diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index caa3163..ace95c4 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -12,7 +12,7 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        php: [7.0, 7.1, 7.2, 7.3, 7.4, 8.0, 8.1, 8.2]
+        php: [7.0, 7.1, 7.2, 7.3, 7.4, 8.0, 8.1, 8.2, 8.3]
 
     name: PHP ${{ matrix.php }}
 
diff --git a/README.md b/README.md
index 8724407..36461f2 100644
--- a/README.md
+++ b/README.md
@@ -3,7 +3,6 @@
 # VDF Converter
 
 ![Tests](https://github.com/exayer/vdf-converter/workflows/Tests/badge.svg)
-[![Total Downloads](https://img.shields.io/packagist/dt/EXayer/vdf-converter)](https://packagist.org/packages/exayer/vdf-converter)
 ![Latest Stable Version](https://img.shields.io/packagist/v/exayer/vdf-converter)
 
 A memory efficient parser for the Valve Data Format (*.vdf) written in PHP.
@@ -89,8 +88,7 @@ $result = iterator_to_array($planets);
 
 Some VDFs are known to contain duplicate keys. 
 To keep the result structure the same as the VDF and since the parser is generator based (not keeping in memory pairs) 
-the duplicated key will be modified - the counter will be concatenated to the end (e.g. 'key__2').
-
+the duplicated key will be modified - the counter will be concatenated to the end (e.g. `key__[2]`).
 
 ```php
 $vdf = '{
@@ -114,9 +112,9 @@ $result = iterator_to_array(VdfConverter::fromString($vdf));
 //    "mercury" => [
 //      "distance" => "58"
 //      "velocity" => "35"
-//      "distance__2" => "92"
+//      "distance__[2]" => "92"
 //    ]
-//    "mercury__2" => [
+//    "mercury__[2]" => [
 //      "distance" => "108"
 //    ]
 //    "earth" => [
@@ -124,6 +122,35 @@ $result = iterator_to_array(VdfConverter::fromString($vdf));
 //    ]
 // ]
 ```
+
+#### Customizing key format
+
+If you want to customize the formatting key process, you can create your own custom formatter. A formatter is any class that implements `EXayer\VdfConverter\UniqueKey\Formatter`.
+
+This is what that interface looks like.
+
+```php
+namespace EXayer\VdfConverter\UniqueKey;
+
+interface Formatter
+{
+    public function buildKeyName(string $key, int $index): string;
+}
+```
+
+After creating your formatter, you can specify its class name in the `uniqueKeyFormatter` method of the `EXayer\VdfConverter\VdfConverterConfig` object. The config can be passed as second argument to any `from` builder method. Your formatter will then be used by default for all duplicate key handling calls.
+
+You can also specify a signer for a specific webhook call:
+
+```php
+$config = VdfConverterConfig::create()
+    ->uniqueKeyFormatter(YourCustomFormatter::class);
+
+$iterator = VdfConverter::fromString($vdf, $config);
+```
+
+**Warning**: Do not create formatters that create a key like `__2`, as some VDFs may have keys in this format.
+
 ## Testing
 
 ```bash
diff --git a/logo.png b/logo.png
index 50b7762..666aa59 100644
Binary files a/logo.png and b/logo.png differ
diff --git a/src/Parser.php b/src/Parser.php
index e5089e6..0aca95c 100644
--- a/src/Parser.php
+++ b/src/Parser.php
@@ -3,6 +3,7 @@
 namespace EXayer\VdfConverter;
 
 use EXayer\VdfConverter\Exception\CouldNotParseException;
+use EXayer\VdfConverter\UniqueKey\UniqueKeyHandler;
 use Traversable;
 
 class Parser implements \IteratorAggregate, PositionAwareInterface, LineColumnAwareInterface
@@ -13,17 +14,18 @@ class Parser implements \IteratorAggregate, PositionAwareInterface, LineColumnAw
     private $lexer;
 
     /**
-     * @var UniqueKey
+     * @var UniqueKeyHandler
      */
     private $uniqueKey;
 
     /**
-     * @param Traversable $lexer
+     * @param Traversable        $lexer
+     * @param VdfConverterConfig $config
      */
-    public function __construct(Traversable $lexer)
+    public function __construct(Traversable $lexer, VdfConverterConfig $config)
     {
         $this->lexer = $lexer;
-        $this->uniqueKey = new UniqueKey();
+        $this->uniqueKey = new UniqueKeyHandler($config->getUniqueKeyFormatter());
     }
 
     /**
diff --git a/src/UniqueKey/DefaultFormatter.php b/src/UniqueKey/DefaultFormatter.php
new file mode 100644
index 0000000..4e23b0a
--- /dev/null
+++ b/src/UniqueKey/DefaultFormatter.php
@@ -0,0 +1,14 @@
+<?php
+
+namespace EXayer\VdfConverter\UniqueKey;
+
+class DefaultFormatter implements Formatter
+{
+    /**
+     * {@inheritDoc}
+     */
+    public function buildKeyName(string $key, int $index): string
+    {
+        return $key . '__[' . $index . ']';
+    }
+}
\ No newline at end of file
diff --git a/src/UniqueKey/Formatter.php b/src/UniqueKey/Formatter.php
new file mode 100644
index 0000000..427efb5
--- /dev/null
+++ b/src/UniqueKey/Formatter.php
@@ -0,0 +1,16 @@
+<?php
+
+namespace EXayer\VdfConverter\UniqueKey;
+
+interface Formatter
+{
+    /**
+     * Returns a formatted key based on original key and duplicate index.
