diff --git a/.gitignore b/.gitignore
index b269b40..1c115db 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 vendor/
 composer.lock
 .phpunit.result.cache
+.phpunit.cache/
 build/
diff --git a/.travis.yml b/.travis.yml
index c327c45..b549d7d 100755
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,10 +1,8 @@
 language: php
 php:
-  - 7.4
-  - 8.0
-  - 8.1
   - 8.2
   - 8.3
+  - 8.4
 
 fast_finish: true
 
diff --git a/README.md b/README.md
index 41b88ef..32a87f5 100755
--- a/README.md
+++ b/README.md
@@ -1,3 +1,27 @@
+## Fork (redcuillin)
+
+Fork of [MARCspec/php-marc-spec](https://github.com/MARCspec/php-marc-spec) for PHP 8.4 and 8.5 support.
+
+### Using this fork with Composer
+
+Add the following to your `composer.json` `repositories` and `require` sections (merge with existing entries as needed):
+
+```json
+"repositories": [
+    {
+        "type": "vcs",
+        "url": "https://github.com/redcuillin/php-marc-spec"
+    }
+],
+"require": {
+    "ck/php-marcspec": "dev-master as 2.999.0"
+}
+```
+
+-----
+
+## Upstream readme:
+
 # PHP-MARCspec
 
 [![Build Status](https://travis-ci.org/MARCspec/php-marc-spec.svg?branch=master)](https://travis-ci.org/MARCspec/php-marc-spec)
@@ -14,7 +38,7 @@ Installation can be done by using [composer](https://getcomposer.org/doc/00-intr
 composer require ck/php-marcspec
 ```
 
-PHP-MARCspec requires PHP 7.4 or later.
+PHP-MARCspec requires PHP 8.2 or later and is compatible with PHP 8.4.
 
 # Usage
 
diff --git a/Test/ComparisonStringTest.php b/Test/ComparisonStringTest.php
index 22f4f55..384ebab 100755
--- a/Test/ComparisonStringTest.php
+++ b/Test/ComparisonStringTest.php
@@ -9,23 +9,21 @@
 namespace CK\MARCspec\Test;
 
 use CK\MARCspec\ComparisonString;
+use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\TestCase;
 
 class ComparisonStringTest extends TestCase
 {
-    /**
-     * @dataProvider invalidFromTestSuiteProvider
-     *
-     * @expectedException Exception
-     */
+    #[DataProvider('invalidFromTestSuiteProvider')]
     public function testInvalidFromTestSuite($test)
     {
+        $this->expectException(\Exception::class);
         new ComparisonString($test);
     }
 
-    public function invalidFromTestSuiteProvider()
+    public static function invalidFromTestSuiteProvider()
     {
-        $invalidTests = json_decode(file_get_contents(__DIR__.'/../'.'vendor/ck/marcspec-test-suite/invalid/invalidComparisonString.json'));
+        $invalidTests = json_decode(file_get_contents(__DIR__ . '/../' . 'vendor/ck/marcspec-test-suite/invalid/invalidComparisonString.json'));
         $data = [];
         foreach ($invalidTests->{'tests'} as $test) {
             $data[0][] = $test->{'data'};
@@ -36,10 +34,10 @@ public function invalidFromTestSuiteProvider()
 
     public function testValidFromTestSuite()
     {
-        $validTests = json_decode(file_get_contents(__DIR__.'/../'.'vendor/ck/marcspec-test-suite/valid/validComparisonString.json'));
+        $validTests = json_decode(file_get_contents(__DIR__ . '/../' . 'vendor/ck/marcspec-test-suite/valid/validComparisonString.json'));
         foreach ($validTests->{'tests'} as $test) {
             new ComparisonString($test->{'data'});
-            $this->assertSame(1, preg_match('/'.$validTests->{'schema'}->{'pattern'}.'/', $test->{'data'}));
+            $this->assertSame(1, preg_match('/' . $validTests->{'schema'}->{'pattern'} . '/', $test->{'data'}));
         }
     }
 
@@ -49,22 +47,22 @@ public function compare($arg)
     }
 
     /****
-    * invalid data types
-    ***/
+     * invalid data types
+     ***/
 
     /**
-     * @expectedException InvalidArgumentException
      */
     public function testInvalidArgument1Decode()
     {
+        $this->expectException(\InvalidArgumentException::class);
         $this->compare(['a']);
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidArgument2Decode()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->compare('.{.');
     }
 
@@ -100,10 +98,10 @@ public function testValidComparisonString2()
     }
 
     /**
-     * @expectedException BadMethodCallException
      */
     public function testOffsetUnset()
     {
+        $this->expectException(\BadMethodCallException::class);
         $escaped_string = '\\.';
         $compare = $this->compare($escaped_string);
         unset($compare['raw']);
diff --git a/Test/FieldTest.php b/Test/FieldTest.php
index 9564d7f..6f9ad54 100755
--- a/Test/FieldTest.php
+++ b/Test/FieldTest.php
@@ -11,23 +11,21 @@
 use CK\MARCspec\Field;
 use CK\MARCspec\MARCspec;
 use CK\MARCspec\SubSpec;
+use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\TestCase;
 
