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diff --git a/include/jsoncons_ext/jsonpath/expression.hpp b/include/jsoncons_ext/jsonpath/expression.hpp
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+++ b/include/jsoncons_ext/jsonpath/expression.hpp
@@ -0,0 +1,3329 @@
+// Copyright 2021 Daniel Parker
+// Distributed under the Boost license, Version 1.0.
+// (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+// See https://github.com/danielaparker/jsoncons for latest version
+
+#ifndef JSONCONS_JSONPATH_EXPRESSION_HPP
+#define JSONCONS_JSONPATH_EXPRESSION_HPP
+
+#include <string> // std::basic_string
+#include <vector> // std::vector
+#include <unordered_map> // std::unordered_map
+#include <unordered_set> // std::unordered_set
+#include <limits> // std::numeric_limits
+#include <set> // std::set
+#include <utility> // std::move
+#if defined(JSONCONS_HAS_STD_REGEX)
+#include <regex>
+#endif
+#include <jsoncons/json_type.hpp>
+#include <jsoncons_ext/jsonpath/json_location.hpp>
+#include <jsoncons_ext/jsonpath/jsonpath_error.hpp>
+
+namespace jsoncons { 
+namespace jsonpath {
+
+    struct reference_arg_t
+    {
+        explicit reference_arg_t() = default;
+    };
+    constexpr reference_arg_t reference_arg{};
+
+    struct const_reference_arg_t
+    {
+        explicit const_reference_arg_t() = default;
+    };
+    constexpr const_reference_arg_t const_reference_arg{};
+
+    struct literal_arg_t
+    {
+        explicit literal_arg_t() = default;
+    };
+    constexpr literal_arg_t literal_arg{};
+
+    struct end_of_expression_arg_t
+    {
+        explicit end_of_expression_arg_t() = default;
+    };
+    constexpr end_of_expression_arg_t end_of_expression_arg{};
+
+    struct separator_arg_t
+    {
+        explicit separator_arg_t() = default;
+    };
+    constexpr separator_arg_t separator_arg{};
+
+    struct lparen_arg_t
+    {
+        explicit lparen_arg_t() = default;
+    };
+    constexpr lparen_arg_t lparen_arg{};
+
+    struct rparen_arg_t
+    {
+        explicit rparen_arg_t() = default;
+    };
+    constexpr rparen_arg_t rparen_arg{};
+
+    struct begin_union_arg_t
+    {
+        explicit begin_union_arg_t() = default;
+    };
+    constexpr begin_union_arg_t begin_union_arg{};
+
+    struct end_union_arg_t
+    {
+        explicit end_union_arg_t() = default;
+    };
+    constexpr end_union_arg_t end_union_arg{};
+
+    struct begin_filter_arg_t
+    {
+        explicit begin_filter_arg_t() = default;
+    };
+    constexpr begin_filter_arg_t begin_filter_arg{};
+
+    struct end_filter_arg_t
+    {
+        explicit end_filter_arg_t() = default;
+    };
+    constexpr end_filter_arg_t end_filter_arg{};
+
+    struct begin_expression_arg_t
+    {
+        explicit begin_expression_arg_t() = default;
+    };
+    constexpr begin_expression_arg_t begin_expression_arg{};
+
+    struct end_index_expression_arg_t
+    {
+        explicit end_index_expression_arg_t() = default;
+    };
+    constexpr end_index_expression_arg_t end_index_expression_arg{};
+
+    struct end_argument_expression_arg_t
+    {
+        explicit end_argument_expression_arg_t() = default;
+    };
+    constexpr end_argument_expression_arg_t end_argument_expression_arg{};
+
+    struct current_node_arg_t
+    {
+        explicit current_node_arg_t() = default;
+    };
+    constexpr current_node_arg_t current_node_arg{};
+
+    struct root_node_arg_t
+    {
+        explicit root_node_arg_t() = default;
+    };
+    constexpr root_node_arg_t root_node_arg{};
+
+    struct end_function_arg_t
+    {
+        explicit end_function_arg_t() = default;
+    };
+    constexpr end_function_arg_t end_function_arg{};
+
+    struct argument_arg_t
+    {
+        explicit argument_arg_t() = default;
+    };
+    constexpr argument_arg_t argument_arg{};
+
+    enum class result_options {value=0, nodups=1, sort=2, path=4};
+
+    using result_type = result_options;
+
+    inline result_options operator~(result_options a)
+    {
+        return static_cast<result_options>(~static_cast<unsigned int>(a));
+    }
+
+    inline result_options operator&(result_options a, result_options b)
+    {
+        return static_cast<result_options>(static_cast<unsigned int>(a) & static_cast<unsigned int>(b));
+    }
+
+    inline result_options operator^(result_options a, result_options b)
+    {
+        return static_cast<result_options>(static_cast<unsigned int>(a) ^ static_cast<unsigned int>(b));
+    }
+
+    inline result_options operator|(result_options a, result_options b)
+    {
+        return static_cast<result_options>(static_cast<unsigned int>(a) | static_cast<unsigned int>(b));
+    }
+
+    inline result_options operator&=(result_options& a, result_options b)
+    {
+        a = a & b;
+        return a;
+    }
+
+    inline result_options operator^=(result_options& a, result_options b)
+    {
+        a = a ^ b;
+        return a;
+    }
+
+    inline result_options operator|=(result_options& a, result_options b)
+    {
+        a = a | b;
+        return a;
+    }
+
+    template <class Json>
+    class parameter;
+
+    template <class Json,class JsonReference>
+    class value_or_pointer
+    {
+    public:
+        friend class parameter<Json>;
+        using value_type = Json;
+        using reference = JsonReference;
+        using pointer = typename std::conditional<std::is_const<typename std::remove_reference<reference>::type>::value,typename Json::const_pointer,typename Json::pointer>::type;
+    private:
+        bool is_value_;
+        union
+        {
+            value_type val_;
+            pointer ptr_;
+        };
+    public:
+        value_or_pointer(value_type&& val)
+            : is_value_(true), val_(std::move(val))
+        {
+        }
+
+        value_or_pointer(pointer ptr)
+            : is_value_(false), ptr_(std::move(ptr))
+        {
+        }
+
+        value_or_pointer(value_or_pointer&& other) noexcept
+            : is_value_(other.is_value_)
+        {
+            if (is_value_)
+            {
+                new(&val_)value_type(std::move(other.val_));
+            }
+            else
+            {
+                ptr_ = other.ptr_;
+            }
+        }
+
+        ~value_or_pointer() noexcept
+        {
+            if (is_value_)
+            {
+                val_.~value_type();
+            }
+        }
+
+        value_or_pointer& operator=(value_or_pointer&& other) noexcept
+        {
+            if (is_value_)
+            {
+                val_.~value_type();
+            }
+            is_value_ = other.is_value_;
+
+            if (is_value_)
+            {
+                new(&val_)value_type(std::move(other.val_));
+            }
+            else
+            {
+                ptr_ = other.ptr_;
+            }
+            return *this;
+        }
+
+        reference value() 
+        {
+            return is_value_ ? val_ : *ptr_;
+        }
+
+        pointer ptr() 
+        {
+            return is_value_ ? &val_ : ptr_;
+        }
+    };
+
+    template <class Json>
+    class parameter
+    {
+        using value_type = Json;
+        using reference = const Json&;
+        using pointer = const Json*;
+    private:
+        value_or_pointer<Json,reference> data_;
+    public:
+        template <class JsonReference>
+        parameter(value_or_pointer<Json,JsonReference>&& data) noexcept
+            : data_(nullptr)
+        {
+            data_.is_value_ = data.is_value_;
+            if (data.is_value_)
+            {
+                data_.val_ = std::move(data.val_);
+            }
+            else
+            {
+                data_.ptr_ = data.ptr_;
+            }
+        }
+
+        parameter(parameter&& other) noexcept = default;
+
+        parameter& operator=(parameter&& other) noexcept = default;
+
+        const Json& value() const
+        {
+            return data_.is_value_ ? data_.val_ : *data_.ptr_;
+        }
+    };
+
+    template <class Json>
+    class custom_function
+    {
+    public:
+        using value_type = Json;
+        using char_type = typename Json::char_type;
+        using parameter_type = parameter<Json>;
+        using function_type = std::function<value_type(jsoncons::span<const parameter_type>, std::error_code& ec)>;
+        using string_type = std::basic_string<char_type>;
+
+        string_type function_name_;
+        optional<std::size_t> arity_;
+        function_type f_;
+
+        custom_function(const string_type& function_name,
+                        const optional<std::size_t>& arity,
+                        const function_type& f)
+            : function_name_(function_name),
+              arity_(arity),
+              f_(f)
+        {
+        }
+
+        custom_function(string_type&& function_name,
+                        optional<std::size_t>&& arity,
+                        function_type&& f)
+            : function_name_(std::move(function_name)),
+              arity_(std::move(arity)),
+              f_(std::move(f))
+        {
+        }
+
+        custom_function(const custom_function&) = default;
+
+        custom_function(custom_function&&) = default;
+
+        const string_type& name() const 
+        {
+            return function_name_;
+        }
+
+        optional<std::size_t> arity() const 
+        {
+            return arity_;
+        }
+
+        const function_type& function() const 
+        {
+            return f_;
+        }
+    };
+
+    template <class Json>
+    class custom_functions
+    {
+        using char_type = typename Json::char_type;
+        using string_type = std::basic_string<char_type>;
+        using value_type = Json;
+        using parameter_type = parameter<Json>;
+        using function_type = std::function<value_type(jsoncons::span<const parameter_type>, std::error_code& ec)>;
+        using const_iterator = typename std::vector<custom_function<Json>>::const_iterator;
+
+        std::vector<custom_function<Json>> functions_;
+    public:
+        void register_function(const string_type& name,
+                               jsoncons::optional<std::size_t> arity,
+                               const function_type& f)
+        {
+            functions_.emplace_back(name, arity, f);
+        }
+
+        const_iterator begin() const
+        {
+            return functions_.begin();
+        }
+
+        const_iterator end() const
+        {
+            return functions_.end();
+        }
+    };
+
+namespace detail {
+
+    template <class Json,class JsonReference>
+    class dynamic_resources;
+
+    template <class Json,class JsonReference>
+    struct unary_operator
+    {
+        std::size_t precedence_level_;
+        bool is_right_associative_;
+
+        unary_operator(std::size_t precedence_level,
+                       bool is_right_associative)
+            : precedence_level_(precedence_level),
+              is_right_associative_(is_right_associative)
+        {
+        }
+
+        virtual ~unary_operator() = default;
+
+        std::size_t precedence_level() const 
+        {
+            return precedence_level_;
+        }
+        bool is_right_associative() const
+        {
+            return is_right_associative_;
+        }
+
+        virtual Json evaluate(JsonReference, 
+                              std::error_code&) const = 0;
+    };
+
+    template <class Json>
+    bool is_false(const Json& val)
+    {
+        return ((val.is_array() && val.empty()) ||
+                 (val.is_object() && val.empty()) ||
+                 (val.is_string() && val.as_string_view().empty()) ||
+                 (val.is_bool() && !val.as_bool()) ||
+                 val.is_null());
+    }
+
+    template <class Json>
+    bool is_true(const Json& val)
+    {
+        return !is_false(val);
+    }
+
+    template <class Json,class JsonReference>
+    class unary_not_operator final : public unary_operator<Json,JsonReference>
+    {
+    public:
+        unary_not_operator()
+            : unary_operator<Json,JsonReference>(1, true)
+        {}
+
+        Json evaluate(JsonReference val, 
+                      std::error_code&) const override
+        {
+            return is_false(val) ? Json(true) : Json(false);
+        }
+    };
+
+    template <class Json,class JsonReference>
+    class unary_minus_operator final : public unary_operator<Json,JsonReference>
+    {
+    public:
+        unary_minus_operator()
+            : unary_operator<Json,JsonReference>(1, true)
+        {}
+
+        Json evaluate(JsonReference val, 
+                      std::error_code&) const override
+        {
+            if (val.is_int64())
+            {
+                return Json(-val.template as<int64_t>());
+            }
+            else if (val.is_double())
+            {
+                return Json(-val.as_double());
+            }
