diff --git a/README.md b/README.md
index 5555423..e13f4fc 100644
--- a/README.md
+++ b/README.md
@@ -280,7 +280,7 @@ composer require macocci7/bash-colorizer
 > <a id="note5"></a>
 *5: Partially effective<br>
 
-e.g.) on VSCode Terminal
+e.g.) on VSCode Terminal (theme:`Dark Modern`)
 
 <img src="arts/available_attributes.png" width="220" height="515" />
 
@@ -302,36 +302,36 @@ See more: [Select Graphic Rendition parameters | ANSI escape code | Wikipedia](h
     - `default`
 
 
-    e.g.) on VSCode Terminal
+    e.g.) on VSCode Terminal (theme:`Dark Modern`)
     |Foregound Colors|Background Colors|
     |---|---|
     |<img src="arts/available_foreground_colors.png" with="240" height="216" />|<img src="arts/available_background_colors.png" with="240" height="216" />|
 
 - 256 colors [ 0 - 255 ]: `foreground`/`background`/`underline`
 
-    e.g.) foreground colors on VSCode Terminal:
+    e.g.) foreground colors on VSCode Terminal (theme:`Dark Modern`):
 
     <img src="arts/foreground_256colors.png" width="500" height="254" />
 
-    e.g.) background colors on VSCode Terminal:
+    e.g.) background colors on VSCode Terminal (theme:`Dark Modern`):
 
     <img src="arts/background_256colors.png" width="500" height="256" />
 
-    e.g.) underline colors on VSCode Terminal:
+    e.g.) underline colors on VSCode Terminal (theme:`Dark Modern`):
 
     <img src="arts/underline_256colors.png" width="500" height="252" />
 
 - 24bit (16777216) colors:
 
-    e.g.) foreground colors on VSCode Terminal:
+    e.g.) foreground colors on VSCode Terminal (theme:`Dark Modern`):
 
     <img src="arts/foreground_24bitcolors.png" width="500" height="412" />
 
-    e.g.) background colors on VSCode Terminal:
+    e.g.) background colors on VSCode Terminal (theme:`Dark Modern`):
 
     <img src="arts/background_24bitcolors.png" width="500" height="406"/>
 
-    e.g.) underline colors on VSCode Terminal:
+    e.g.) underline colors on VSCode Terminal (theme:`Dark Modern`):
 
     <img src="arts/underline_24bitcolors.png" width="500" height="410"/>
 
@@ -350,6 +350,7 @@ Example codes are in [playground](playground/) directory.
 - [underline_256colors.php](playground/underline_256colors.php)
 - [underline_24bitcolors.php](playground/underline_24bitcolors.php)
 - [readable.php](playground/readable.php)
+- [converter.php](playground/converter.php)
 
 ## 8. LICENSE
 
diff --git a/playground/converter.php b/playground/converter.php
new file mode 100644
index 0000000..96bb0b4
--- /dev/null
+++ b/playground/converter.php
@@ -0,0 +1,30 @@
+<?php
+
+require_once __DIR__ . '/../vendor/autoload.php';
+
+use Macocci7\BashColorizer\Colorizer;
+use Macocci7\BashColorizer\Converter;
+
+// hex to rgb
+$hex = '#abcdef';
+$rgb = Converter::decimal($hex);
+
+echo sprintf(
+    "%s => [%s, %s, %s]\n",
+    Colorizer::foreground($hex)->encode($hex),
+    Colorizer::foreground('red')->encode($rgb[0]),
+    Colorizer::foreground('green')->encode($rgb[1]),
+    Colorizer::foreground('blue')->encode($rgb[2]),
+);
+
+// decimal to rgb
+$rgb = [254, 220, 186];
+$hex = Converter::hex($rgb);
+
+echo sprintf(
+    "[%s, %s, %s] => %s\n",
+    Colorizer::foreground('red')->encode($rgb[0]),
+    Colorizer::foreground('green')->encode($rgb[1]),
+    Colorizer::foreground('blue')->encode($rgb[2]),
+    Colorizer::foreground($hex)->encode($hex),
+);
diff --git a/src/Converter.php b/src/Converter.php
index d843757..e0cf175 100644
--- a/src/Converter.php
+++ b/src/Converter.php
@@ -2,6 +2,8 @@
 
 namespace Macocci7\BashColorizer;
 