 class FieldTest extends TestCase
 {
-    /**
-     * @dataProvider invalidFromTestSuiteProvider
-     *
-     * @expectedException Exception
-     */
+    #[DataProvider('invalidFromTestSuiteProvider')]
     public function testInvalidFromTestSuite($test)
     {
+        $this->expectException(\Exception::class);
         new Field($test);
     }
 
-    public function invalidFromTestSuiteProvider()
+    public static function invalidFromTestSuiteProvider()
     {
-        $invalidTests = json_decode(file_get_contents(__DIR__.'/../'.'vendor/ck/marcspec-test-suite/invalid/invalidFieldTag.json'));
+        $invalidTests = json_decode(file_get_contents(__DIR__ . '/../' . 'vendor/ck/marcspec-test-suite/invalid/invalidFieldTag.json'));
         $data = [];
         foreach ($invalidTests->{'tests'} as $test) {
             $data[0][] = $test->{'data'};
@@ -38,7 +36,7 @@ public function invalidFromTestSuiteProvider()
 
     public function testValidFromTestSuite()
     {
-        $validTests = json_decode(file_get_contents(__DIR__.'/../'.'vendor/ck/marcspec-test-suite/valid/validFieldTag.json'));
+        $validTests = json_decode(file_get_contents(__DIR__ . '/../' . 'vendor/ck/marcspec-test-suite/valid/validFieldTag.json'));
         foreach ($validTests->{'tests'} as $test) {
             $this->assertInstanceOf('CK\MARCspec\FieldInterface', new Field($test->{'data'}));
         }
@@ -50,146 +48,146 @@ public function fieldspec($arg)
     }
 
     /**
-     * @expectedException InvalidArgumentException
      */
     public function testInvalidArgument2Decode()
     {
+        $this->expectException(\InvalidArgumentException::class);
         $this->fieldspec(['245']);
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidFieldSpec13()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->fieldspec('007/');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidFieldSpec14()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->fieldspec('007/1-2-');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidFieldSpec15()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->fieldspec('24#');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidFieldSpec16()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->fieldspec('007/-2');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidFieldSpec17()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->fieldspec('245[-2]');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidFieldSpec18()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->fieldspec('245[1-2-]');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidFieldSpec19()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->fieldspec('245[1-2');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidFieldSpec110()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->fieldspec('007/1-X');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidFieldSpec111()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->fieldspec('007/#-');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidFieldSpec112()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->fieldspec('245[0-2a]');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidFieldSpec113()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->fieldspec('300[1-]');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidFieldSpec114()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $fieldSpec = $this->fieldspec('aA0');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidFieldSpec31()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->fieldspec('245^1');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidFieldSpec32()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->fieldspec('245_$');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidFieldSpec33()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->fieldspec('245^');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidArgument310Decode()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->fieldspec('245{$c=$d}$a');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidArgument311Decode()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->fieldspec('245[1]/1_01');
     }
 
@@ -274,9 +272,7 @@ public function testValidFieldSpec22()
     }
 
     /**
-     * @covers CK\MARCspec\Field::offsetSet
-     * @covers CK\MARCspec\Field::offsetExists
-     * @covers CK\MARCspec\Field::addSubSpec
+     * Field offset API and subspecs.
      */
     public function testValidFieldSpec24()
     {
@@ -391,7 +387,7 @@ public function testEncode()
     }
 
     /**
-     * @covers CK\MARCspec\Field::jsonSerialize
+     * JSON encoding of a field spec.