+            else
+            {
+                return Json::null();
+            }
+        }
+    };
+
+    template <class Json,class JsonReference>
+    class regex_operator final : public unary_operator<Json,JsonReference>
+    {
+        using char_type = typename Json::char_type;
+        using string_type = std::basic_string<char_type>;
+        std::basic_regex<char_type> pattern_;
+    public:
+        regex_operator(std::basic_regex<char_type>&& pattern)
+            : unary_operator<Json,JsonReference>(2, true),
+              pattern_(std::move(pattern))
+        {
+        }
+
+        regex_operator(regex_operator&&) = default;
+        regex_operator& operator=(regex_operator&&) = default;
+
+        Json evaluate(JsonReference val, 
+                             std::error_code&) const override
+        {
+            if (!val.is_string())
+            {
+                return Json::null();
+            }
+            return std::regex_search(val.as_string(), pattern_) ? Json(true) : Json(false);
+        }
+    };
+
+    template <class Json,class JsonReference>
+    struct binary_operator
+    {
+        std::size_t precedence_level_;
+        bool is_right_associative_;
+
+        binary_operator(std::size_t precedence_level,
+                        bool is_right_associative = false)
+            : precedence_level_(precedence_level),
+              is_right_associative_(is_right_associative)
+        {
+        }
+
+        std::size_t precedence_level() const 
+        {
+            return precedence_level_;
+        }
+        bool is_right_associative() const
+        {
+            return is_right_associative_;
+        }
+
+        virtual Json evaluate(JsonReference, 
+                             JsonReference, 
+
+                             std::error_code&) const = 0;
+
+        virtual std::string to_string(int = 0) const
+        {
+            return "binary operator";
+        }
+
+    protected:
+        ~binary_operator() = default;
+    };
+
+    // Implementations
+
+    template <class Json,class JsonReference>
+    class or_operator final : public binary_operator<Json,JsonReference>
+    {
+    public:
+        or_operator()
+            : binary_operator<Json,JsonReference>(9)
+        {
+        }
+
+        Json evaluate(JsonReference lhs, JsonReference rhs, std::error_code&) const override
+        {
+            if (lhs.is_null() && rhs.is_null())
+            {
+                return Json::null();
+            }
+            if (!is_false(lhs))
+            {
+                return lhs;
+            }
+            else
+            {
+                return rhs;
+            }
+        }
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                //s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("or operator");
+            return s;
+        }
+    };
+
+    template <class Json,class JsonReference>
+    class and_operator final : public binary_operator<Json,JsonReference>
+    {
+    public:
+        and_operator()
+            : binary_operator<Json,JsonReference>(8)
+        {
+        }
+
+        Json evaluate(JsonReference lhs, JsonReference rhs, std::error_code&) const override
+        {
+            if (is_true(lhs))
+            {
+                return rhs;
+            }
+            else
+            {
+                return lhs;
+            }
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("and operator");
+            return s;
+        }
+    };
+
+    template <class Json,class JsonReference>
+    class eq_operator final : public binary_operator<Json,JsonReference>
+    {
+    public:
+        eq_operator()
+            : binary_operator<Json,JsonReference>(6)
+        {
+        }
+
+        Json evaluate(JsonReference lhs, JsonReference rhs, std::error_code&) const override 
+        {
+            return lhs == rhs ? Json(true) : Json(false);
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("equal operator");
+            return s;
+        }
+    };
+
+    template <class Json,class JsonReference>
+    class ne_operator final : public binary_operator<Json,JsonReference>
+    {
+    public:
+        ne_operator()
+            : binary_operator<Json,JsonReference>(6)
+        {
+        }
+
+        Json evaluate(JsonReference lhs, JsonReference rhs, std::error_code&) const override 
+        {
+            return lhs != rhs ? Json(true) : Json(false);
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("not equal operator");
+            return s;
+        }
+    };
+
+    template <class Json,class JsonReference>
+    class lt_operator final : public binary_operator<Json,JsonReference>
+    {
+    public:
+        lt_operator()
+            : binary_operator<Json,JsonReference>(5)
+        {
+        }
+
+        Json evaluate(JsonReference lhs, JsonReference rhs, std::error_code&) const override 
+        {
+            if (lhs.is_number() && rhs.is_number())
+            {
+                return lhs < rhs ? Json(true) : Json(false);
+            }
+            else if (lhs.is_string() && rhs.is_string())
+            {
+                return lhs < rhs ? Json(true) : Json(false);
+            }
+            return Json::null();
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("less than operator");
+            return s;
+        }
+    };
+
+    template <class Json,class JsonReference>
+    class lte_operator final : public binary_operator<Json,JsonReference>
+    {
+    public:
+        lte_operator()
+            : binary_operator<Json,JsonReference>(5)
+        {
+        }
+
+        Json evaluate(JsonReference lhs, JsonReference rhs, std::error_code&) const override 
+        {
+            if (lhs.is_number() && rhs.is_number())
+            {
+                return lhs <= rhs ? Json(true) : Json(false);
+            }
+            else if (lhs.is_string() && rhs.is_string())
+            {
+                return lhs <= rhs ? Json(true) : Json(false);
+            }
+            return Json::null();
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("less than or equal operator");
+            return s;
+        }
+    };
+
+    template <class Json,class JsonReference>
+    class gt_operator final : public binary_operator<Json,JsonReference>
+    {
+    public:
+        gt_operator()
+            : binary_operator<Json,JsonReference>(5)
+        {
+        }
+
+        Json evaluate(JsonReference lhs, JsonReference rhs, std::error_code&) const override
+        {
+            //std::cout << "operator> lhs: " << lhs << ", rhs: " << rhs << "\n";
+
+            if (lhs.is_number() && rhs.is_number())
+            {
+                return lhs > rhs ? Json(true) : Json(false);
+            }
+            else if (lhs.is_string() && rhs.is_string())
+            {
+                return lhs > rhs ? Json(true) : Json(false);
+            }
+            return Json::null();
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("greater than operator");
+            return s;
+        }
+    };
+
+    template <class Json,class JsonReference>
+    class gte_operator final : public binary_operator<Json,JsonReference>
+    {
+    public:
+        gte_operator()
+            : binary_operator<Json,JsonReference>(5)
+        {
+        }
+
+        Json evaluate(JsonReference lhs, JsonReference rhs, std::error_code&) const override
+        {
+            if (lhs.is_number() && rhs.is_number())
+            {
+                return lhs >= rhs ? Json(true) : Json(false);
+            }
+            else if (lhs.is_string() && rhs.is_string())
+            {
+                return lhs >= rhs ? Json(true) : Json(false);
+            }
+            return Json::null();
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("greater than or equal operator");
+            return s;
+        }
+    };
+
+    template <class Json,class JsonReference>
+    class plus_operator final : public binary_operator<Json,JsonReference>
+    {
+    public:
+        plus_operator()
+            : binary_operator<Json,JsonReference>(4)
+        {
+        }
+
+        Json evaluate(JsonReference lhs, JsonReference rhs, std::error_code&) const override
+        {
+            if (!(lhs.is_number() && rhs.is_number()))
+            {
+                return Json::null();
+            }
+            else if (lhs.is_int64() && rhs.is_int64())
+            {
+                return Json(((lhs.template as<int64_t>() + rhs.template as<int64_t>())));
+            }
+            else if (lhs.is_uint64() && rhs.is_uint64())
+            {
+                return Json((lhs.template as<uint64_t>() + rhs.template as<uint64_t>()));
+            }
+            else
+            {
+                return Json((lhs.as_double() + rhs.as_double()));
+            }
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("plus operator");
+            return s;
+        }
+    };
+
+    template <class Json,class JsonReference>
+    class minus_operator final : public binary_operator<Json,JsonReference>
+    {
+    public:
+        minus_operator()
+            : binary_operator<Json,JsonReference>(4)
+        {
+        }
+
+        Json evaluate(JsonReference lhs, JsonReference rhs, std::error_code&) const override
+        {
+            if (!(lhs.is_number() && rhs.is_number()))
+            {
+                return Json::null();
+            }
+            else if (lhs.is_int64() && rhs.is_int64())
+            {
+                return Json(((lhs.template as<int64_t>() - rhs.template as<int64_t>())));
+            }
+            else if (lhs.is_uint64() && rhs.is_uint64())
+            {
+                return Json((lhs.template as<uint64_t>() - rhs.template as<uint64_t>()));
+            }
+            else
+            {
+                return Json((lhs.as_double() - rhs.as_double()));
+            }
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("minus operator");
+            return s;
+        }
+    };
+
+    template <class Json,class JsonReference>
+    class mult_operator final : public binary_operator<Json,JsonReference>
+    {
+    public:
+        mult_operator()
+            : binary_operator<Json,JsonReference>(3)
+        {
+        }
+
+        Json evaluate(JsonReference lhs, JsonReference rhs, std::error_code&) const override
+        {
+            if (!(lhs.is_number() && rhs.is_number()))
+            {
+                return Json::null();
+            }
+            else if (lhs.is_int64() && rhs.is_int64())
+            {
+                return Json(((lhs.template as<int64_t>() * rhs.template as<int64_t>())));
+            }
+            else if (lhs.is_uint64() && rhs.is_uint64())
+            {
+                return Json((lhs.template as<uint64_t>() * rhs.template as<uint64_t>()));
+            }
+            else
+            {
+                return Json((lhs.as_double() * rhs.as_double()));
+            }
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("multiply operator");
+            return s;
+        }
+    };
+
+    template <class Json,class JsonReference>
+    class div_operator final : public binary_operator<Json,JsonReference>
+    {
+    public:
+        div_operator()
+            : binary_operator<Json,JsonReference>(3)
+        {
+        }
+
+        Json evaluate(JsonReference lhs, JsonReference rhs, std::error_code&) const override
+        {
+            //std::cout << "operator/ lhs: " << lhs << ", rhs: " << rhs << "\n";
+
+            if (!(lhs.is_number() && rhs.is_number()))
+            {
+                return Json::null();
+            }
+            else if (lhs.is_int64() && rhs.is_int64())
+            {
+                return Json(((lhs.template as<int64_t>() / rhs.template as<int64_t>())));
+            }
+            else if (lhs.is_uint64() && rhs.is_uint64())
+            {
+                return Json((lhs.template as<uint64_t>() / rhs.template as<uint64_t>()));
+            }
+            else
+            {
+                return Json((lhs.as_double() / rhs.as_double()));
+            }
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("divide operator");
+            return s;
+        }
+    };
+
+    template <class Json,class JsonReference>
+    class modulus_operator final : public binary_operator<Json,JsonReference>
+    {
+    public:
+        modulus_operator()
+            : binary_operator<Json,JsonReference>(3)
+        {
+        }
+
+        Json evaluate(JsonReference lhs, JsonReference rhs, std::error_code&) const override
+        {
+            //std::cout << "operator/ lhs: " << lhs << ", rhs: " << rhs << "\n";
+
+            if (!(lhs.is_number() && rhs.is_number()))
+            {
+                return Json::null();
+            }
+            else if (lhs.is_int64() && rhs.is_int64())
+            {
