diff --git a/src/Invoice/NumberGenerator/ConfigurableNumberGenerator.php b/src/Invoice/NumberGenerator/ConfigurableNumberGenerator.php
index f72db9e2..c1a12f3c 100644
--- a/src/Invoice/NumberGenerator/ConfigurableNumberGenerator.php
+++ b/src/Invoice/NumberGenerator/ConfigurableNumberGenerator.php
@@ -62,103 +62,135 @@ final class ConfigurableNumberGenerator implements NumberGeneratorInterface
 
         preg_match_all('/{[^}]*?}/', $format, $matches);
         foreach ($matches[0] as $part) {
-            $formatterLength = null;
-            $increaseBy = 1;
-
-            $tmp = str_replace(['{', '}'], '', $part);
-
-            $parts = preg_split('/[,]+/', $tmp);
-            $tmp = $parts[0];
-            if (\count($parts) === 2) {
-                $formatterLength = \intval($parts[1]);
-                if ((string) $formatterLength !== $parts[1]) {
-                    $formatterLength = null;
-                }
-            }
-
-            $parts = preg_split("/[\+]+/", $tmp);
-            $tmp = $parts[0];
-            if (\count($parts) === 2) {
-                $increaseBy = \intval($parts[1]);
-                if ($increaseBy <= 0) {
-                    $increaseBy = 1;
-                }
-            }
-
-            switch ($tmp) {
-                case 'Y':
-                    $partialResult = $invoiceDate->format('Y');
-                    break;
-
-                case 'y':
-                    $partialResult = $invoiceDate->format('y');
-                    break;
-
-                case 'M':
-                    $partialResult = $invoiceDate->format('m');
-                    break;
-
-                case 'm':
-                    $partialResult = $invoiceDate->format('n');
-                    break;
-
-                case 'D':
-                    $partialResult = $invoiceDate->format('d');
-                    break;
-
-                case 'd':
-                    $partialResult = $invoiceDate->format('j');
-                    break;
-
-                case 'date':
-                    $partialResult = $invoiceDate->format('ymd');
-                    break;
-
-                // for customer
-                case 'cc':
-                    $partialResult = $this->repository->getCounterForAllTime($invoiceDate, $this->model->getCustomer()) + 1;
-                    break;
-
-                case 'ccy':
-                    $partialResult = $this->repository->getCounterForYear($invoiceDate, $this->model->getCustomer()) + 1;
-                    break;
-
-                case 'ccm':
-                    $partialResult = $this->repository->getCounterForMonth($invoiceDate, $this->model->getCustomer()) + 1;
-                    break;
-
-                case 'ccd':
-                    $partialResult = $this->repository->getCounterForDay($invoiceDate, $this->model->getCustomer()) + 1;
-                    break;
-
-                // across all invoices
-                case 'c':
-                    $partialResult = $this->repository->getCounterForAllTime($invoiceDate) + $increaseBy;
-                    break;
-
-                case 'cy':
-                    $partialResult = $this->repository->getCounterForYear($invoiceDate) + $increaseBy;
-                    break;
-
-                case 'cm':
-                    $partialResult = $this->repository->getCounterForMonth($invoiceDate) + $increaseBy;
-                    break;
-
-                case 'cd':
-                    $partialResult = $this->repository->getCounterForDay($invoiceDate) + $increaseBy;
-                    break;
-
-                default:
-                    $partialResult = $part;
-            }
-
-            if (null !== $formatterLength) {
-                $partialResult = str_pad($partialResult, $formatterLength, '0', STR_PAD_LEFT);
-            }
-
+            $partialResult = $this->parseReplacer($invoiceDate, $part);
             $result = str_replace($part, $partialResult, $result);
         }
 
         return (string) $result;
     }
+
