but got '", show_char(c), "'."));
+        }
+        ++(loc.iter()); // update location
+
+        return ok(region<Cont>(loc, first, loc.iter()));
+    }
+
+    static std::string pattern() {return show_char(target);}
+};
+template<char C>
+constexpr char character<C>::target;
+
+// closed interval [Low, Up]. both Low and Up are included.
+template<char Low, char Up>
+struct in_range
+{
+    // assuming ascii part of UTF-8...
+    static_assert(Low <= Up, "lower bound should be less than upper bound.");
+
+    static constexpr char upper = Up;
+    static constexpr char lower = Low;
+
+    template<typename Cont>
+    static result<region<Cont>, std::string> invoke(location<Cont>& loc)
+    {
+        static_assert(std::is_same<char, typename Cont::value_type>::value,
+                      "internal error: container::value_type should be `char`.");
+
+        if(loc.iter() == loc.end()) {return err("empty input");}
+        const auto first = loc.iter();
+
+        const char c = *(loc.iter());
+        if(c < lower || upper < c)
+        {
+            return err(concat_to_string("expected character in range "
+                "[", show_char(lower), ", ", show_char(upper), "] but got ",
+                "'", show_char(c), "'."));
+        }
+
+        ++(loc.iter());
+        return ok(region<Cont>(loc, first, loc.iter()));
+    }
+
+    static std::string pattern()
+    {
+        return concat_to_string("[",show_char(lower),"-",show_char(upper),"]");
+    }
+};
+template<char L, char U> constexpr char in_range<L, U>::upper;
+template<char L, char U> constexpr char in_range<L, U>::lower;
+
+// keep iterator if `Combinator` matches. otherwise, increment `iter` by 1 char.
+// for detecting invalid characters, like control sequences in toml string.
+template<typename Combinator>
+struct exclude
+{
+    template<typename Cont>
+    static result<region<Cont>, std::string> invoke(location<Cont>& loc)
+    {
+        static_assert(std::is_same<char, typename Cont::value_type>::value,
+                      "internal error: container::value_type should be `char`.");
+
+        if(loc.iter() == loc.end()) {return err("empty input");}
+        auto first = loc.iter();
+
+        auto rslt = Combinator::invoke(loc);
+        if(rslt.is_ok())
+        {
+            loc.iter() = first; // rollback
+            return err(concat_to_string(
+                "invalid pattern (", Combinator::pattern(), ") appeared ",
+                rslt.unwrap().str()));
+        }
+        loc.iter() = std::next(first);
+        return ok(region<Cont>(loc, first, loc.iter()));
+    }
+
+    static std::string pattern()
+    {
+        return concat_to_string("^(", Combinator::pattern(), ')');
+    }
+};
+
+// increment `iter`, if matches. otherwise, just return empty string.
+template<typename Combinator>
+struct maybe
+{
+    template<typename Cont>
+    static result<region<Cont>, std::string> invoke(location<Cont>& loc)
+    {
+        static_assert(std::is_same<char, typename Cont::value_type>::value,
+                      "internal error: container::value_type should be `char`.");
+
+        const auto rslt = Combinator::invoke(loc);
+        if(rslt.is_ok())
+        {
+            return rslt;
+        }
+        return ok(region<Cont>(loc));
+    }
+
+    static std::string pattern()
+    {
+        return concat_to_string('(', Combinator::pattern(), ")?");
+    }
+};
+
+template<typename ... Ts>
+struct sequence;
+
+template<typename Head, typename ... Tail>
+struct sequence<Head, Tail...>
+{
+    template<typename Cont>
+    static result<region<Cont>, std::string> invoke(location<Cont>& loc)
+    {
+        static_assert(std::is_same<char, typename Cont::value_type>::value,
+                      "internal error: container::value_type should be `char`.");
+
+        const auto first = loc.iter();
+        const auto rslt = Head::invoke(loc);
+        if(rslt.is_err())
+        {
+            loc.iter() = first;
+            return err(rslt.unwrap_err());
+        }
+        return sequence<Tail...>::invoke(loc, std::move(rslt.unwrap()), first);
+    }
+
+    // called from the above function only, recursively.