+                return Json(((lhs.template as<int64_t>() % rhs.template as<int64_t>())));
+            }
+            else if (lhs.is_uint64() && rhs.is_uint64())
+            {
+                return Json((lhs.template as<uint64_t>() % rhs.template as<uint64_t>()));
+            }
+            else
+            {
+                return Json(fmod(lhs.as_double(), rhs.as_double()));
+            }
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("modulus operator");
+            return s;
+        }
+    };
+
+    // function_base
+    template <class Json>
+    class function_base
+    {
+        jsoncons::optional<std::size_t> arg_count_;
+    public:
+        using value_type = Json;
+        using parameter_type = parameter<Json>;
+
+        function_base(jsoncons::optional<std::size_t> arg_count)
+            : arg_count_(arg_count)
+        {
+        }
+
+        virtual ~function_base() noexcept = default;
+
+        jsoncons::optional<std::size_t> arity() const
+        {
+            return arg_count_;
+        }
+
+        virtual value_type evaluate(const std::vector<parameter_type>& args, 
+                                    std::error_code& ec) const = 0;
+
+        virtual std::string to_string(int level = 0) const
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("function");
+            return s;
+        }
+    };  
+
+    template <class Json>
+    class decorator_function : public function_base<Json>
+    {
+    public:
+        using value_type = Json;
+        using parameter_type = parameter<Json>;
+        using string_view_type = typename Json::string_view_type;
+        using function_type = std::function<value_type(jsoncons::span<const parameter_type>, std::error_code& ec)>;
+    private:
+        function_type f_;
+    public:
+        decorator_function(jsoncons::optional<std::size_t> arity,
+            const function_type& f)
+            : function_base<Json>(arity), f_(f)
+        {
+        }
+
+        value_type evaluate(const std::vector<parameter_type>& args,
+            std::error_code& ec) const override
+        {
+            return f_(args, ec);
+        }
+    };
+
+    template <class Json>
+    class contains_function : public function_base<Json>
+    {
+    public:
+        using value_type = Json;
+        using parameter_type = parameter<Json>;
+        using string_view_type = typename Json::string_view_type;
+
+        contains_function()
+            : function_base<Json>(2)
+        {
+        }
+
+        value_type evaluate(const std::vector<parameter_type>& args, 
+                            std::error_code& ec) const override
+        {
+            if (args.size() != *this->arity())
+            {
+                ec = jsonpath_errc::invalid_arity;
+                return value_type::null();
+            }
+
+            auto arg0= args[0].value();
+            auto arg1= args[1].value();
+
+            switch (arg0.type())
+            {
+                case json_type::array_value:
+                    for (auto& j : arg0.array_range())
+                    {
+                        if (j == arg1)
+                        {
+                            return value_type(true);
+                        }
+                    }
+                    return value_type(false);
+                case json_type::string_value:
+                {
+                    if (!arg1.is_string())
+                    {
+                        ec = jsonpath_errc::invalid_type;
+                        return value_type::null();
+                    }
+                    auto sv0 = arg0.template as<string_view_type>();
+                    auto sv1 = arg1.template as<string_view_type>();
+                    return sv0.find(sv1) != string_view_type::npos ? value_type(true) : value_type(false);
+                }
+                default:
+                {
+                    ec = jsonpath_errc::invalid_type;
+                    return value_type::null();
+                }
+            }
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("contains function");
+            return s;
+        }
+    };
+
+    template <class Json>
+    class ends_with_function : public function_base<Json>
+    {
+    public:
+        using value_type = Json;
+        using parameter_type = parameter<Json>;
+        using string_view_type = typename Json::string_view_type;
+
+        ends_with_function()
+            : function_base<Json>(2)
+        {
+        }
+
+        value_type evaluate(const std::vector<parameter_type>& args, 
+                            std::error_code& ec) const override
+        {
+            if (args.size() != *this->arity())
+            {
+                ec = jsonpath_errc::invalid_arity;
+                return value_type::null();
+            }
+
+            auto arg0= args[0].value();
+            if (!arg0.is_string())
+            {
+                ec = jsonpath_errc::invalid_type;
+                return value_type::null();
+            }
+
+            auto arg1= args[1].value();
+            if (!arg1.is_string())
+            {
+                ec = jsonpath_errc::invalid_type;
+                return value_type::null();
+            }
+
+            auto sv0 = arg0.template as<string_view_type>();
+            auto sv1 = arg1.template as<string_view_type>();
+
+            if (sv1.length() <= sv0.length() && sv1 == sv0.substr(sv0.length() - sv1.length()))
+            {
+                return value_type(true);
+            }
+            else
+            {
+                return value_type(false);
+            }
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("ends_with function");
+            return s;
+        }
+    };
+
+    template <class Json>
+    class starts_with_function : public function_base<Json>
+    {
+    public:
+        using value_type = Json;
+        using parameter_type = parameter<Json>;
+        using string_view_type = typename Json::string_view_type;
+
+        starts_with_function()
+            : function_base<Json>(2)
+        {
+        }
+
+        value_type evaluate(const std::vector<parameter_type>& args, 
+                            std::error_code& ec) const override
+        {
+            if (args.size() != *this->arity())
+            {
+                ec = jsonpath_errc::invalid_arity;
+                return value_type::null();
+            }
+
+            auto arg0= args[0].value();
+            if (!arg0.is_string())
+            {
+                ec = jsonpath_errc::invalid_type;
+                return value_type::null();
+            }
+
+            auto arg1= args[1].value();
+            if (!arg1.is_string())
+            {
+                ec = jsonpath_errc::invalid_type;
+                return value_type::null();
+            }
+
+            auto sv0 = arg0.template as<string_view_type>();
+            auto sv1 = arg1.template as<string_view_type>();
+
+            if (sv1.length() <= sv0.length() && sv1 == sv0.substr(0, sv1.length()))
+            {
+                return value_type(true);
+            }
+            else
+            {
+                return value_type(false);
+            }
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("starts_with function");
+            return s;
+        }
+    };
+
+    template <class Json>
+    class sum_function : public function_base<Json>
+    {
+    public:
+        using value_type = Json;
+        using parameter_type = parameter<Json>;
+
+        sum_function()
+            : function_base<Json>(1)
+        {
+        }
+
+        value_type evaluate(const std::vector<parameter_type>& args, 
+                            std::error_code& ec) const override
+        {
+            if (args.size() != *this->arity())
+            {
+                ec = jsonpath_errc::invalid_arity;
+                return value_type::null();
+            }
+
+            auto arg0= args[0].value();
+            if (!arg0.is_array())
+            {
+                //std::cout << "arg: " << arg0 << "\n";
+                ec = jsonpath_errc::invalid_type;
+                return value_type::null();
+            }
+            //std::cout << "sum function arg: " << arg0 << "\n";
+
+            double sum = 0;
+            for (auto& j : arg0.array_range())
+            {
+                if (!j.is_number())
+                {
+                    ec = jsonpath_errc::invalid_type;
+                    return value_type::null();
+                }
+                sum += j.template as<double>();
+            }
+
+            return value_type(sum);
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("sum function");
+            return s;
+        }
+    };
+
+#if defined(JSONCONS_HAS_STD_REGEX)
+
+    template <class Json>
+    class tokenize_function : public function_base<Json>
+    {
+    public:
+        using value_type = Json;
+        using parameter_type = parameter<Json>;
+        using char_type = typename Json::char_type;
+        using string_type = std::basic_string<char_type>;
+
+        tokenize_function()
+            : function_base<Json>(2)
+        {
+        }
+
+        value_type evaluate(const std::vector<parameter_type>& args, 
+                            std::error_code& ec) const override
+        {
+            if (args.size() != *this->arity())
+            {
+                ec = jsonpath_errc::invalid_arity;
+                return value_type::null();
+            }
+
+            if (!args[0].value().is_string() || !args[1].value().is_string())
+            {
+                //std::cout << "arg: " << arg0 << "\n";
+                ec = jsonpath_errc::invalid_type;
+                return value_type::null();
+            }
+            auto arg0 = args[0].value().template as<string_type>();
+            auto arg1 = args[1].value().template as<string_type>();
+
+            std::regex::flag_type options = std::regex_constants::ECMAScript; 
+            std::basic_regex<char_type> pieces_regex(arg1, options);
+
+            std::regex_token_iterator<typename string_type::const_iterator> rit ( arg0.begin(), arg0.end(), pieces_regex, -1);
+            std::regex_token_iterator<typename string_type::const_iterator> rend;
+
+            value_type j(json_array_arg);
+            while (rit != rend) 
+            {
+                j.emplace_back(rit->str());
+                ++rit;
+            }
+            return j;
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("tokenize function");
+            return s;
+        }
+    };
+
+#endif // defined(JSONCONS_HAS_STD_REGEX)
+
+    template <class Json>
+    class ceil_function : public function_base<Json>
+    {
+    public:
+        using value_type = Json;
+        using parameter_type = parameter<Json>;
+
+        ceil_function()
+            : function_base<Json>(1)
+        {
+        }
+
+        value_type evaluate(const std::vector<parameter_type>& args, 
+                            std::error_code& ec) const override
+        {
+            if (args.size() != *this->arity())
+            {
+                ec = jsonpath_errc::invalid_arity;
+                return value_type::null();
+            }
+
+            auto arg0= args[0].value();
+            switch (arg0.type())
+            {
+                case json_type::uint64_value:
+                case json_type::int64_value:
+                {
+                    return value_type(arg0.template as<double>());
+                }
+                case json_type::double_value:
+                {
+                    return value_type(std::ceil(arg0.template as<double>()));
+                }
+                default:
+                    ec = jsonpath_errc::invalid_type;
+                    return value_type::null();
+            }
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("ceil function");
+            return s;
+        }
+    };
+
+    template <class Json>
+    class floor_function : public function_base<Json>
+    {
+    public:
+        using value_type = Json;
+        using parameter_type = parameter<Json>;
+
+        floor_function()
+            : function_base<Json>(1)
+        {
+        }
+
+        value_type evaluate(const std::vector<parameter_type>& args, 
+                            std::error_code& ec) const override
+        {
+            if (args.size() != *this->arity())
+            {
+                ec = jsonpath_errc::invalid_arity;
+                return value_type::null();
+            }
+
+            auto arg0= args[0].value();
+            switch (arg0.type())
+            {
+                case json_type::uint64_value:
+                case json_type::int64_value:
+                {
+                    return value_type(arg0.template as<double>());
+                }
+                case json_type::double_value:
+                {
