diff --git a/src/main.cc b/src/main.cc
index 7242ff96..e3b0098e 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -147,6 +147,11 @@ void register_env_vars()
 
 void register_registers()
 {
+    RegisterManager& register_manager = RegisterManager::instance();
+
+    for (auto c : "abcdefghijklmnopqrstuvwxyz/\"|^@")
+        register_manager.add_register(c, make_unique<StaticRegister>());
+
     using StringList = Vector<String, MemoryDomain::Registers>;
     static const struct {
         char name;
@@ -165,20 +170,21 @@ void register_registers()
         } }
     };
 
-    RegisterManager& register_manager = RegisterManager::instance();
     for (auto& dyn_reg : dyn_regs)
-        register_manager.register_dynamic_register(dyn_reg.name, dyn_reg.func);
+        register_manager.add_register(dyn_reg.name, make_unique<DynamicRegister>(dyn_reg.func));
 
     for (size_t i = 0; i < 10; ++i)
     {
-        register_manager.register_dynamic_register('0'+i,
+        register_manager.add_register('0'+i, make_unique<DynamicRegister>(
             [i](const Context& context) {
                 StringList result;
                 for (auto& sel : context.selections())
                     result.emplace_back(i < sel.captures().size() ? sel.captures()[i] : "");
                 return result;
-            });
+            }));
     }
+
+    register_manager.add_register('_', make_unique<NullRegister>());
 }
 
 void register_options()
diff --git a/src/register_manager.cc b/src/register_manager.cc
index 64e56926..c026f3fc 100644
--- a/src/register_manager.cc
+++ b/src/register_manager.cc
@@ -1,62 +1,12 @@
 #include "register_manager.hh"
 
 #include "assert.hh"
-#include "exception.hh"
 #include "id_map.hh"
 
 namespace Kakoune
 {
 
-// static value register, which can be modified
-// using operator=, so should be user modifiable
-class StaticRegister : public Register
-{
-public:
-    Register& operator=(ConstArrayView<String> values) override
-    {
-        m_content = Vector<String, MemoryDomain::Registers>(values.begin(), values.end());
-        return *this;
-    }
-
-    ConstArrayView<String> values(const Context&) override
-    {
-        if (m_content.empty())
-            return ConstArrayView<String>(ms_empty);
-        else
-            return ConstArrayView<String>(m_content);
-    }
-protected:
-    Vector<String, MemoryDomain::Registers> m_content;
-
-    static const String ms_empty;
-};
-
-const String StaticRegister::ms_empty;
-
-// Dynamic value register, use it's RegisterRetriever
-// to get it's value when needed.
-class DynamicRegister : public StaticRegister
-{
-public:
-    DynamicRegister(RegisterRetriever function)
-        : m_function(std::move(function)) {}
-
-    Register& operator=(ConstArrayView<String> values) override
-    {
-        throw runtime_error("this register is not assignable");
-    }
-
-    ConstArrayView<String> values(const Context& context) override
-    {
-        m_content = m_function(context);
-        return StaticRegister::values(context);
-    }
-
-private:
-    RegisterRetriever m_function;
-};
-
-Register& RegisterManager::operator[](StringView reg)
+Register& RegisterManager::operator[](StringView reg) const
 {
     if (reg.length() == 1)
         return (*this)[reg[0_byte]];
@@ -74,23 +24,21 @@ Register& RegisterManager::operator[](StringView reg)
     return (*this)[it->value];
 }
 
-Register& RegisterManager::operator[](Codepoint c)
+Register& RegisterManager::operator[](Codepoint c) const
 {
     c = to_lower(c);
-    if (c < 32 or c > 127)
-        throw runtime_error(format("invalid register name: '{}'", c));
+    auto it = m_registers.find(c);
+    if (it == m_registers.end())
+        throw runtime_error(format("no such register: '{}'", c));
 
-    auto& reg_ptr = m_registers[c];
-    if (not reg_ptr)
-        reg_ptr.reset(new StaticRegister());
-    return *reg_ptr;
+    return *(it->second);
 }
 
-void RegisterManager::register_dynamic_register(char reg, RegisterRetriever function)
+void RegisterManager::add_register(char c, std::unique_ptr<Register> reg)
 {
-    auto& reg_ptr = m_registers[reg];
+    auto& reg_ptr = m_registers[c];
     kak_assert(not reg_ptr);
-    reg_ptr.reset(new DynamicRegister(std::move(function)));
+    reg_ptr = std::move(reg);
 }
 
 }
diff --git a/src/register_manager.hh b/src/register_manager.hh
index df829763..355bec1f 100644
--- a/src/register_manager.hh
+++ b/src/register_manager.hh
@@ -2,6 +2,7 @@
 #define register_manager_hh_INCLUDED
 
 #include "array_view.hh"
+#include "exception.hh"
 #include "utils.hh"
 #include "unordered_map.hh"
 #include "string.hh"
@@ -23,14 +24,73 @@ public:
     virtual ConstArrayView<String> values(const Context& context) = 0;
 };
 
+// static value register, which can be modified
+// using operator=, so should be user modifiable
+class StaticRegister : public Register
+{
+public:
+    Register& operator=(ConstArrayView<String> values) override
+    {
+        m_content = Vector<String, MemoryDomain::Registers>(values.begin(), values.end());
+        return *this;
+    }
+
+    ConstArrayView<String> values(const Context&) override
+    {
+        if (m_content.empty())
+            return ConstArrayView<String>(String::ms_empty);
+        else
+            return ConstArrayView<String>(m_content);
+    }
+protected:
+    Vector<String, MemoryDomain::Registers> m_content;
+};
+
 using RegisterRetriever = std::function<Vector<String, MemoryDomain::Registers> (const Context&)>;
 
+// Dynamic value register, use it's RegisterRetriever
+// to get it's value when needed.
+class DynamicRegister : public StaticRegister
+{
+public:
+    DynamicRegister(RegisterRetriever function)
+        : m_function(std::move(function)) {}
+
+    Register& operator=(ConstArrayView<String> values) override
+    {
+        throw runtime_error("this register is not assignable");
+    }
+
+    ConstArrayView<String> values(const Context& context) override
+    {
+        m_content = m_function(context);
+        return StaticRegister::values(context);
+    }
+
+private:
+    RegisterRetriever m_function;
+};
+
+class NullRegister : public Register
+{
+public:
+    Register& operator=(ConstArrayView<String> values) override
+    {
+        return *this;
+    }
+
+    ConstArrayView<String> values(const Context& context) override
+    {
+        return ConstArrayView<String>(String::ms_empty);
+    }
+};
+
 class RegisterManager : public Singleton<RegisterManager>
 {
 public:
-    Register& operator[](StringView reg);
-    Register& operator[](Codepoint c);
-    void register_dynamic_register(char reg, RegisterRetriever function);
+    Register& operator[](StringView reg) const;
+    Register& operator[](Codepoint c) const;
+    void add_register(char c, std::unique_ptr<Register> reg);
 
 protected:
     UnorderedMap<char, std::unique_ptr<Register>, MemoryDomain::Registers> m_registers;