      */
     public function testJson()
     {
@@ -407,10 +403,10 @@ public function testJson()
     }
 
     /**
-     * @expectedException BadMethodCallException
      */
     public function testOffsetUnset()
     {
+        $this->expectException(\BadMethodCallException::class);
         $fieldSpec = $this->fieldspec('245');
         unset($fieldSpec['tag']);
     }
diff --git a/Test/IndicatorTest.php b/Test/IndicatorTest.php
index 0a826d0..ea4e774 100644
--- a/Test/IndicatorTest.php
+++ b/Test/IndicatorTest.php
@@ -11,6 +11,7 @@
 use CK\MARCspec\Indicator;
 use CK\MARCspec\MARCspec;
 use CK\MARCspec\SubSpec;
+use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\TestCase;
 
 class IndicatorTest extends TestCase
@@ -22,19 +23,16 @@ protected function indicatorPos($arg)
         return $ind->getPos();
     }
 
-    /**
-     * @dataProvider invalidFromTestSuiteProvider
-     *
-     * @expectedException Exception
-     */
+    #[DataProvider('invalidFromTestSuiteProvider')]
     public function testInvalidFromTestSuite($test)
     {
+        $this->expectException(\Exception::class);
         new Indicator($test);
     }
 
-    public function invalidFromTestSuiteProvider()
+    public static function invalidFromTestSuiteProvider()
     {
-        $invalidTests = json_decode(file_get_contents(__DIR__.'/../'.'vendor/ck/marcspec-test-suite/invalid/invalidIndicators.json'));
+        $invalidTests = json_decode(file_get_contents(__DIR__ . '/../' . 'vendor/ck/marcspec-test-suite/invalid/invalidIndicators.json'));
         $data = [];
         foreach ($invalidTests->{'tests'} as $test) {
             $data[0][] = $test->{'data'};
@@ -45,7 +43,7 @@ public function invalidFromTestSuiteProvider()
 
     public function testValidFromTestSuite()
     {
-        $validTests = json_decode(file_get_contents(__DIR__.'/../'.'vendor/ck/marcspec-test-suite/valid/validIndicators.json'));
+        $validTests = json_decode(file_get_contents(__DIR__ . '/../' . 'vendor/ck/marcspec-test-suite/valid/validIndicators.json'));
         foreach ($validTests->{'tests'} as $test) {
             $pos = $this->indicatorPos($test->{'data'});
             $this->assertTrue($pos === '1' or $pos === '2');
@@ -53,10 +51,10 @@ public function testValidFromTestSuite()
     }
 
     /**
-     * @expectedException InvalidArgumentException
      */
     public function testPosFail()
     {
+        $this->expectException(\InvalidArgumentException::class);
         $ind = new Indicator('3');
     }
 
@@ -72,34 +70,34 @@ public function testSetPos()
     }
 
     /**
-     * @expectedException InvalidArgumentException
      */
     public function testIndicatorFail1()
     {
+        $this->expectException(\InvalidArgumentException::class);
         $ind = new Indicator(3);
     }
 
     /**
-     * @expectedException InvalidArgumentException
      */
     public function testIndicatorFail2()
     {
+        $this->expectException(\InvalidArgumentException::class);
         $ind = new Indicator(['3']);
     }
 
     /**
-     * @expectedException InvalidArgumentException
      */
     public function testIndicatorFail3()
     {
+        $this->expectException(\InvalidArgumentException::class);
         $ind = new Indicator('12');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testIndicatorFail4()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $ind = new Indicator('1{$a}');
     }
 
@@ -115,7 +113,7 @@ public function testSubspecsAndArrayAccessAndToString()
     }
 
     /**
-     * @covers CK\MARCspec\Indicator::jsonSerialize
+     * JSON encoding of an indicator spec.