+                    return value_type(std::floor(arg0.template as<double>()));
+                }
+                default:
+                    ec = jsonpath_errc::invalid_type;
+                    return value_type::null();
+            }
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("floor function");
+            return s;
+        }
+    };
+
+    template <class Json>
+    class to_number_function : public function_base<Json>
+    {
+    public:
+        using value_type = Json;
+        using parameter_type = parameter<Json>;
+
+        to_number_function()
+            : function_base<Json>(1)
+        {
+        }
+
+        value_type evaluate(const std::vector<parameter_type>& args, 
+                            std::error_code& ec) const override
+        {
+            if (args.size() != *this->arity())
+            {
+                ec = jsonpath_errc::invalid_arity;
+                return value_type::null();
+            }
+
+            auto arg0= args[0].value();
+            switch (arg0.type())
+            {
+                case json_type::int64_value:
+                case json_type::uint64_value:
+                case json_type::double_value:
+                    return arg0;
+                case json_type::string_value:
+                {
+                    auto sv = arg0.as_string_view();
+                    uint64_t un{0};
+                    auto result1 = jsoncons::detail::to_integer(sv.data(), sv.length(), un);
+                    if (result1)
+                    {
+                        return value_type(un);
+                    }
+                    int64_t sn{0};
+                    auto result2 = jsoncons::detail::to_integer(sv.data(), sv.length(), sn);
+                    if (result2)
+                    {
+                        return value_type(sn);
+                    }
+                    jsoncons::detail::chars_to to_double;
+                    try
+                    {
+                        auto s = arg0.as_string();
+                        double d = to_double(s.c_str(), s.length());
+                        return value_type(d);
+                    }
+                    catch (const std::exception&)
+                    {
+                        return value_type::null();
+                    }
+                }
+                default:
+                    ec = jsonpath_errc::invalid_type;
+                    return value_type::null();
+            }
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("to_number function");
+            return s;
+        }
+    };
+
+    template <class Json>
+    class prod_function : public function_base<Json>
+    {
+    public:
+        using value_type = Json;
+        using parameter_type = parameter<Json>;
+
+        prod_function()
+            : function_base<Json>(1)
+        {
+        }
+
+        value_type evaluate(const std::vector<parameter_type>& args, 
+                            std::error_code& ec) const override
+        {
+            if (args.size() != *this->arity())
+            {
+                ec = jsonpath_errc::invalid_arity;
+                return value_type::null();
+            }
+
+            auto arg0= args[0].value();
+            if (!arg0.is_array() || arg0.empty())
+            {
+                //std::cout << "arg: " << arg0 << "\n";
+                ec = jsonpath_errc::invalid_type;
+                return value_type::null();
+            }
+            double prod = 1;
+            for (auto& j : arg0.array_range())
+            {
+                if (!j.is_number())
+                {
+                    ec = jsonpath_errc::invalid_type;
+                    return value_type::null();
+                }
+                prod *= j.template as<double>();
+            }
+
+            return value_type(prod);
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("prod function");
+            return s;
+        }
+    };
+
+    template <class Json>
+    class avg_function : public function_base<Json>
+    {
+    public:
+        using value_type = Json;
+        using parameter_type = parameter<Json>;
+
+        avg_function()
+            : function_base<Json>(1)
+        {
+        }
+
+        value_type evaluate(const std::vector<parameter_type>& args, 
+                            std::error_code& ec) const override
+        {
+            if (args.size() != *this->arity())
+            {
+                ec = jsonpath_errc::invalid_arity;
+                return value_type::null();
+            }
+
+            auto arg0= args[0].value();
+            if (!arg0.is_array())
+            {
+                ec = jsonpath_errc::invalid_type;
+                return value_type::null();
+            }
+            if (arg0.empty())
+            {
+                return value_type::null();
+            }
+            double sum = 0;
+            for (auto& j : arg0.array_range())
+            {
+                if (!j.is_number())
+                {
+                    ec = jsonpath_errc::invalid_type;
+                    return value_type::null();
+                }
+                sum += j.template as<double>();
+            }
+
+            return value_type(sum / static_cast<double>(arg0.size()));
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("to_string function");
+            return s;
+        }
+    };
+
+    template <class Json>
+    class min_function : public function_base<Json>
+    {
+    public:
+        using value_type = Json;
+        using parameter_type = parameter<Json>;
+
+        min_function()
+            : function_base<Json>(1)
+        {
+        }
+
+        value_type evaluate(const std::vector<parameter_type>& args, 
+                            std::error_code& ec) const override
+        {
+            if (args.size() != *this->arity())
+            {
+                ec = jsonpath_errc::invalid_arity;
+                return value_type::null();
+            }
+
+            auto arg0= args[0].value();
+            if (!arg0.is_array())
+            {
+                //std::cout << "arg: " << arg0 << "\n";
+                ec = jsonpath_errc::invalid_type;
+                return value_type::null();
+            }
+            if (arg0.empty())
+            {
+                return value_type::null();
+            }
+            bool is_number = arg0.at(0).is_number();
+            bool is_string = arg0.at(0).is_string();
+            if (!is_number && !is_string)
+            {
+                ec = jsonpath_errc::invalid_type;
+                return value_type::null();
+            }
+
+            std::size_t index = 0;
+            for (std::size_t i = 1; i < arg0.size(); ++i)
+            {
+                if (!(arg0.at(i).is_number() == is_number && arg0.at(i).is_string() == is_string))
+                {
+                    ec = jsonpath_errc::invalid_type;
+                    return value_type::null();
+                }
+                if (arg0.at(i) < arg0.at(index))
+                {
+                    index = i;
+                }
+            }
+
+            return arg0.at(index);
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("min function");
+            return s;
+        }
+    };
+
+    template <class Json>
+    class max_function : public function_base<Json>
+    {
+    public:
+        using value_type = Json;
+        using parameter_type = parameter<Json>;
+
+        max_function()
+            : function_base<Json>(1)
+        {
+        }
+
+        value_type evaluate(const std::vector<parameter_type>& args, 
+                            std::error_code& ec) const override
+        {
+            if (args.size() != *this->arity())
+            {
+                ec = jsonpath_errc::invalid_arity;
+                return value_type::null();
+            }
+
+            auto arg0= args[0].value();
+            if (!arg0.is_array())
+            {
+                //std::cout << "arg: " << arg0 << "\n";
+                ec = jsonpath_errc::invalid_type;
+                return value_type::null();
+            }
+            if (arg0.empty())
+            {
+                return value_type::null();
+            }
+
+            bool is_number = arg0.at(0).is_number();
+            bool is_string = arg0.at(0).is_string();
+            if (!is_number && !is_string)
+            {
+                ec = jsonpath_errc::invalid_type;
+                return value_type::null();
+            }
+
+            std::size_t index = 0;
+            for (std::size_t i = 1; i < arg0.size(); ++i)
+            {
+                if (!(arg0.at(i).is_number() == is_number && arg0.at(i).is_string() == is_string))
+                {
+                    ec = jsonpath_errc::invalid_type;
+                    return value_type::null();
+                }
+                if (arg0.at(i) > arg0.at(index))
+                {
+                    index = i;
+                }
+            }
+
+            return arg0.at(index);
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("max function");
+            return s;
+        }
+    };
+
+    template <class Json>
+    class abs_function : public function_base<Json>
+    {
+    public:
+        using value_type = Json;
+        using parameter_type = parameter<Json>;
+
+        abs_function()
+            : function_base<Json>(1)
+        {
+        }
+
+        value_type evaluate(const std::vector<parameter_type>& args, 
+                            std::error_code& ec) const override
+        {
+            if (args.size() != *this->arity())
+            {
+                ec = jsonpath_errc::invalid_arity;
+                return value_type::null();
+            }
+
+            auto arg0= args[0].value();
+            switch (arg0.type())
+            {
+                case json_type::uint64_value:
+                    return arg0;
+                case json_type::int64_value:
+                {
+                    return arg0.template as<int64_t>() >= 0 ? arg0 : value_type(std::abs(arg0.template as<int64_t>()));
+                }
+                case json_type::double_value:
+                {
+                    return arg0.template as<double>() >= 0 ? arg0 : value_type(std::abs(arg0.template as<double>()));
+                }
+                default:
+                {
+                    ec = jsonpath_errc::invalid_type;
+                    return value_type::null();
+                }
+            }
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("abs function");
+            return s;
+        }
+    };
+
+    template <class Json>
+    class length_function : public function_base<Json>
+    {
+    public:
+        using value_type = Json;
+        using string_view_type = typename Json::string_view_type;
+        using parameter_type = parameter<Json>;
+
+        length_function()
+            : function_base<Json>(1)
+        {
+        }
+
+        value_type evaluate(const std::vector<parameter_type>& args, 
+                            std::error_code& ec) const override
+        {
+            if (args.size() != *this->arity())
+            {
+                ec = jsonpath_errc::invalid_arity;
+                return value_type::null();
+            }
+
+            auto arg0= args[0].value();
+            //std::cout << "length function arg: " << arg0 << "\n";
+
+            switch (arg0.type())
+            {
+                case json_type::object_value:
+                case json_type::array_value:
+                    return value_type(arg0.size());
+                case json_type::string_value:
+                {
+                    auto sv0 = arg0.template as<string_view_type>();
+                    auto length = unicode_traits::count_codepoints(sv0.data(), sv0.size());
+                    return value_type(length);
+                }
+                default:
+                {
+                    ec = jsonpath_errc::invalid_type;
+                    return value_type::null();
+                }
+            }
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("length function");
+            return s;
+        }
+    };
+
+    template <class Json>
+    class keys_function : public function_base<Json>
+    {
+    public:
+        using value_type = Json;
+        using parameter_type = parameter<Json>;
+        using string_view_type = typename Json::string_view_type;
+
+        keys_function()
+            : function_base<Json>(1)
+        {
+        }
+
+        value_type evaluate(const std::vector<parameter_type>& args, 
+                            std::error_code& ec) const override
+        {
+            if (args.size() != *this->arity())
+            {
+                ec = jsonpath_errc::invalid_arity;
+                return value_type::null();
+            }
+
+            auto arg0= args[0].value();