+    private function parseReplacer(\DateTime $invoiceDate, string $originalFormat): string
+    {
+        $formatterLength = null;
+        $increaseBy = 0;
+        $formatPattern = str_replace(['{', '}'], '', $originalFormat);
+
+        $parts = preg_split('/([+\-,])+/', $formatPattern, -1, PREG_SPLIT_DELIM_CAPTURE);
+        $format = array_shift($parts);
+
+        if (\count($parts) % 2 !== 0) {
+            throw new \InvalidArgumentException('Invalid configuration found');
+        }
+
+        while (null !== ($tmp = array_shift($parts))) {
+            switch ($tmp) {
+                case '+':
+                    $local = array_shift($parts);
+                    if (!is_numeric($local)) {
+                        throw new \InvalidArgumentException('Unknown increment found');
+                    }
+                    $increaseBy = $increaseBy + \intval($local);
+                    break;
+
+                case '-':
+                    $local = array_shift($parts);
+                    if (!is_numeric($local)) {
+                        throw new \InvalidArgumentException('Unknown decrement found');
+                    }
+                    $increaseBy = $increaseBy - \intval($local);
+                    break;
+
+                case ',':
+                    $local = array_shift($parts);
+                    if (!is_numeric($local)) {
+                        throw new \InvalidArgumentException('Unknown format length found');
+                    }
+                    $formatterLength = \intval($local);
+                    if ((string) $formatterLength !== $local) {
+                        throw new \InvalidArgumentException('Unknown format length found');
+                    }
+                    break;
+
+                default:
+                    throw new \InvalidArgumentException('Unknown pattern found');
+            }
+        }
+
+        if ($increaseBy === 0) {
+            $increaseBy = 1;
+        }
+
+        switch ($format) {
+            case 'Y':
+                $partialResult = $invoiceDate->format('Y');
+                break;
+
+            case 'y':
+                $partialResult = $invoiceDate->format('y');
+                break;
+
+            case 'M':
+                $partialResult = $invoiceDate->format('m');
+                break;
+
+            case 'm':
+                $partialResult = $invoiceDate->format('n');
+                break;
+
+            case 'D':
+                $partialResult = $invoiceDate->format('d');
+                break;
+
+            case 'd':
+                $partialResult = $invoiceDate->format('j');
+                break;
+
+            case 'date':
+                $partialResult = $invoiceDate->format('ymd');
+                break;
+
+            // for customer
+            case 'cc':
+                $partialResult = $this->repository->getCounterForAllTime($invoiceDate, $this->model->getCustomer()) + $increaseBy;
+                break;
+
+            case 'ccy':
+                $partialResult = $this->repository->getCounterForYear($invoiceDate, $this->model->getCustomer()) + $increaseBy;
+                break;
+
+            case 'ccm':
+                $partialResult = $this->repository->getCounterForMonth($invoiceDate, $this->model->getCustomer()) + $increaseBy;
+                break;
+
+            case 'ccd':
+                $partialResult = $this->repository->getCounterForDay($invoiceDate, $this->model->getCustomer()) + $increaseBy;
+                break;
+
+            // across all invoices
+            case 'c':
+                $partialResult = $this->repository->getCounterForAllTime($invoiceDate) + $increaseBy;
+                break;
+
+            case 'cy':
+                $partialResult = $this->repository->getCounterForYear($invoiceDate) + $increaseBy;
+                break;
+
+            case 'cm':
+                $partialResult = $this->repository->getCounterForMonth($invoiceDate) + $increaseBy;
+                break;
+
+            case 'cd':