      */
     public function testJson()
     {
diff --git a/Test/MarcSpecTest.php b/Test/MarcSpecTest.php
index bf15190..e0faa7b 100755
--- a/Test/MarcSpecTest.php
+++ b/Test/MarcSpecTest.php
@@ -10,23 +10,21 @@
 
 use CK\MARCspec\Field;
 use CK\MARCspec\MARCspec;
+use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\TestCase;
 
 class MarcSpecTest extends TestCase
 {
-    /**
-     * @dataProvider invalidFromTestSuiteProvider
-     *
-     * @expectedException Exception
-     */
+    #[DataProvider('invalidFromTestSuiteProvider')]
     public function testInvalidFromTestSuite($test)
     {
+        $this->expectException(\Exception::class);
         new MARCspec($test);
     }
 
-    public function invalidFromTestSuiteProvider()
+    public static function invalidFromTestSuiteProvider()
     {
-        foreach (glob(__DIR__.'/../'.'vendor/ck/marcspec-test-suite/invalid/wildCombination_*.json') as $filename) {
+        foreach (glob(__DIR__ . '/../' . 'vendor/ck/marcspec-test-suite/invalid/wildCombination_*.json') as $filename) {
             $invalidTests = json_decode(file_get_contents($filename));
         }
         $data = [];
@@ -39,7 +37,7 @@ public function invalidFromTestSuiteProvider()
 
     public function testValidFromTestSuite()
     {
-        foreach (glob(__DIR__.'/../'.'vendor/ck/marcspec-test-suite/valid/wildCombination_*.json') as $filename) {
+        foreach (glob(__DIR__ . '/../' . 'vendor/ck/marcspec-test-suite/valid/wildCombination_*.json') as $filename) {
             $validTests = json_decode(file_get_contents($filename));
         }
         foreach ($validTests->{'tests'} as $test) {
@@ -53,38 +51,38 @@ public function marcspec($arg)
     }
 
     /****
-    * invalid data types
-    ***/
+     * invalid data types
+     ***/
 
     /**
-     * @expectedException RuntimeException
      */
     public function testInvalidArgument01Decode()
     {
+        $this->expectException(\RuntimeException::class);
         $this->marcspec('24');
     }
 
     /**
-     * @expectedException InvalidArgumentException
      */
     public function testInvalidArgument1Decode()
     {
+        $this->expectException(\InvalidArgumentException::class);
         $this->marcspec((int) '245$a');
     }
 
     /**
-     * @expectedException InvalidArgumentException
      */
     public function testInvalidArgument2Decode()
     {
+        $this->expectException(\InvalidArgumentException::class);
         $this->marcspec(['245$a']);
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidArgument3Decode()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->marcspec('245/#$a');
     }
 
@@ -268,10 +266,10 @@ public function testToString()
     }
 
     /**
-     * @expectedException BadMethodCallException
      */
     public function testOffsetUnset()
     {
+        $this->expectException(\BadMethodCallException::class);
         $ms = $this->marcspec('245');
         unset($ms['field']);
     }
diff --git a/Test/SubSpecTest.php b/Test/SubSpecTest.php
index e778b6c..34cc21f 100755
--- a/Test/SubSpecTest.php
+++ b/Test/SubSpecTest.php
@@ -27,30 +27,30 @@ protected function marcspec($arg)
     }
 
     /****
-    * invalid data types
-    ***/
+     * invalid data types
+     ***/
 
     /**
-     * @expectedException InvalidArgumentException
      */
     public function testInvalidSubSpec1Decode()
     {
+        $this->expectException(\InvalidArgumentException::class);
         $this->subspec('245', '=', '300');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidSubSpec2Decode()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->subspec(new Field('245'), '=', new Subfield('245'));
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidSubSpec3Decode()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->marcspec('...{$a{$b}}');
     }
 
diff --git a/Test/SubfieldTest.php b/Test/SubfieldTest.php
index 1cc6ac6..9fdf08d 100755
--- a/Test/SubfieldTest.php
+++ b/Test/SubfieldTest.php
@@ -11,23 +11,21 @@
 use CK\MARCspec\MARCspec;
 use CK\MARCspec\Subfield;
 use CK\MARCspec\SubSpec;
+use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\TestCase;
 
 class SubfieldTest extends TestCase
 {
-    /**
-     * @dataProvider invalidFromTestSuiteProvider
-     *
-     * @expectedException Exception
-     */
+    #[DataProvider('invalidFromTestSuiteProvider')]
     public function testInvalidFromTestSuite($test)
     {
+        $this->expectException(\Exception::class);
         new Subfield($test);
     }
 
-    public function invalidFromTestSuiteProvider()
+    public static function invalidFromTestSuiteProvider()
     {
-        $invalidTests = json_decode(file_get_contents(__DIR__.'/../'.'vendor/ck/marcspec-test-suite/invalid/invalidSubfieldTag.json'));
