diff --git a/src/color.cc b/src/color.cc
index dfe2e955..145a5381 100644
--- a/src/color.cc
+++ b/src/color.cc
@@ -71,9 +71,9 @@ String to_string(Color color)
     {
         char buffer[14];
         if (color.a == 255)
-            sprintf(buffer, "rgb:%02x%02x%02x", color.r, color.g, color.b);
+            format_to(buffer, "rgb:{:02}{:02}{:02}", hex(color.r), hex(color.g), hex(color.b));
         else
-            sprintf(buffer, "rgba:%02x%02x%02x%02x", color.r, color.g, color.b, color.a);
+            format_to(buffer, "rgba:{:02}{:02}{:02}{:02}", hex(color.r), hex(color.g), hex(color.b), hex(color.a));
         return buffer;
     }
     else
diff --git a/src/json.cc b/src/json.cc
index 3661ac80..82fc0f28 100644
--- a/src/json.cc
+++ b/src/json.cc
@@ -29,7 +29,7 @@ String to_json(StringView str)
 
         char buf[7] = {'\\', *next, 0};
         if (*next >= 0 and *next <= 0x1F)
-            sprintf(buf, "\\u%04x", *next);
+            format_to(buf, "\\u{:04}", hex(*next));
 
         res += buf;
         it = next+1;
@@ -188,4 +188,11 @@ UnitTest test_json_parser{[]()
     }
 }};
 
+UnitTest test_to_json{[]()
+{
+    kak_assert(to_json(true) == "true");
+    kak_assert(to_json(false) == "false");
+    kak_assert(to_json(HashMap<String, Vector<int>>{{"foo", {1,2,3}}, {"\033", {3, 4, 5}}}) == R"({"foo": [1, 2, 3],"\u001b": [3, 4, 5]})");
+}};
+
 }
diff --git a/src/json.hh b/src/json.hh
index 1f76739c..84c14d36 100644
--- a/src/json.hh
+++ b/src/json.hh
@@ -11,6 +11,10 @@ namespace Kakoune
 using JsonArray = Vector<Value>;
 using JsonObject = HashMap<String, Value>;
 
+String to_json(int i);
+String to_json(bool b);
+String to_json(StringView str);
+
 template<typename T>
 String to_json(ArrayView<const T> array)
 {
@@ -27,10 +31,6 @@ String to_json(const HashMap<K, V, D>& map)
                       ',', false) + "}";
 }
 
-String to_json(int i);
-String to_json(bool b);
-String to_json(StringView str);
-
 struct JsonResult { Value value; const char* new_pos; };
 
 JsonResult parse_json(const char* pos, const char* end);
diff --git a/src/json_ui.cc b/src/json_ui.cc
index 1200ed60..01768897 100644
--- a/src/json_ui.cc
+++ b/src/json_ui.cc
@@ -27,7 +27,7 @@ String to_json(Color color)
     if (color.color == Kakoune::Color::RGB)
     {
         char buffer[10];
-        sprintf(buffer, R"("#%02x%02x%02x")", color.r, color.g, color.b);
+        format_to(buffer, R"("#{:02}{:02}{:02}")", hex(color.r), hex(color.g), hex(color.b));
         return buffer;
     }
     return to_json(to_string(color));
diff --git a/src/regex_impl.cc b/src/regex_impl.cc
index a380e7f2..a02554c7 100644
--- a/src/regex_impl.cc
+++ b/src/regex_impl.cc
@@ -1080,7 +1080,7 @@ String dump_regex(const CompiledRegex& program)
     for (auto& inst : program.instructions)
     {
         char buf[20];
-        sprintf(buf, " %03d     ", count++);
+        format_to(buf, " {:03}     ", count++);
         res += buf;
         switch (inst.op)
         {
@@ -1148,7 +1148,7 @@ String dump_regex(const CompiledRegex& program)
             if (desc.map[c])
             {
                 if (c < 32)
-                    res += format("<0x{}>", Hex{c});
+                    res += format("<0x{}>", hex(c));
                 else
                     res += (char)c;
             }
diff --git a/src/string_utils.cc b/src/string_utils.cc
index bc368978..53d2d455 100644
--- a/src/string_utils.cc
+++ b/src/string_utils.cc
@@ -215,7 +215,7 @@ InplaceString<23> to_string(float val)
     return to_string_impl<23>(val, std::chars_format::general);
 #else
     InplaceString<23> res;
-    res.m_length = sprintf(res.m_data, "%f", val);
+    res.m_length = snprintf(res.m_data, 23, "%f", val);
     return res;
 #endif
 }
@@ -360,9 +360,15 @@ void format_impl(StringView fmt, ArrayView<const StringView> params, AppendFunc
 
             if (format != closing)
             {
-                for (ColumnCount width = str_to_int({format+1, closing}), len = params[index].column_length();
+                char padding = ' ';
+                if (*(++format) == '0')
+                {
+                    padding = '0';
+                    ++format;
+                }
+                for (ColumnCount width = str_to_int({format, closing}), len = params[index].column_length();
                      width > len; --width)
-                    append(' ');
+                    append(padding);
             }
 
             append(params[index]);
@@ -475,7 +481,7 @@ UnitTest test_string{[]()
     kak_assert(subsequence_match("tchou kanaky", "tchou kanaky"));
     kak_assert(not subsequence_match("tchou kanaky", "tchou  kanaky"));
 
-    kak_assert(format("Youhou {1} {} '{0:4}' \\{}", 10, "hehe", 5) == "Youhou hehe 5 '  10' {}");
+    kak_assert(format("Youhou {1} {} '{0:4}' {2:04} \\{}", 10, "hehe", 5) == "Youhou hehe 5 '  10' 0005 {}");
 
     char buffer[20];
     kak_assert(format_to(buffer, "Hey {}", 15) == "Hey 15");