+            if (!arg0.is_object())
+            {
+                ec = jsonpath_errc::invalid_type;
+                return value_type::null();
+            }
+
+            value_type result(json_array_arg);
+            result.reserve(args.size());
+
+            for (auto& item : arg0.object_range())
+            {
+                result.emplace_back(item.key());
+            }
+            return result;
+        }
+
+        std::string to_string(int level = 0) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("keys function");
+            return s;
+        }
+    };
+
+    enum class jsonpath_token_kind
+    {
+        root_node,
+        current_node,
+        expression,
+        lparen,
+        rparen,
+        begin_union,
+        end_union,
+        begin_filter,
+        end_filter,
+        begin_expression,
+        end_index_expression,
+        end_argument_expression,
+        separator,
+        literal,
+        selector,
+        function,
+        end_function,
+        argument,
+        unary_operator,
+        binary_operator
+    };
+
+    inline
+    std::string to_string(jsonpath_token_kind kind)
+    {
+        switch (kind)
+        {
+            case jsonpath_token_kind::root_node:
+                return "root_node";
+            case jsonpath_token_kind::current_node:
+                return "current_node";
+            case jsonpath_token_kind::lparen:
+                return "lparen";
+            case jsonpath_token_kind::rparen:
+                return "rparen";
+            case jsonpath_token_kind::begin_union:
+                return "begin_union";
+            case jsonpath_token_kind::end_union:
+                return "end_union";
+            case jsonpath_token_kind::begin_filter:
+                return "begin_filter";
+            case jsonpath_token_kind::end_filter:
+                return "end_filter";
+            case jsonpath_token_kind::begin_expression:
+                return "begin_expression";
+            case jsonpath_token_kind::end_index_expression:
+                return "end_index_expression";
+            case jsonpath_token_kind::end_argument_expression:
+                return "end_argument_expression";
+            case jsonpath_token_kind::separator:
+                return "separator";
+            case jsonpath_token_kind::literal:
+                return "literal";
+            case jsonpath_token_kind::selector:
+                return "selector";
+            case jsonpath_token_kind::function:
+                return "function";
+            case jsonpath_token_kind::end_function:
+                return "end_function";
+            case jsonpath_token_kind::argument:
+                return "argument";
+            case jsonpath_token_kind::unary_operator:
+                return "unary_operator";
+            case jsonpath_token_kind::binary_operator:
+                return "binary_operator";
+            default:
+                return "";
+        }
+    }
+
+    template <class Json,class JsonReference>
+    struct path_value_pair
+    {
+        using char_type = typename Json::char_type;
+        using string_type = std::basic_string<char_type,std::char_traits<char_type>>;
+        using value_type = Json;
+        using reference = JsonReference;
+        using value_pointer = typename std::conditional<std::is_const<typename std::remove_reference<JsonReference>::type>::value,typename Json::const_pointer,typename Json::pointer>::type;
+        using json_location_node_type = json_location_node<char_type>;
+        using json_location_type = json_location<char_type>;
+        using path_pointer = const json_location_node_type*;
+
+        json_location_type path_;
+        value_pointer value_ptr_;
+
+        path_value_pair(const json_location_type& path, reference value) noexcept
+            : path_(path), value_ptr_(std::addressof(value))
+        {
+        }
+
+        path_value_pair(json_location_type&& path, value_pointer valp) noexcept
+            : path_(std::move(path)), value_ptr_(valp)
+        {
+        }
+
+        path_value_pair(const path_value_pair&) = default;
+        path_value_pair(path_value_pair&& other) = default;
+        path_value_pair& operator=(const path_value_pair&) = default;
+        path_value_pair& operator=(path_value_pair&& other) = default;
+
+        json_location_type path() const
+        {
+            return path_;
+        }
+
+        reference value() 
+        {
+            return *value_ptr_;
+        }
+    };
+ 
+    template <class Json,class JsonReference>
+    struct path_value_pair_less
+    {
+        bool operator()(const path_value_pair<Json,JsonReference>& lhs,
+                        const path_value_pair<Json,JsonReference>& rhs) const noexcept
+        {
+            return lhs.path() < rhs.path();
+        }
+    };
+
+    template <class Json,class JsonReference>
+    struct path_value_pair_equal
+    {
+        bool operator()(const path_value_pair<Json,JsonReference>& lhs,
+                        const path_value_pair<Json,JsonReference>& rhs) const noexcept
+        {
+            return lhs.path() == rhs.path();
+        }
+    };
+
+    template <class Json,class JsonReference>
+    struct path_component_value_pair
+    {
+        using char_type = typename Json::char_type;
+        using string_type = std::basic_string<char_type,std::char_traits<char_type>>;
+        using value_type = Json;
+        using reference = JsonReference;
+        using value_pointer = typename std::conditional<std::is_const<typename std::remove_reference<JsonReference>::type>::value,typename Json::const_pointer,typename Json::pointer>::type;
+        using json_location_node_type = json_location_node<char_type>;
+        using json_location_type = json_location<char_type>;
+        using path_pointer = const json_location_node_type*;
+    private:
+        const json_location_node_type* last_ptr_;
+        value_pointer value_ptr_;
+    public:
+        path_component_value_pair(const json_location_node_type& last, reference value) noexcept
+            : last_ptr_(std::addressof(last)), value_ptr_(std::addressof(value))
+        {
+        }
+
+        const json_location_node_type& last() const
+        {
+            return *last_ptr_;
+        }
+
+        reference value() const
+        {
+            return *value_ptr_;
+        }
+    };
+
+    template <class Json,class JsonReference>
+    class node_receiver
+    {
+    public:
+        using char_type = typename Json::char_type;
+        using reference = JsonReference;
+        using json_location_node_type = json_location_node<char_type>;
+
+        virtual ~node_receiver() noexcept = default;
+
+        virtual void add(const json_location_node_type& path_tail, 
+                         reference value) = 0;
+    };
+
+    template <class Json,class JsonReference>
+    class path_value_receiver : public node_receiver<Json,JsonReference>
+    {
+    public:
+        using reference = JsonReference;
+        using char_type = typename Json::char_type;
+        using json_location_node_type = json_location_node<char_type>;
+        using json_location_type = json_location<char_type>;
+        using path_value_pair_type = path_value_pair<Json,JsonReference>;
+
+        std::vector<path_value_pair_type> nodes;
+
+        void add(const json_location_node_type& path_tail, 
+                 reference value) override
+        {
+            nodes.emplace_back(json_location_type(path_tail), std::addressof(value));
+        }
+    };
+
+    template <class Json,class JsonReference>
+    class path_stem_value_receiver : public node_receiver<Json,JsonReference>
+    {
+    public:
+        using reference = JsonReference;
+        using char_type = typename Json::char_type;
+        using json_location_node_type = json_location_node<char_type>;
+        using path_stem_value_pair_type = path_component_value_pair<Json,JsonReference>;
+
+        std::vector<path_stem_value_pair_type> nodes;
+
+        void add(const json_location_node_type& path_tail, 
+                 reference value) override
+        {
+            nodes.emplace_back(path_tail, value);
+        }
+    };
+
+    template <class Json, class JsonReference>
+    class dynamic_resources
+    {
+        using reference = JsonReference;
+        using pointer = typename std::conditional<std::is_const<typename std::remove_reference<reference>::type>::value,typename Json::const_pointer,typename Json::pointer>::type;
+        using json_location_node_type = json_location_node<typename Json::char_type>;
+        using path_stem_value_pair_type = path_component_value_pair<Json,JsonReference>;
+        std::vector<std::unique_ptr<Json>> temp_json_values_;
+        std::vector<std::unique_ptr<json_location_node_type>> temp_path_node_values_;
+        std::unordered_map<std::size_t,pointer> cache_;
+    public:
+        bool is_cached(std::size_t id) const
+        {
+            return cache_.find(id) != cache_.end();
+        }
+        void add_to_cache(std::size_t id, reference val) 
+        {
+            cache_.emplace(id, std::addressof(val));
+        }
+        reference retrieve_from_cache(std::size_t id) 
+        {
+            return *cache_[id];
+        }
+
+        reference null_value()
+        {
+            static Json j{ null_type{} };
+            return j;
+        }
+
+        template <typename... Args>
+        Json* create_json(Args&& ... args)
+        {
+            auto temp = jsoncons::make_unique<Json>(std::forward<Args>(args)...);
+            Json* ptr = temp.get();
+            temp_json_values_.emplace_back(std::move(temp));
+            return ptr;
+        }
+
+        const json_location_node_type& root_path_node() const
+        {
+            static json_location_node_type root('$');
+            return root;
+        }
+
+        const json_location_node_type& current_path_node() const
+        {
+            static json_location_node_type root('@');
+            return root;
+        }
+
+        template <typename... Args>
+        const json_location_node_type* create_path_node(Args&& ... args)
+        {
+            auto temp = jsoncons::make_unique<json_location_node_type>(std::forward<Args>(args)...);
+            json_location_node_type* ptr = temp.get();
+            temp_path_node_values_.emplace_back(std::move(temp));
+            return ptr;
+        }
+    };
+
+    template <class Json,class JsonReference>
+    struct node_less
+    {
+        bool operator()(const path_value_pair<Json,JsonReference>& a, const path_value_pair<Json,JsonReference>& b) const
+        {
+            return *(a.ptr) < *(b.ptr);
+        }
+    };
+
+    template <class Json,class JsonReference>
+    class jsonpath_selector
+    {
+        bool is_path_;
+        std::size_t precedence_level_;
+
+    public:
+        using char_type = typename Json::char_type;
+        using string_type = std::basic_string<char_type,std::char_traits<char_type>>;
+        using string_view_type = jsoncons::basic_string_view<char_type, std::char_traits<char_type>>;
+        using value_type = Json;
+        using reference = JsonReference;
+        using pointer = typename std::conditional<std::is_const<typename std::remove_reference<JsonReference>::type>::value,typename Json::const_pointer,typename Json::pointer>::type;
+        using path_value_pair_type = path_value_pair<Json,JsonReference>;
+        using json_location_node_type = json_location_node<char_type>;
+        using json_location_type = json_location<char_type>;
+        using node_receiver_type = node_receiver<Json,JsonReference>;
+        using selector_type = jsonpath_selector<Json,JsonReference>;
+
+        jsonpath_selector(bool is_path,
+                          std::size_t precedence_level = 0)
+            : is_path_(is_path), 
+              precedence_level_(precedence_level)
+        {
+        }
+
+        virtual ~jsonpath_selector() noexcept = default;
+
+        bool is_path() const 
+        {
+            return is_path_;
+        }
+
+        std::size_t precedence_level() const
+        {
+            return precedence_level_;
+        }
+
+        bool is_right_associative() const
+        {
+            return true;
+        }
+
+        virtual void select(dynamic_resources<Json,JsonReference>& resources,
+                            reference root,
+                            const json_location_node_type& path_tail, 
+                            reference val, 
+                            node_receiver_type& receiver,
+                            result_options options) const = 0;
+
+        virtual reference evaluate(dynamic_resources<Json,JsonReference>& resources,
+                                   reference root,
+                                   const json_location_node_type& path_tail, 