+                $partialResult = $this->repository->getCounterForDay($invoiceDate) + $increaseBy;
+                break;
+
+            default:
+                $partialResult = $originalFormat;
+        }
+
+        if (null !== $formatterLength) {
+            $partialResult = str_pad($partialResult, $formatterLength, '0', STR_PAD_LEFT);
+        }
+
+        return $partialResult;
+    }
 }
diff --git a/tests/Invoice/NumberGenerator/ConfigurableNumberGeneratorTest.php b/tests/Invoice/NumberGenerator/ConfigurableNumberGeneratorTest.php
index 443bd072..d8b7a067 100644
--- a/tests/Invoice/NumberGenerator/ConfigurableNumberGeneratorTest.php
+++ b/tests/Invoice/NumberGenerator/ConfigurableNumberGeneratorTest.php
@@ -22,7 +22,7 @@ use PHPUnit\Framework\TestCase;
  */
 class ConfigurableNumberGeneratorTest extends TestCase
 {
-    private function getSut(string $format)
+    private function getSut(string $format, int $counter = 1)
     {
         $config = $this->createMock(SystemConfiguration::class);
         $config->expects($this->any())
@@ -33,19 +33,19 @@ class ConfigurableNumberGeneratorTest extends TestCase
         $repository
             ->expects($this->any())
             ->method('getCounterForAllTime')
-            ->willReturn(1);
+            ->willReturn($counter);
         $repository
             ->expects($this->any())
             ->method('getCounterForYear')
-            ->willReturn(1);
+            ->willReturn($counter);
         $repository
             ->expects($this->any())
             ->method('getCounterForMonth')
-            ->willReturn(1);
+            ->willReturn($counter);
         $repository
             ->expects($this->any())
             ->method('getCounterForDay')
-            ->willReturn(1);
+            ->willReturn($counter);
 
         return new ConfigurableNumberGenerator($repository, $config);
     }
@@ -68,12 +68,14 @@ class ConfigurableNumberGeneratorTest extends TestCase
             ['{cy}', '2', $invoiceDate],
             ['{cm}', '2', $invoiceDate],
             ['{cd}', '2', $invoiceDate],
+            ['{cc}', '2', $invoiceDate],
+            ['{ccy}', '2', $invoiceDate],
+            ['{ccm}', '2', $invoiceDate],
+            ['{ccd}', '2', $invoiceDate],
             // number formatting (not testing the lower case versions, as the tests might break depending on the date)
             ['{date,10}', '0000' . $invoiceDate->format('ymd'), $invoiceDate],
-            ['{date,a}', $invoiceDate->format('ymd'), $invoiceDate], // invalid formatter length
             ['{Y,6}', '00' . $invoiceDate->format('Y'), $invoiceDate],
             ['{M,3}', '0' . $invoiceDate->format('m'), $invoiceDate],
-            ['{M,#}', $invoiceDate->format('m'), $invoiceDate], // invalid formatter length
             ['{D,3}', '0' . $invoiceDate->format('d'), $invoiceDate],
             // counter across all invoices
             ['{c,2}', '02', $invoiceDate],
@@ -86,32 +88,50 @@ class ConfigurableNumberGeneratorTest extends TestCase
             ['{ccm,2}', '02', $invoiceDate],
             ['{ccd,2}', '02', $invoiceDate],
             // with incrementing counter
+            ['{c+13,3}', '014', $invoiceDate],
             ['{c+13,2}', '14', $invoiceDate],
-            ['{ccy+1,2}', '02', $invoiceDate],
-            ['{cm+-1,2}', '02', $invoiceDate], // negative is not allowed and set to 1
-            ['{cm+0,2}', '02', $invoiceDate], // zero is not allowed and set to 1
-            ['{cd+111,2}', '112', $invoiceDate],
             ['{c+13}', '14', $invoiceDate],
-            ['{cy+1}', '2', $invoiceDate],
-            ['{cm+-1}', '2', $invoiceDate], // negative is not allowed and set to 1
-            ['{cm+0}', '2', $invoiceDate], // zero is not allowed and set to 1
             ['{cd+111}', '112', $invoiceDate],
+            ['{cd+111,5}', '00112', $invoiceDate],
+            ['{cd+111,2}', '113', $invoiceDate, 2],
+            ['{cm+0,2}', '03', $invoiceDate, 2], // zero is not allowed and set to 1