+     *
+     * @param string $key Original key.
+     * @param int    $index Duplicate key index.
+     *
+     * @return string
+     */
+    public function buildKeyName(string $key, int $index): string;
+}
\ No newline at end of file
diff --git a/src/UniqueKey.php b/src/UniqueKey/UniqueKeyHandler.php
similarity index 67%
rename from src/UniqueKey.php
rename to src/UniqueKey/UniqueKeyHandler.php
index ac64db4..f0426e2 100644
--- a/src/UniqueKey.php
+++ b/src/UniqueKey/UniqueKeyHandler.php
@@ -1,13 +1,13 @@
 <?php
 
-namespace EXayer\VdfConverter;
+namespace EXayer\VdfConverter\UniqueKey;
 
-class UniqueKey
+class UniqueKeyHandler
 {
     /**
-     * @const
+     * @var Formatter
      */
-    const DELIMITER = '__';
+    private $formatter;
 
     /**
      * Holds duplicate key counts for each level of nesting.
@@ -17,6 +17,14 @@ class UniqueKey
      */
     private $storage = [];
 
+    /**
+     * @param Formatter $formatter
+     */
+    public function __construct(Formatter $formatter)
+    {
+        $this->formatter = $formatter;
+    }
+
     /**
      * @param int $level Buffer nesting level.
      * @param string $key
@@ -30,7 +38,7 @@ public function get(int $level, string $key): string
             return $key;
         }
 
-        return $this->buildName($key, ++$this->storage[$level][$key]);
+        return $this->formatter->buildKeyName($key, ++$this->storage[$level][$key]);
     }
 
     /**
@@ -42,14 +50,4 @@ public function clear(int $level)
             $this->storage[$level] = [];
         }
     }
-
-    /**
-     * @param string $key
-     * @param int $index
-     * @return string
-     */
-    private function buildName(string $key, int $index): string
-    {
-        return $key . self::DELIMITER . $index;
-    }
 }
diff --git a/src/VdfConverter.php b/src/VdfConverter.php
index 48d55c6..4d320e8 100644
--- a/src/VdfConverter.php
+++ b/src/VdfConverter.php
@@ -19,52 +19,59 @@ class VdfConverter implements \IteratorAggregate, PositionAwareInterface
     private $parser;
 
     /**
-     * @param iterable $bytesIterator
+     * @param iterable                $bytesIterator
+     * @param VdfConverterConfig|null $config
      */
-    public function __construct($bytesIterator)
+    public function __construct($bytesIterator, VdfConverterConfig $config = null)
     {
         $this->bytesIterator = $bytesIterator;
-        $this->parser = new Parser(new Lexer($this->bytesIterator));
+
+        $config = $config ?: new VdfConverterConfig();
+        $this->parser = new Parser(new Lexer($this->bytesIterator), $config);
     }
 
     /**
-     * @param string $string
+     * @param string                  $string
+     * @param VdfConverterConfig|null $config
      *
      * @return self
      */
-    public static function fromString(string $string): self
+    public static function fromString(string $string, VdfConverterConfig $config = null): self
     {
-        return new static(new StringChunks($string));
+        return new static(new StringChunks($string), $config);
     }
 
     /**
-     * @param string $fileName
+     * @param string                  $fileName
+     * @param VdfConverterConfig|null $config
      *
      * @return self
      */
-    public static function fromFile(string $fileName): self
+    public static function fromFile(string $fileName, VdfConverterConfig $config = null): self
     {
-        return new static(new FileChunks($fileName));
+        return new static(new FileChunks($fileName), $config);
     }
 
     /**
-     * @param resource $stream
+     * @param resource                $stream
+     * @param VdfConverterConfig|null $config
      *
      * @return self
      */
-    public static function fromStream($stream): self
+    public static function fromStream($stream, VdfConverterConfig $config = null): self
     {
-        return new static(new StreamChunks($stream));
+        return new static(new StreamChunks($stream), $config);
     }
 
     /**
-     * @param \Traversable|array $iterable
+     * @param \Traversable|array      $iterable
+     * @param VdfConverterConfig|null $config