+        $invalidTests = json_decode(file_get_contents(__DIR__ . '/../' . 'vendor/ck/marcspec-test-suite/invalid/invalidSubfieldTag.json'));
         $data = [];
         foreach ($invalidTests->{'tests'} as $test) {
             $data[0][] = $test->{'data'};
@@ -38,7 +36,7 @@ public function invalidFromTestSuiteProvider()
 
     public function testValidFromTestSuite()
     {
-        $validTests = json_decode(file_get_contents(__DIR__.'/../'.'vendor/ck/marcspec-test-suite/valid/validSubfieldTag.json'));
+        $validTests = json_decode(file_get_contents(__DIR__ . '/../' . 'vendor/ck/marcspec-test-suite/valid/validSubfieldTag.json'));
         foreach ($validTests->{'tests'} as $test) {
             $this->assertInstanceOf('CK\MARCspec\SubfieldInterface', new Subfield($test->{'data'}));
         }
@@ -50,146 +48,146 @@ public function subfieldspec($arg)
     }
 
     /****
-    * invalid data types
-    ***/
+     * invalid data types
+     ***/
 
     /**
-     * @expectedException InvalidArgumentException
      */
     public function testInvalidArgument1Decode()
     {
+        $this->expectException(\InvalidArgumentException::class);
         $this->subfieldspec((int) '$a');
     }
 
     /**
-     * @expectedException InvalidArgumentException
      */
     public function testInvalidArgument2Decode()
     {
+        $this->expectException(\InvalidArgumentException::class);
         $this->subfieldspec(['$a']);
     }
 
     /****
-    * invalid subfield tags
-    ***/
+     * invalid subfield tags
+     ***/
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidSubfieldSpec1()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->subfieldspec(' $a ');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidSubfieldSpec11()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->subfieldspec('$a/');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidSubfieldSpec12()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->subfieldspec('$a$b');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidSubfieldSpec13()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->subfieldspec('|');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidSubfieldSpec14()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->subfieldspec('$a/1-2-');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidSubfieldSpec15()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->subfieldspec('$|');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidSubfieldSpec16()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->subfieldspec('$a/-2');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidSubfieldSpec17()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->subfieldspec('$a[-2]');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidSubfieldSpec18()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->subfieldspec('$a[1-2-]');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidSubfieldSpec19()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->subfieldspec('$a[1-2');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidSubfieldSpec110()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->subfieldspec('$a/1-X');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidSubfieldSpec111()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->subfieldspec('$a/#-');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidSubfieldSpec112()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->subfieldspec('$a[0-2a]');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidSubfieldSpec113()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->subfieldspec('$[1-]');
     }
 
     /**
-     * @expectedException CK\MARCspec\Exception\InvalidMARCspecException
      */
     public function testInvalidSubfieldSpec114()
     {
+        $this->expectException(\CK\MARCspec\Exception\InvalidMARCspecException::class);
         $this->subfieldspec('$a{$b}');
     }
 
@@ -215,10 +213,10 @@ public function testValidSubfieldSpec01()
     }
 
     /**
-     * @expectedException InvalidArgumentException
      */
     public function testInvalidSubSpecAdd1()
     {
+        $this->expectException(\InvalidArgumentException::class);
         $Subfield = $this->subfieldspec('a');
         $Subspec = new SubSpec('245$b', '!', '245$b');
         $Subfield->addSubSpec($Subspec);
@@ -384,10 +382,10 @@ public function testEncode()
     }
 
     /**
-     * @expectedException BadMethodCallException
      */
     public function testOffsetUnset()
     {
+        $this->expectException(\BadMethodCallException::class);
         $Subfield = $this->subfieldspec('$a');
         unset($Subfield['tag']);
     }
diff --git a/composer.json b/composer.json
index b8b395d..ef6ad78 100644
--- a/composer.json
+++ b/composer.json