+            ['{cm+0}', '2', $invoiceDate], // zero is not allowed and set to 1
+            ['{cy+2}', '3', $invoiceDate],
+            ['{cc+4}', '5', $invoiceDate],
+            ['{ccy+2,2}', '03', $invoiceDate],
+            ['{ccm+2,2}', '03', $invoiceDate],
+            ['{ccd+2,2}', '03', $invoiceDate],
             // mixing identifiers
             ['{Y}{cy}', $invoiceDate->format('Y') . '2', $invoiceDate],
             ['{Y}{cy}{m}', $invoiceDate->format('Y') . '2' . $invoiceDate->format('n'), $invoiceDate],
             ['{Y}-{cy}/{m}', $invoiceDate->format('Y') . '-2/' . $invoiceDate->format('n'), $invoiceDate],
             ['{Y}-{cy}/{m}', $invoiceDate->format('Y') . '-2/' . $invoiceDate->format('n'), $invoiceDate],
             ['{Y,5}/{cy,5}', '0' . $invoiceDate->format('Y') . '/00002', $invoiceDate],
-            ['{Y,!}/{cy,o}', $invoiceDate->format('Y') . '/2', $invoiceDate], // invalid formatter length
+            // with decrementing counter
+            ['{c-1,2}', '00', $invoiceDate],
+            ['{c-2,2}', '-1', $invoiceDate],
+            // with incrementing and decrementing counter
+            ['{c-5+13,1}', '9', $invoiceDate],
+            ['{c+13-5,2}', '09', $invoiceDate],
+            // undefined behaviour - can change at any time
+            ['{cm+-1,2}', '00', $invoiceDate],
+            ['{cm+-1}', '0', $invoiceDate],
+            ['{cm-+1,2}', '02', $invoiceDate],
+            ['{cm-+1}', '2', $invoiceDate],
+            ['{a-+1+-1+++2}', '{a-+1+-1+++2}', $invoiceDate],
+            ['{date+2+2+2}', $invoiceDate->format('ymd'), $invoiceDate],
+            ['{cm+22+2+2-1-13-4}', '9', $invoiceDate],
+            ['{cm+22+2-2-22}', '6', $invoiceDate, 5],
+            ['{cm-21+22+2-2}', '6', $invoiceDate, 5],
         ];
     }
 
     /**
      * @dataProvider getTestData
      */
-    public function testGetInvoiceNumber(string $format, string $expectedInvoiceNumber, \DateTime $invoiceDate)
+    public function testGetInvoiceNumber(string $format, string $expectedInvoiceNumber, \DateTime $invoiceDate, int $counter = 1)
     {
-        $sut = $this->getSut($format);
+        $sut = $this->getSut($format, $counter);
         $model = new InvoiceModel(new DebugFormatter());
         $model->setInvoiceDate($invoiceDate);
         $model->setCustomer(new Customer());
@@ -120,4 +140,43 @@ class ConfigurableNumberGeneratorTest extends TestCase
         $this->assertEquals($expectedInvoiceNumber, $sut->getInvoiceNumber());
         $this->assertEquals('default', $sut->getId());
     }
+
+    public function getInvalidTestData()
+    {
+        $invoiceDate = new \DateTime();
+
+        return [
+            ['{cm-}', $invoiceDate, 'decrement'],
+            ['{cm+}', $invoiceDate, 'increment'],
+            ['{cm+-}', $invoiceDate, 'decrement'],
+            ['{cm-+}', $invoiceDate, 'increment'],
+            ['{cy,}', $invoiceDate, 'format length'],
+            ['{date,a}', $invoiceDate, 'format length'],
+            ['{M,#}', $invoiceDate, 'format length'],
+            ['{,a}', $invoiceDate, 'format length'],
+            ['{Y,!}/{cy,o}', $invoiceDate, 'format length'],
+            ['{cm,}', $invoiceDate, 'format length'],
+            ['{cd+111,050}', $invoiceDate,  'format length'],
+            ['{-+}', $invoiceDate, 'increment'],
+            ['{+}', $invoiceDate, 'increment'],
+            ['{+-}', $invoiceDate, 'decrement'],
+        ];
+    }
+
+    /**
+     * @dataProvider getInvalidTestData
+     */
+    public function testInvalidGetInvoiceNumber(string $format, \DateTime $invoiceDate, string $brokenPart)
+    {
+        $this->expectException(\InvalidArgumentException::class);
+        $this->expectExceptionMessage(sprintf('Unknown %s found', $brokenPart));
+
+        $sut = $this->getSut($format);
+        $model = new InvoiceModel(new DebugFormatter());
+        $model->setInvoiceDate($invoiceDate);
+        $model->setCustomer(new Customer());
+        $sut->setModel($model);
+
+        $sut->getInvoiceNumber();
+    }
 }