      *
      * @return self
      */
-    public static function fromIterable($iterable): self
+    public static function fromIterable($iterable, VdfConverterConfig $config = null): self
     {
-        return new static($iterable);
+        return new static($iterable, $config);
     }
 
     /**
diff --git a/src/VdfConverterConfig.php b/src/VdfConverterConfig.php
new file mode 100644
index 0000000..d1379fe
--- /dev/null
+++ b/src/VdfConverterConfig.php
@@ -0,0 +1,42 @@
+<?php
+
+namespace EXayer\VdfConverter;
+
+use EXayer\VdfConverter\UniqueKey\DefaultFormatter;
+use EXayer\VdfConverter\UniqueKey\Formatter;
+
+class VdfConverterConfig
+{
+    /**
+     * @var string
+     */
+    private $uniqueKeyFormatter = DefaultFormatter::class;
+
+    /**
+     * @return self
+     */
+    public static function create(): self
+    {
+        return new self();
+    }
+
+    /**
+     * @param string $formatterClassName
+     *
+     * @return $this
+     */
+    public function uniqueKeyFormatter(string $formatterClassName): self
+    {
+        $this->uniqueKeyFormatter = $formatterClassName;
+
+        return $this;
+    }
+
+    /**
+     * @return Formatter
+     */
+    public function getUniqueKeyFormatter(): Formatter
+    {
+        return new $this->uniqueKeyFormatter;
+    }
+}
diff --git a/tests/ParserTest.php b/tests/ParserTest.php
index b4db7be..6d8cc52 100644
--- a/tests/ParserTest.php
+++ b/tests/ParserTest.php
@@ -5,6 +5,7 @@
 use EXayer\VdfConverter\Exception\CouldNotParseException;
 use EXayer\VdfConverter\Lexer;
 use EXayer\VdfConverter\Parser;
+use EXayer\VdfConverter\VdfConverterConfig;
 use PHPUnit\Framework\TestCase;
 
 class ParserTest extends TestCase
@@ -16,7 +17,7 @@ class ParserTest extends TestCase
      */
     public function testSyntax($vdf, $expectedResult)
     {
-        $parser = new Parser(new Lexer(new \ArrayIterator([$vdf])));
+        $parser = new Parser(new Lexer(new \ArrayIterator([$vdf])), new VdfConverterConfig());
         $result = iterator_to_array($parser);
 
         $this->assertEquals($expectedResult, $result);
@@ -87,13 +88,13 @@ public function syntaxDataProvider()
                 [
                     'a' => [
                         'b' => 1,
-                        'x' => ['x' => 1, 'x__2' => 2],
+                        'x' => ['x' => 1, 'x__[2]' => 2],
                         'e' => [
-                            'f' => ['x' => 1, '' => 1, '__2' => 2, 'x__2' => 2],
-                            'f__2' => [],
+                            'f' => ['x' => 1, '' => 1, '__[2]' => 2, 'x__[2]' => 2],
+                            'f__[2]' => [],
                         ],
-                        'x__2' => [],
-                        'e__2' => [],
+                        'x__[2]' => [],
+                        'e__[2]' => [],
                     ],
                 ],
             ],
@@ -103,14 +104,14 @@ public function syntaxDataProvider()
 
                 [
                     'a' => 1,
-                    'b' => ['c' => 1, 'c__2' => 2],
-                    'a__2' => [
-                        'b' => ['c' => 1, 'c__2' => 2],
-                        'b__2' => [],
+                    'b' => ['c' => 1, 'c__[2]' => 2],
+                    'a__[2]' => [
+                        'b' => ['c' => 1, 'c__[2]' => 2],
+                        'b__[2]' => [],
                     ],
-                    'a__3' => [],
+                    'a__[3]' => [],
                     'c' => [],
-                    'c__2' => [],
+                    'c__[2]' => [],
                 ],
             ],
             // all duplicates
@@ -118,13 +119,13 @@ public function syntaxDataProvider()
                 '{"x" "1" "x" {"x" "1" "x" "2"} "x" {"x" {"x" "1" "x" "2"} "x" {}} "x" {} "x" {}}',
                 [
                     'x' => 1,
-                    'x__2' => ['x' => 1, 'x__2' => 2],
-                    'x__3' => [
-                        'x' => ['x' => 1, 'x__2' => 2],
-                        'x__2' => [],
+                    'x__[2]' => ['x' => 1, 'x__[2]' => 2],