+    template<typename Cont, typename Iterator>
+    static result<region<Cont>, std::string>
+    invoke(location<Cont>& loc, region<Cont> reg, Iterator first)
+    {
+        const auto rslt = Head::invoke(loc);
+        if(rslt.is_err())
+        {
+            loc.iter() = first;
+            return err(rslt.unwrap_err());
+        }
+        reg += rslt.unwrap(); // concat regions
+        return sequence<Tail...>::invoke(loc, std::move(reg), first);
+    }
+
+    static std::string pattern()
+    {
+        return concat_to_string(Head::pattern(), sequence<Tail...>::pattern());
+    }
+};
+
+template<typename Head>
+struct sequence<Head>
+{
+    // would be called from sequence<T ...>::invoke only.
+    template<typename Cont, typename Iterator>
+    static result<region<Cont>, std::string>
+    invoke(location<Cont>& loc, region<Cont> reg, Iterator first)
+    {
+        const auto rslt = Head::invoke(loc);
+        if(rslt.is_err())
+        {
+            loc.iter() = first;
+            return err(rslt.unwrap_err());
+        }
+        reg += rslt.unwrap(); // concat regions
+        return ok(reg);
+    }
+    static std::string pattern() {return Head::pattern();}
+};
+
+template<typename ... Ts>
+struct either;
+
+template<typename Head, typename ... Tail>
+struct either<Head, Tail...>
+{
+    template<typename Cont>
+    static result<region<Cont>, std::string> invoke(location<Cont>& loc)
+    {
+        static_assert(std::is_same<char, typename Cont::value_type>::value,
+                      "internal error: container::value_type should be `char`.");
+
+        const auto rslt = Head::invoke(loc);
+        if(rslt.is_ok()) {return rslt;}
+        return either<Tail...>::invoke(loc);
+    }
+
+    static std::string pattern()
+    {
+        return concat_to_string('(', Head::pattern(), ")|", either<Tail...>::pattern());
+    }
+};
+template<typename Head>
+struct either<Head>
+{
+    template<typename Cont>
+    static result<region<Cont>, std::string> invoke(location<Cont>& loc)
+    {
+        static_assert(std::is_same<char, typename Cont::value_type>::value,
+                      "internal error: container::value_type should be `char`.");
+        return Head::invoke(loc);
+    }
+    static std::string pattern()
+    {
+        return concat_to_string('(', Head::pattern(), ')');
+    }
+};
+
+template<typename T, typename N>
+struct repeat;
+
+template<std::size_t N> struct exactly{};
+template<std::size_t N> struct at_least{};
+struct unlimited{};
+
+template<typename T, std::size_t N>
+struct repeat<T, exactly<N>>
+{
+    template<typename Cont>
+    static result<region<Cont>, std::string> invoke(location<Cont>& loc)
+    {
+        region<Cont> retval(loc);
+        const auto first = loc.iter();
+        for(std::size_t i=0; i<N; ++i)
+        {
+            auto rslt = T::invoke(loc);
+            if(rslt.is_err())
+            {
+                loc.iter() = first;
+                return err(rslt.unwrap_err());
+            }
+            retval += rslt.unwrap();
+        }
+        return ok(std::move(retval));
+    }
+    static std::string pattern()
+    {
+        return concat_to_string('(', T::pattern(), "){", N, '}');
+    }
+};
+
+template<typename T, std::size_t N>
+struct repeat<T, at_least<N>>
+{
+    template<typename Cont>
+    static result<region<Cont>, std::string> invoke(location<Cont>& loc)
+    {
+        region<Cont> retval(loc);
+
+        const auto first = loc.iter();
+        for(std::size_t i=0; i<N; ++i)
+        {
+            auto rslt = T::invoke(loc);
+            if(rslt.is_err())
+            {
+                loc.iter() = first;
+                return err(rslt.unwrap_err());
+            }
+            retval += rslt.unwrap();
+        }
+        while(true)
+        {
+            auto rslt = T::invoke(loc);
+            if(rslt.is_err())
+            {
+                return ok(std::move(retval));
+            }
+            retval += rslt.unwrap();
+        }
+    }
+    static std::string pattern()
+    {
+        return concat_to_string('(',T::pattern(), "){", N, ",}");
+    }
+};
+
+template<typename T>
+struct repeat<T, unlimited>
+{
+    template<typename Cont>
+    static result<region<Cont>, std::string> invoke(location<Cont>& loc)
+    {
+        region<Cont> retval(loc);
+        while(true)
+        {
+            auto rslt = T::invoke(loc);
+            if(rslt.is_err())
+            {
+                return ok(std::move(retval));
+            }
+            retval += rslt.unwrap();
+        }
+    }
+    static std::string pattern() {return concat_to_string('(', T::pattern(), ")*");}
+};
+
+} // detail
+} // toml
+#endif// TOML11_COMBINATOR_HPP