+use Macocci7\BashColorizer\Filters\Filter;
+
 class Converter
 {
     /**
@@ -11,14 +13,14 @@ public static function decimal(string $rgb): array
     {
         $patternS = '/^#([0-9a-f])([0-9a-f])([0-9a-f])$/';
         $patternL = '/^#([0-9a-f]{2})([0-9a-f]{2})([0-9a-f]{2})$/';
-        if (preg_match($patternS, $rgb, $matches)) {
+        if (preg_match($patternS, strtolower($rgb), $matches)) {
             // short format (#rgb)
             return [
                 hexdec($matches[1] . $matches[1]),
                 hexdec($matches[2] . $matches[2]),
                 hexdec($matches[3] . $matches[3]),
             ];
-        } elseif (preg_match($patternL, $rgb, $matches)) {
+        } elseif (preg_match($patternL, strtolower($rgb), $matches)) {
             // long format (#rrggbb)
             return [
                 hexdec($matches[1]),
@@ -28,4 +30,21 @@ public static function decimal(string $rgb): array
         }
         return [];
     }
+
+    /**
+     * @param   int[]   $rgb
+     */
+    public static function hex(array $rgb): string|null
+    {
+        if (empty($rgb)) {
+            return null;
+        }
+        $rgb = Filter::rgb($rgb);
+        return sprintf(
+            "#%02s%02s%02s",
+            dechex($rgb[0]),
+            dechex($rgb[1]),
+            dechex($rgb[2]),
+        );
+    }
 }
diff --git a/tests/ConverterTest.php b/tests/ConverterTest.php
index 4096330..02a721c 100644
--- a/tests/ConverterTest.php
+++ b/tests/ConverterTest.php
@@ -16,9 +16,15 @@ public static function provide_decimal_can_return_correct_array(): array
             '#000' => ['rgb' => '#000', 'expected' => [0, 0, 0]],
             '#123' => ['rgb' => '#123', 'expected' => [17, 34, 51]],
             '#fed' => ['rgb' => '#fed', 'expected' => [255, 238, 221]],
+            '#FED' => ['rgb' => '#FED', 'expected' => [255, 238, 221]],
+            '#19f' => ['rgb' => '#19f', 'expected' => [17, 153, 255]],
+            '#19F' => ['rgb' => '#19F', 'expected' => [17, 153, 255]],
             '#000000' => ['rgb' => '#000000', 'expected' => [0, 0, 0]],
             '#123456' => ['rgb' => '#123456', 'expected' => [18, 52, 86]],
             '#ffeedd' => ['rgb' => '#ffeedd', 'expected' => [255, 238, 221]],
+            '#FFEEDD' => ['rgb' => '#FFEEDD', 'expected' => [255, 238, 221]],
+            '#123abc' => ['rgb' => '#123abc', 'expected' => [18, 58, 188]],
+            '#123ABC' => ['rgb' => '#123abc', 'expected' => [18, 58, 188]],
 
             'fff' => ['rgb' => 'fff', 'expected' => []],
             'fffffff' => ['rgb' => 'fffffff', 'expected' => []],
@@ -26,9 +32,11 @@ public static function provide_decimal_can_return_correct_array(): array
             '#1' => ['rgb' => '#1', 'expected' => []],
             '#12' => ['rgb' => '#12', 'expected' => []],
             '#efg' => ['rgb' => '#efg', 'expected' => []],
+            '#EFG' => ['rgb' => '#EFG', 'expected' => []],
             '#1234' => ['rgb' => '#1234', 'expected' => []],
             '#12345' => ['rgb' => '#12345', 'expected' => []],
             '#bcdefg' => ['rgb' => '#bcdefg', 'expected' => []],
+            '#BCDEFG' => ['rgb' => '#BCDEFG', 'expected' => []],
             '#1234567' => ['rgb' => '#1234567', 'expected' => []],
             '#abcdefg' => ['rgb' => '#abcdefg', 'expected' => []],
         ];
@@ -39,4 +47,27 @@ public function test_decimal_can_return_correct_array(string $rgb, array $expect
     {
         $this->assertSame($expected, Converter::decimal($rgb));
     }
+
+    public static function provide_hex_can_return_correct_hex_code(): array
+    {
+        return [
+            'empty' => ['rgb' => [], 'expected' => null],
+            '[0]' => ['rgb' => [0], 'expected' => '#000000'],
+            '[1]' => ['rgb' => [1], 'expected' => '#010000'],
+            '[1, 2]' => ['rgb' => [1, 2], 'expected' => '#010200'],
+            '[1, 2, 3]' => ['rgb' => [1, 2, 3], 'expected' => '#010203'],
+            '[0, 0, 10]' => ['rgb' => [0, 0, 10], 'expected' => '#00000a'],
+            '[0, 0, 171]' => ['rgb' => [0, 0, 171], 'expected' => '#0000ab'],
+            '[0, 171, 205]' => ['rgb' => [0, 171, 205], 'expected' => '#00abcd'],
+            '[171, 205, 239]' => ['rgb' => [171, 205, 239], 'expected' => '#abcdef'],
+            '[-1, 0, 1]' => ['rgb' => [-1, 0, 1], 'expected' => '#000001'],
+            '[254, 255, 256]' => ['rgb' => [254, 255, 256], 'expected' => '#feffff'],
+        ];
+    }
+
+    #[DataProvider('provide_hex_can_return_correct_hex_code')]
+    public function test_hex_can_return_correct_hex_code(array $rgb, string|null $expected): void
+    {
+        $this->assertSame($expected, Converter::hex($rgb));
+    }
 }
diff --git a/tests/Filter/FilterTest.php b/tests/Filters/FilterTest.php
similarity index 100%
rename from tests/Filter/FilterTest.php
rename to tests/Filters/FilterTest.php