+                    'x__[3]' => [
+                        'x' => ['x' => 1, 'x__[2]' => 2],
+                        'x__[2]' => [],
                     ],
-                    'x__4' => [],
-                    'x__5' => [],
+                    'x__[4]' => [],
+                    'x__[5]' => [],
                 ],
             ],
         ];
@@ -138,7 +139,7 @@ public function testSyntaxError(string $brokenVdf)
     {
         $this->expectException(CouldNotParseException::class);
 
-        iterator_to_array(new Parser(new Lexer(new \ArrayIterator([$brokenVdf]))));
+        iterator_to_array(new Parser(new Lexer(new \ArrayIterator([$brokenVdf])), new VdfConverterConfig()));
     }
 
     public function syntaxErrorDataProvider()
@@ -162,7 +163,7 @@ public function testUnexpectedEndError(string $vdf)
     {
         $this->expectExceptionMessage(CouldNotParseException::unexpectedEnding()->getMessage());
 
-        iterator_to_array(new Parser(new Lexer(new \ArrayIterator([$vdf]))));
+        iterator_to_array(new Parser(new Lexer(new \ArrayIterator([$vdf])), new VdfConverterConfig()));
     }
 
     public function unexpectedEndExceptionDataProvider()
diff --git a/tests/UniqueKey/DefaultFormatterTest.php b/tests/UniqueKey/DefaultFormatterTest.php
new file mode 100644
index 0000000..98285a7
--- /dev/null
+++ b/tests/UniqueKey/DefaultFormatterTest.php
@@ -0,0 +1,17 @@
+<?php
+
+namespace EXayer\VdfConverter\Tests\UniqueKey;
+
+use EXayer\VdfConverter\UniqueKey\DefaultFormatter;
+use PHPUnit\Framework\TestCase;
+
+class DefaultFormatterTest extends TestCase
+{
+    public function testBuildName()
+    {
+        $formatter = new DefaultFormatter();
+        $formattedKey = $formatter->buildKeyName('sample_key', 9);
+
+        $this->assertEquals($formattedKey, 'sample_key__[9]');
+    }
+}
\ No newline at end of file
diff --git a/tests/UniqueKeyTest.php b/tests/UniqueKey/UniqueKeyTest.php
similarity index 81%
rename from tests/UniqueKeyTest.php
rename to tests/UniqueKey/UniqueKeyTest.php
index 3824bf3..188baa7 100644
--- a/tests/UniqueKeyTest.php
+++ b/tests/UniqueKey/UniqueKeyTest.php
@@ -1,8 +1,9 @@
 <?php
 
-namespace EXayer\VdfConverter\Tests;
+namespace EXayer\VdfConverter\Tests\UniqueKey;
 
-use EXayer\VdfConverter\UniqueKey;
+use EXayer\VdfConverter\UniqueKey\DefaultFormatter;
+use EXayer\VdfConverter\UniqueKey\UniqueKeyHandler;
 use PHPUnit\Framework\TestCase;
 
 class UniqueKeyTest extends TestCase
@@ -15,7 +16,7 @@ class UniqueKeyTest extends TestCase
      */
     public function testStorage(array $initStorage, array $levelsToClear, array $expected)
     {
-        $uniqueKey = new UniqueKey();
+        $uniqueKey = new UniqueKeyHandler(new DefaultFormatter());
 
         foreach ($initStorage as $pairs) {
             foreach ($pairs as $level => $key) {
@@ -44,7 +45,7 @@ public function storageDataProvider()
             [
                 [[1 => 'key'], [1 => 'key']],
                 [],
-                [1, 'key', 'key__3']
+                [1, 'key', 'key__[3]']
             ],
             [
                 [[1 => 'key'], [1 => 'key'], [2 => 'key'], [3 => 'key']],
@@ -54,7 +55,7 @@ public function storageDataProvider()
             [
                 [[1 => 'key'], [2 => 'key'], [3 => 'key']],
                 [2, 3],
-                [1, 'key', 'key__2']
+                [1, 'key', 'key__[2]']
             ],
         ];
     }
diff --git a/tests/VdfConverterConfigTest.php b/tests/VdfConverterConfigTest.php
new file mode 100644
index 0000000..78fb014
--- /dev/null
+++ b/tests/VdfConverterConfigTest.php
@@ -0,0 +1,16 @@
+<?php
+
+use EXayer\VdfConverter\UniqueKey\DefaultFormatter;
+use EXayer\VdfConverter\VdfConverterConfig;
+use PHPUnit\Framework\TestCase;
+
+class VdfConverterConfigTest extends TestCase
+{
+    public function testUniqueKeyFormatterCreation()
+    {
+        $config = VdfConverterConfig::create()
+            ->uniqueKeyFormatter(DefaultFormatter::class);
+
+        $this->assertEquals(new DefaultFormatter(), $config->getUniqueKeyFormatter());
+    }
+}
\ No newline at end of file