+                                   reference current, 
+                                   result_options options,
+                                   std::error_code& ec) const = 0;
+
+        virtual void append_selector(jsonpath_selector*) 
+        {
+        }
+
+        virtual std::string to_string(int = 0) const
+        {
+            return std::string();
+        }
+    };
+
+    template <class Json, class JsonReference>
+    struct static_resources
+    {
+        using char_type = typename Json::char_type;
+        using string_type = std::basic_string<char_type>;
+        using value_type = Json;
+        using reference = JsonReference;
+        using function_base_type = function_base<Json>;
+        using selector_type = jsonpath_selector<Json,JsonReference>;
+
+        std::vector<std::unique_ptr<selector_type>> selectors_;
+        std::vector<std::unique_ptr<Json>> temp_json_values_;
+        std::vector<std::unique_ptr<unary_operator<Json,JsonReference>>> unary_operators_;
+        std::unordered_map<string_type,std::unique_ptr<function_base_type>> custom_functions_;
+
+        static_resources()
+        {
+        }
+
+        static_resources(const custom_functions<Json>& functions)
+        {
+            for (const auto& item : functions)
+            {
+                custom_functions_.emplace(item.name(),
+                                          jsoncons::make_unique<decorator_function<Json>>(item.arity(),item.function()));
+            }
+        }
+
+        static_resources(const static_resources&) = default;
+
+        static_resources(static_resources&& other) noexcept 
+            : selectors_(std::move(other.selectors_)),
+              temp_json_values_(std::move(other.temp_json_values_)),
+              unary_operators_(std::move(other.unary_operators_)),
+              custom_functions_(std::move(other.custom_functions_))
+        {
+        }
+
+        const function_base_type* get_function(const string_type& name, std::error_code& ec) const
+        {
+            static abs_function<Json> abs_func;
+            static contains_function<Json> contains_func;
+            static starts_with_function<Json> starts_with_func;
+            static ends_with_function<Json> ends_with_func;
+            static ceil_function<Json> ceil_func;
+            static floor_function<Json> floor_func;
+            static to_number_function<Json> to_number_func;
+            static sum_function<Json> sum_func;
+            static prod_function<Json> prod_func;
+            static avg_function<Json> avg_func;
+            static min_function<Json> min_func;
+            static max_function<Json> max_func;
+            static length_function<Json> length_func;
+            static keys_function<Json> keys_func;
+#if defined(JSONCONS_HAS_STD_REGEX)
+            static tokenize_function<Json> tokenize_func;
+#endif
+
+            static std::unordered_map<string_type,const function_base_type*> functions =
+            {
+                {string_type{'a','b','s'}, &abs_func},
+                {string_type{'c','o','n','t','a','i','n','s'}, &contains_func},
+                {string_type{'s','t','a','r','t','s','_','w','i','t','h'}, &starts_with_func},
+                {string_type{'e','n','d','s','_','w','i','t','h'}, &ends_with_func},
+                {string_type{'c','e','i','l'}, &ceil_func},
+                {string_type{'f','l','o','o','r'}, &floor_func},
+                {string_type{'t','o','_','n','u','m','b','e','r'}, &to_number_func},
+                {string_type{'s','u','m'}, &sum_func},
+                {string_type{'p','r','o', 'd'}, &prod_func},
+                {string_type{'a','v','g'}, &avg_func},
+                {string_type{'m','i','n'}, &min_func},
+                {string_type{'m','a','x'}, &max_func},
+                {string_type{'l','e','n','g','t','h'}, &length_func},
+                {string_type{'k','e','y','s'}, &keys_func},
+#if defined(JSONCONS_HAS_STD_REGEX)
+                {string_type{'t','o','k','e','n','i','z','e'}, &tokenize_func},
+#endif
+                {string_type{'c','o','u','n','t'}, &length_func}
+            };
+
+            auto it = functions.find(name);
+            if (it == functions.end())
+            {
+                auto it2 = custom_functions_.find(name);
+                if (it2 == custom_functions_.end())
+                {
+                    ec = jsonpath_errc::unknown_function;
+                    return nullptr;
+                }
+                else
+                {
+                    return it2->second.get();
+                }
+            }
+            else
+            {
+                return it->second;
+            }
+        }
+
+        const unary_operator<Json,JsonReference>* get_unary_not() const
+        {
+            static unary_not_operator<Json,JsonReference> oper;
+            return &oper;
+        }
+
+        const unary_operator<Json,JsonReference>* get_unary_minus() const
+        {
+            static unary_minus_operator<Json,JsonReference> oper;
+            return &oper;
+        }
+
+        const unary_operator<Json,JsonReference>* get_regex_operator(std::basic_regex<char_type>&& pattern) 
+        {
+            unary_operators_.push_back(jsoncons::make_unique<regex_operator<Json,JsonReference>>(std::move(pattern)));
+            return unary_operators_.back().get();
+        }
+
+        const binary_operator<Json,JsonReference>* get_or_operator() const
+        {
+            static or_operator<Json,JsonReference> oper;
+
+            return &oper;
+        }
+
+        const binary_operator<Json,JsonReference>* get_and_operator() const
+        {
+            static and_operator<Json,JsonReference> oper;
+
+            return &oper;
+        }
+
+        const binary_operator<Json,JsonReference>* get_eq_operator() const
+        {
+            static eq_operator<Json,JsonReference> oper;
+            return &oper;
+        }
+
+        const binary_operator<Json,JsonReference>* get_ne_operator() const
+        {
+            static ne_operator<Json,JsonReference> oper;
+            return &oper;
+        }
+
+        const binary_operator<Json,JsonReference>* get_lt_operator() const
+        {
+            static lt_operator<Json,JsonReference> oper;
+            return &oper;
+        }
+
+        const binary_operator<Json,JsonReference>* get_lte_operator() const
+        {
+            static lte_operator<Json,JsonReference> oper;
+            return &oper;
+        }
+
+        const binary_operator<Json,JsonReference>* get_gt_operator() const
+        {
+            static gt_operator<Json,JsonReference> oper;
+            return &oper;
+        }
+
+        const binary_operator<Json,JsonReference>* get_gte_operator() const
+        {
+            static gte_operator<Json,JsonReference> oper;
+            return &oper;
+        }
+
+        const binary_operator<Json,JsonReference>* get_plus_operator() const
+        {
+            static plus_operator<Json,JsonReference> oper;
+            return &oper;
+        }
+
+        const binary_operator<Json,JsonReference>* get_minus_operator() const
+        {
+            static minus_operator<Json,JsonReference> oper;
+            return &oper;
+        }
+
+        const binary_operator<Json,JsonReference>* get_mult_operator() const
+        {
+            static mult_operator<Json,JsonReference> oper;
+            return &oper;
+        }
+
+        const binary_operator<Json,JsonReference>* get_div_operator() const
+        {
+            static div_operator<Json,JsonReference> oper;
+            return &oper;
+        }
+
+        const binary_operator<Json,JsonReference>* get_modulus_operator() const
+        {
+            static modulus_operator<Json,JsonReference> oper;
+            return &oper;
+        }
+
+        template <typename T>
+        selector_type* new_selector(T&& val)
+        {
+            selectors_.emplace_back(jsoncons::make_unique<T>(std::forward<T>(val)));
+            return selectors_.back().get();
+        }
+
+        template <typename... Args>
+        Json* create_json(Args&& ... args)
+        {
+            auto temp = jsoncons::make_unique<Json>(std::forward<Args>(args)...);
+            Json* ptr = temp.get();
+            temp_json_values_.emplace_back(std::move(temp));
+            return ptr;
+        }
+    };
+
+    template <class Json, class JsonReference>
+    class expression_base
+    {
+    public:
+        using char_type = typename Json::char_type;
+        using string_type = std::basic_string<char_type,std::char_traits<char_type>>;
+        using string_view_type = jsoncons::basic_string_view<char_type, std::char_traits<char_type>>;
+        using value_type = Json;
+        using reference = JsonReference;
+        using pointer = typename std::conditional<std::is_const<typename std::remove_reference<JsonReference>::type>::value,typename Json::const_pointer,typename Json::pointer>::type;
+        using path_value_pair_type = path_value_pair<Json,JsonReference>;
+        using json_location_node_type = json_location_node<char_type>;
+
+        virtual ~expression_base() noexcept = default;
+
+        virtual value_type evaluate(dynamic_resources<Json,JsonReference>& resources,
+                                           reference root,
+                                           //const json_location_node_type& path, 
+                                           reference val, 
+                                           result_options options,
+                                           std::error_code& ec) const = 0;
+
+        virtual std::string to_string(int level = 0) const = 0;
+    };
+
+    template <class Json,class JsonReference>
+    class token
+    {
+    public:
+        using selector_type = jsonpath_selector<Json,JsonReference>;
+        using expression_base_type = expression_base<Json,JsonReference>;
+
+        jsonpath_token_kind token_kind_;
+
+        union
+        {
+            selector_type* selector_;
+            std::unique_ptr<expression_base_type> expression_;
+            const unary_operator<Json,JsonReference>* unary_operator_;
+            const binary_operator<Json,JsonReference>* binary_operator_;
+            const function_base<Json>* function_;
+            Json value_;
+        };
+    public:
+
+        token(const unary_operator<Json,JsonReference>* expr) noexcept
+            : token_kind_(jsonpath_token_kind::unary_operator),
+              unary_operator_(expr)
+        {
+        }
+
+        token(const binary_operator<Json,JsonReference>* expr) noexcept
+            : token_kind_(jsonpath_token_kind::binary_operator),
+              binary_operator_(expr)
+        {
+        }
+
+        token(current_node_arg_t) noexcept
+            : token_kind_(jsonpath_token_kind::current_node)
+        {
+        }
+
+        token(root_node_arg_t) noexcept
+            : token_kind_(jsonpath_token_kind::root_node)
+        {
+        }
+
+        token(end_function_arg_t) noexcept
+            : token_kind_(jsonpath_token_kind::end_function)
+        {
+        }
+
+        token(separator_arg_t) noexcept
+            : token_kind_(jsonpath_token_kind::separator)
+        {
+        }
+
+        token(lparen_arg_t) noexcept
+            : token_kind_(jsonpath_token_kind::lparen)
+        {
+        }
+
+        token(rparen_arg_t) noexcept
+            : token_kind_(jsonpath_token_kind::rparen)
+        {
+        }
+
+        token(begin_union_arg_t) noexcept
+            : token_kind_(jsonpath_token_kind::begin_union)
+        {
+        }
+
+        token(end_union_arg_t) noexcept
+            : token_kind_(jsonpath_token_kind::end_union)
+        {
+        }
+
+        token(begin_filter_arg_t) noexcept
+            : token_kind_(jsonpath_token_kind::begin_filter)
+        {
+        }
+
+        token(end_filter_arg_t) noexcept
+            : token_kind_(jsonpath_token_kind::end_filter)
+        {
+        }
+
+        token(begin_expression_arg_t) noexcept
+            : token_kind_(jsonpath_token_kind::begin_expression)
+        {
+        }
+
+        token(end_index_expression_arg_t) noexcept
+            : token_kind_(jsonpath_token_kind::end_index_expression)
+        {
+        }
+
+        token(end_argument_expression_arg_t) noexcept
+            : token_kind_(jsonpath_token_kind::end_argument_expression)
+        {
+        }
+
+        token(selector_type* selector)
+            : token_kind_(jsonpath_token_kind::selector), selector_(selector)
+        {
+        }
+
+        token(std::unique_ptr<expression_base_type>&& expr)
+            : token_kind_(jsonpath_token_kind::expression)
+        {
+            new (&expression_) std::unique_ptr<expression_base_type>(std::move(expr));
+        }
+
+        token(const function_base<Json>* function) noexcept
+            : token_kind_(jsonpath_token_kind::function),
+              function_(function)
+        {
+        }
+
+        token(argument_arg_t) noexcept
+            : token_kind_(jsonpath_token_kind::argument)
+        {
+        }
+
+        token(literal_arg_t, Json&& value) noexcept
+            : token_kind_(jsonpath_token_kind::literal), value_(std::move(value))
+        {
+        }
+
+        token(token&& other) noexcept
+        {
+            construct(std::forward<token>(other));
+        }
+
+        const Json& get_value(const_reference_arg_t, dynamic_resources<Json,JsonReference>&) const
+        {
+            return value_;
+        }
+
+        Json& get_value(reference_arg_t, dynamic_resources<Json,JsonReference>& resources) const
+        {
+            return *resources.create_json(value_);
+        }
+
+        token& operator=(token&& other)
+        {
+            if (&other != this)
+            {
+                if (token_kind_ == other.token_kind_)
+                {
+                    switch (token_kind_)
+                    {
+                        case jsonpath_token_kind::selector:
+                            selector_ = other.selector_;
+                            break;
+                        case jsonpath_token_kind::expression:
+                            expression_ = std::move(other.expression_);
+                            break;
+                        case jsonpath_token_kind::unary_operator:
+                            unary_operator_ = other.unary_operator_;
+                            break;
+                        case jsonpath_token_kind::binary_operator:
+                            binary_operator_ = other.binary_operator_;
+                            break;
+                        case jsonpath_token_kind::function:
+                            function_ = other.function_;
+                            break;
+                        case jsonpath_token_kind::literal:
+                            value_ = std::move(other.value_);
+                            break;
+                        default:
+                            break;
+                    }
+                }
+                else
+                {
+                    destroy();
+                    construct(std::forward<token>(other));
+                }
+            }
+            return *this;
+        }
+
+        ~token() noexcept
+        {
+            destroy();
+        }
+
+        jsonpath_token_kind token_kind() const
+        {
+            return token_kind_;
+        }
+
+        bool is_lparen() const
+        {
+            return token_kind_ == jsonpath_token_kind::lparen; 
+        }
+
+        bool is_rparen() const
+        {
+            return token_kind_ == jsonpath_token_kind::rparen; 
+        }
+
+        bool is_current_node() const
+        {
+            return token_kind_ == jsonpath_token_kind::current_node; 
+        }
+
+        bool is_path() const
+        {
+            return token_kind_ == jsonpath_token_kind::selector && selector_->is_path(); 
+        }
+
+        bool is_operator() const
+        {
+            return token_kind_ == jsonpath_token_kind::unary_operator || 
+                   token_kind_ == jsonpath_token_kind::binary_operator; 
+        }
+
+        std::size_t precedence_level() const
+        {
+            switch(token_kind_)
+            {
+                case jsonpath_token_kind::selector:
+                    return selector_->precedence_level();
+                case jsonpath_token_kind::unary_operator:
+                    return unary_operator_->precedence_level();
+                case jsonpath_token_kind::binary_operator:
+                    return binary_operator_->precedence_level();
+                default:
+                    return 0;
+            }
+        }
+
+        jsoncons::optional<std::size_t> arity() const
+        {
+            return token_kind_ == jsonpath_token_kind::function ? function_->arity() : jsoncons::optional<std::size_t>();
+        }
+
+        bool is_right_associative() const
+        {
+            switch(token_kind_)
+            {
+                case jsonpath_token_kind::selector:
+                    return selector_->is_right_associative();
+                case jsonpath_token_kind::unary_operator:
+                    return unary_operator_->is_right_associative();
+                case jsonpath_token_kind::binary_operator:
+                    return binary_operator_->is_right_associative();
+                default:
+                    return false;
+            }
+        }
+
+        void construct(token&& other)
+        {
+            token_kind_ = other.token_kind_;
+            switch (token_kind_)
+            {
+                case jsonpath_token_kind::selector:
+                    selector_ = other.selector_;
+                    break;
+                case jsonpath_token_kind::expression:
+                    new (&expression_) std::unique_ptr<expression_base_type>(std::move(other.expression_));
+                    break;
+                case jsonpath_token_kind::unary_operator:
+                    unary_operator_ = other.unary_operator_;
+                    break;
+                case jsonpath_token_kind::binary_operator:
+                    binary_operator_ = other.binary_operator_;
+                    break;
+                case jsonpath_token_kind::function:
+                    function_ = other.function_;
+                    break;
+                case jsonpath_token_kind::literal:
+                    new (&value_) Json(std::move(other.value_));
+                    break;
+                default:
+                    break;
+            }
+        }
+
+        void destroy() noexcept 
+        {
+            switch(token_kind_)
+            {
+                case jsonpath_token_kind::expression:
+                    expression_.~unique_ptr();
+                    break;
+                case jsonpath_token_kind::literal:
+                    value_.~Json();
+                    break;
+                default:
+                    break;
+            }
+        }
+
+        std::string to_string(int level = 0) const
+        {
+            std::string s;
+            switch (token_kind_)
+            {
+                case jsonpath_token_kind::root_node:
+                    if (level > 0)
+                    {
+                        s.append("\n");
+                        s.append(level*2, ' ');
+                    }
+                    s.append("root node");
+                    break;
+                case jsonpath_token_kind::current_node:
+                    if (level > 0)
+                    {
+                        s.append("\n");
+                        s.append(level*2, ' ');
+                    }
+                    s.append("current node");
+                    break;
+                case jsonpath_token_kind::argument:
+                    if (level > 0)
+                    {
+                        s.append("\n");
+                        s.append(level*2, ' ');
+                    }
+                    s.append("argument");
+                    break;
+                case jsonpath_token_kind::selector:
+                    s.append(selector_->to_string(level));
+                    break;
+                case jsonpath_token_kind::expression:
+                    s.append(expression_->to_string(level));
+                    break;
+                case jsonpath_token_kind::literal:
+                {
+                    if (level > 0)
+                    {
+                        s.append("\n");
+                        s.append(level*2, ' ');
+                    }
+                    auto sbuf = value_.to_string();
+                    unicode_traits::convert(sbuf.data(), sbuf.size(), s);
+                    break;
+                }
+                case jsonpath_token_kind::binary_operator:
+                    s.append(binary_operator_->to_string(level));
+                    break;
+                case jsonpath_token_kind::function:
+                    s.append(function_->to_string(level));
+                    break;
+                default:
+                    if (level > 0)
+                    {
+                        s.append("\n");
+                        s.append(level*2, ' ');
+                    }
+                    s.append("token kind: ");
+                    s.append(jsoncons::jsonpath::detail::to_string(token_kind_));
+                    break;
+            }
+            //s.append("\n");
+            return s;
+        }
+    };
+
+    template <class Callback, class Json,class JsonReference>
+    class callback_receiver : public node_receiver<Json,JsonReference>
+    {
+        Callback& callback_;
+    public:
+        using reference = JsonReference;
+        using char_type = typename Json::char_type;
+        using json_location_node_type = json_location_node<char_type>;
+        using json_location_type = json_location<char_type>;
+
+        callback_receiver(Callback& callback)
+            : callback_(callback)
+        {
+        }
+
+        void add(const json_location_node_type& path_tail, 
+                 reference value) override
+        {
+            callback_(json_location_type(path_tail), value);
+        }
+    };
+
+    template <class Json,class JsonReference>
+    class path_expression
+    {
+    public:
+        using char_type = typename Json::char_type;
+        using string_type = std::basic_string<char_type,std::char_traits<char_type>>;
+        using string_view_type = typename Json::string_view_type;
+        using path_value_pair_type = path_value_pair<Json,JsonReference>;
+        using path_value_pair_less_type = path_value_pair_less<Json,JsonReference>;
+        using path_value_pair_equal_type = path_value_pair_equal<Json,JsonReference>;
+        using value_type = Json;
+        using reference = typename path_value_pair_type::reference;
+        using pointer = typename path_value_pair_type::value_pointer;
+        using token_type = token<Json,JsonReference>;
+        using reference_arg_type = typename std::conditional<std::is_const<typename std::remove_reference<JsonReference>::type>::value,
+            const_reference_arg_t,reference_arg_t>::type;
+        using json_location_node_type = json_location_node<char_type>;
+        using json_location_type = json_location<char_type>;
+        using selector_type = jsonpath_selector<Json,JsonReference>;
+    private:
+        selector_type* selector_;
+        result_options required_options_;
+    public:
+
+        path_expression()
+            : required_options_()
+        {
+        }
+
+        path_expression(path_expression&& expr) = default;
+
+        path_expression(selector_type* selector, bool paths_required)
+            : selector_(selector), required_options_()
+        {
+            if (paths_required)
+            {
+                required_options_ |= result_options::path;
+            }
+        }
+
+        path_expression& operator=(path_expression&& expr) = default;
+
+        Json evaluate(dynamic_resources<Json,JsonReference>& resources, 
+                      reference root,
+                      const json_location_node_type& path, 
+                      reference instance,
+                      result_options options) const
+        {
+            Json result(json_array_arg);
+
+            if ((options & result_options::path) == result_options::path)
+            {
+                auto callback = [&result](const json_location_type& path, reference)
+                {
+                    result.emplace_back(path.to_string());
+                };
+                evaluate(resources, root, path, instance, callback, options);
+            }
+            else
+            {
+                auto callback = [&result](const json_location_type&, reference val)
+                {
+                    result.push_back(val);
+                };
+                evaluate(resources, root, path, instance, callback, options);
+            }
+
+            return result;
+        }
+
+        template <class Callback>
+        typename std::enable_if<type_traits::is_binary_function_object<Callback,const json_location_type&,reference>::value,void>::type
+        evaluate(dynamic_resources<Json,JsonReference>& resources, 
+                 reference root,
+                 const json_location_node_type& path, 
+                 reference current, 
+                 Callback callback,
+                 result_options options) const
+        {
+            std::error_code ec;
+
+            options |= required_options_;
+
+            const result_options require_more = result_options::nodups | result_options::sort;
+
+            if ((options & require_more) != result_options())
+            {
+                path_value_receiver<Json,JsonReference> receiver;
+                selector_->select(resources, root, path, current, receiver, options);
+
+                if (receiver.nodes.size() > 1 && (options & result_options::sort) == result_options::sort)
+                {
+                    std::sort(receiver.nodes.begin(), receiver.nodes.end(), path_value_pair_less_type());
+                }
+
+                if (receiver.nodes.size() > 1 && (options & result_options::nodups) == result_options::nodups)
+                {
+                    if ((options & result_options::sort) == result_options::sort)
+                    {
+                        auto last = std::unique(receiver.nodes.begin(),receiver.nodes.end(),path_value_pair_equal_type());
+                        receiver.nodes.erase(last,receiver.nodes.end());
+                        for (auto& node : receiver.nodes)
+                        {
+                            callback(node.path(), node.value());
+                        }
+                    }
+                    else
+                    {
+                        std::vector<path_value_pair_type> index(receiver.nodes);
+                        std::sort(index.begin(), index.end(), path_value_pair_less_type());
+                        auto last = std::unique(index.begin(),index.end(),path_value_pair_equal_type());
+                        index.erase(last,index.end());
+
+                        std::vector<path_value_pair_type> temp2;
+                        temp2.reserve(index.size());
+                        for (auto&& node : receiver.nodes)
+                        {
+                            auto it = std::lower_bound(index.begin(),index.end(),node, path_value_pair_less_type());
+                            if (it != index.end() && it->path() == node.path()) 
+                            {
+                                temp2.emplace_back(std::move(node));
+                                index.erase(it);
+                            }
+                        }
+                        for (auto& node : temp2)
+                        {
+                            callback(node.path(), node.value());
+                        }
+                    }
+                }
+                else
+                {
+                    for (auto& node : receiver.nodes)
+                    {
+                        callback(node.path(), node.value());
+                    }
+                }
+            }
+            else
+            {
+                callback_receiver<Callback,Json,JsonReference> receiver(callback);
+                selector_->select(resources, root, path, current, receiver, options);
+            }
+        }
+
+        std::string to_string(int level) const
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("expression ");
+            s.append(selector_->to_string(level+1));
+
+            return s;
+
+        }
+    };
+
+    template <class Json,class JsonReference>
+    class expression : public expression_base<Json,JsonReference>
+    {
+    public:
+        using path_value_pair_type = path_value_pair<Json,JsonReference>;
+        using value_type = Json;
+        using reference = typename path_value_pair_type::reference;
+        using pointer = typename path_value_pair_type::value_pointer;
+        using const_pointer = const value_type*;
+        using char_type = typename Json::char_type;
+        using string_type = std::basic_string<char_type,std::char_traits<char_type>>;
+        using string_view_type = typename Json::string_view_type;
+        using path_value_pair_less_type = path_value_pair_less<Json,reference>;
+        using path_value_pair_equal_type = path_value_pair_equal<Json,reference>;
+        using parameter_type = parameter<Json>;
+        using token_type = token<Json,reference>;
+        using reference_arg_type = typename std::conditional<std::is_const<typename std::remove_reference<reference>::type>::value,
+            const_reference_arg_t,reference_arg_t>::type;
+        using json_location_node_type = json_location_node<char_type>;
+        using stack_item_type = value_or_pointer<Json,JsonReference>;
+    private:
+        std::vector<token_type> token_list_;
+    public:
+
+        expression()
+        {
+        }
+
+        expression(expression&& expr)
+            : token_list_(std::move(expr.token_list_))
+        {
+        }
+
+        expression(std::vector<token_type>&& token_stack)
+            : token_list_(std::move(token_stack))
+        {
+        }
+
+        expression& operator=(expression&& expr) = default;
+
+        value_type evaluate(dynamic_resources<Json,reference>& resources, 
+                                   reference root,
+                                   reference current,
+                                   result_options options,
+                                   std::error_code& ec) const override
+        {
+            std::vector<stack_item_type> stack;
+            std::vector<parameter_type> arg_stack;
+
+            //std::cout << "EVALUATE TOKENS\n";
+            //for (auto& tok : token_list_)
+            //{
+            //    std::cout << tok.to_string() << "\n";
+            //}
+            //std::cout << "\n";
+
+            if (!token_list_.empty())
+            {
+                for (auto& tok : token_list_)
+                {
+                    //std::cout << "Token: " << tok.to_string() << "\n";
+                    switch (tok.token_kind())
+                    { 
+                        case jsonpath_token_kind::literal:
+                        {
+                            stack.emplace_back(std::addressof(tok.get_value(reference_arg_type(), resources)));
+                            break;
+                        }
+                        case jsonpath_token_kind::unary_operator:
+                        {
+                            JSONCONS_ASSERT(stack.size() >= 1);
+                            auto item = std::move(stack.back());
+                            stack.pop_back();
+
+                            auto val = tok.unary_operator_->evaluate(item.value(), ec);
+                            stack.emplace_back(std::move(val));
+                            break;
+                        }
+                        case jsonpath_token_kind::binary_operator:
+                        {
+                            //std::cout << "binary operator: " << stack.size() << "\n";
+                            JSONCONS_ASSERT(stack.size() >= 2);
+                            auto rhs = std::move(stack.back());
+                            //std::cout << "rhs: " << *rhs << "\n";
+                            stack.pop_back();
+                            auto lhs = std::move(stack.back());
+                            //std::cout << "lhs: " << *lhs << "\n";
+                            stack.pop_back();
+
+                            auto val = tok.binary_operator_->evaluate(lhs.value(), rhs.value(), ec);
+                            //std::cout << "Evaluate binary expression: " << r << "\n";
+                            stack.emplace_back(std::move(val));
+                            break;
+                        }
+                        case jsonpath_token_kind::root_node:
+                            //std::cout << "root: " << root << "\n";
+                            stack.emplace_back(std::addressof(root));
+                            break;
+                        case jsonpath_token_kind::current_node:
+                            //std::cout << "current: " << current << "\n";
+                            stack.emplace_back(std::addressof(current));
+                            break;
+                        case jsonpath_token_kind::argument:
+                            JSONCONS_ASSERT(!stack.empty());
+                            //std::cout << "argument stack items " << stack.size() << "\n";
+                            //for (auto& item : stack)
+                            //{
+                            //    std::cout << *item.to_pointer(resources) << "\n";
+                            //}
+                            //std::cout << "\n";
+                            arg_stack.emplace_back(std::move(stack.back()));
+                            //for (auto& item : arg_stack)
+                            //{
+                            //    std::cout << *item << "\n";
+                            //}
+                            //std::cout << "\n";
+                            stack.pop_back();
+                            break;
+                        case jsonpath_token_kind::function:
+                        {
+                            if (tok.function_->arity() && *(tok.function_->arity()) != arg_stack.size())
+                            {
+                                ec = jsonpath_errc::invalid_arity;
+                                return Json::null();
+                            }
+                            //std::cout << "function arg stack:\n";
+                            //for (auto& item : arg_stack)
+                            //{
+                            //    std::cout << *item << "\n";
+                            //}
+                            //std::cout << "\n";
+
+                            value_type val = tok.function_->evaluate(arg_stack, ec);
+                            if (ec)
+                            {
+                                return Json::null();
+                            }
+                            //std::cout << "function result: " << val << "\n";
+                            arg_stack.clear();
+                            stack.emplace_back(std::move(val));
+                            break;
+                        }
+                        case jsonpath_token_kind::expression:
+                        {
+                            value_type val = tok.expression_->evaluate(resources, root, current, options, ec);
+                            stack.emplace_back(std::move(val));
+                            break;
+                        }
+                        case jsonpath_token_kind::selector:
+                        {
+                            JSONCONS_ASSERT(!stack.empty());
+                            auto item = std::move(stack.back());
+                            //for (auto& item : stack)
+                            //{
+                                //std::cout << "selector stack input:\n";
+                                //switch (item.tag)
+                                //{
+                                //    case node_set_tag::single:
+                                //        std::cout << "single: " << *(item.node.ptr) << "\n";
+                                //        break;
+                                //    case node_set_tag::multi:
+                                //        for (auto& node : stack.back().ptr().nodes)
+                                //        {
+                                //            std::cout << "multi: " << *node.ptr << "\n";
+                                //        }
+                                //        break;
+                                //    default:
+                                //        break;
+                            //}
+                            //std::cout << "\n";
+                            //}
+                            //std::cout << "selector item: " << *ptr << "\n";
+
+                            reference val = tok.selector_->evaluate(resources, root, resources.current_path_node(), item.value(), options, ec);
+
+                            stack.pop_back();
+                            stack.emplace_back(stack_item_type(std::addressof(val)));
+                            break;
+                        }
+                        default:
+                            break;
+                    }
+                }
+            }
+
+            //if (stack.size() != 1)
+            //{
+            //    std::cout << "Stack size: " << stack.size() << "\n";
+            //}
+            return stack.empty() ? Json::null() : stack.back().value();
+        }
+ 
+        std::string to_string(int level) const override
+        {
+            std::string s;
+            if (level > 0)
+            {
+                s.append("\n");
+                s.append(level*2, ' ');
+            }
+            s.append("expression ");
+            for (const auto& item : token_list_)
+            {
+                s.append(item.to_string(level+1));
+            }
+
+            return s;
+
+        }
+    private:
+    };
+
+} // namespace detail
+} // namespace jsonpath
+} // namespace jsoncons
+
+#endif // JSONCONS_JSONPATH_JSONPATH_EXPRESSION_HPP