@@ -2,7 +2,10 @@
     "name": "ck/php-marcspec",
     "type": "library",
     "description": "PHP based MARCspec parser and validator",
-    "keywords": [ "MARCspec", "MARC 21" ],
+    "keywords": [
+        "MARCspec",
+        "MARC 21"
+    ],
     "license": "MIT",
     "authors": [
         {
@@ -12,11 +15,11 @@
         }
     ],
     "require": {
-        "php": ">=7.4"
+        "php": ">=8.2"
     },
     "require-dev": {
-         "phpunit/phpunit": "~8.5.52",
-         "ck/marcspec-test-suite": "@dev"
+        "phpunit/phpunit": "^11.5",
+        "ck/marcspec-test-suite": "@dev"
     },
     "autoload": {
         "psr-4": {
@@ -24,4 +27,4 @@
             "CK\\MARCspec\\Exception\\": "src/Exception/"
         }
     }
-}
+}
\ No newline at end of file
diff --git a/phpunit.xml b/phpunit.xml
index 90adbb8..d58fe9b 100755
--- a/phpunit.xml
+++ b/phpunit.xml
@@ -1,33 +1,16 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<phpunit backupGlobals="false"
-backupStaticAttributes="false"
-colors="true"
-convertErrorsToExceptions="true"
-convertNoticesToExceptions="true"
-convertWarningsToExceptions="true"
-processIsolation="false"
-stopOnFailure="false"
-bootstrap="Test/autoload.php"
->
-    <testsuites>
-        <testsuite name="PHP-MARCspec Test Suite">
-            <directory suffix="Test.php">./Test/</directory>
-        </testsuite>
-    </testsuites>
-    <filter>
-        <whitelist processUncoveredFilesFromWhitelist="true">
-            <directory>./</directory>
-            <exclude>
-                <directory>./Test</directory>
-                <directory>./vendor</directory>
-            </exclude>
-        </whitelist>
-    </filter>
-    <php>
-        <ini name="error_reporting" value="-1" />
-    </php>
-    <logging>
-        <log type="coverage-text" target="php://stdout" showUncoveredFiles="true" />
-        <log type="coverage-clover" target="build/logs/clover.xml"/>
-    </logging>
-</phpunit>
\ No newline at end of file
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/11.5/phpunit.xsd" bootstrap="Test/autoload.php" colors="true" cacheDirectory=".phpunit.cache" displayDetailsOnTestsThatTriggerDeprecations="true">
+  <testsuites>
+    <testsuite name="PHP-MARCspec Test Suite">
+      <directory suffix="Test.php">Test</directory>
+    </testsuite>
+  </testsuites>
+  <php>
+    <ini name="error_reporting" value="-1"/>
+  </php>
+  <source restrictNotices="true" restrictWarnings="true" ignoreIndirectDeprecations="true">
+    <include>
+      <directory suffix=".php">src</directory>
+    </include>
+  </source>
+</phpunit>
diff --git a/src/ComparisonString.php b/src/ComparisonString.php
index 9092cf4..75f499c 100755
--- a/src/ComparisonString.php
+++ b/src/ComparisonString.php
@@ -1,4 +1,5 @@
 <?php
+
 /**
  * MARCspec is the specification of a reference, encoded as string, to a set of data
  * from within a MARC record.
@@ -32,13 +33,13 @@ public function __construct($raw)
     {
         if (!is_string($raw)) {
             throw new \InvalidArgumentException('Argument must be of type string. Got '
-                .gettype($raw).'.');
+                . gettype($raw) . '.');
         }
 
         if (false !== strpos($raw, ' ')) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::CS.
-                InvalidMARCspecException::SPACE,
+                InvalidMARCspecException::CS .
+                    InvalidMARCspecException::SPACE,
                 $raw
             );
         }
@@ -46,8 +47,8 @@ public function __construct($raw)
         /* char of list ${}!=~?|\s must be escaped if not at index 0*/
         if (!preg_match('/^(.(?:[^${}!=~?| ]|(?<=\\\\)[${}!=~?|])*)$/', $raw)) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::CS.
-                InvalidMARCspecException::ESCAPE,
+                InvalidMARCspecException::CS .
+                    InvalidMARCspecException::ESCAPE,
                 $raw
             );
         }
@@ -80,7 +81,7 @@ public static function escape($arg)
     {
         $specialChars = ['{', '}', '!', '=', '~', '?'];
         for ($i = 0; $i < count($specialChars); $i++) {
-            $arg = str_replace($specialChars[$i], '\\'.$specialChars[$i], $arg);
+            $arg = str_replace($specialChars[$i], '\\' . $specialChars[$i], $arg);
         }
 
         return $arg = str_replace(' ', '\s', $arg);
@@ -91,13 +92,13 @@ public static function escape($arg)
      */
     public function __toString()
     {
-        return '\\'.$this->raw;
+        return '\\' . $this->raw;
     }
 
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize(): mixed
     {
         return ['comparisonString' => $this->raw];
     }
@@ -109,13 +110,13 @@ public function jsonSerialize()
      *
      * @param string $offset Key raw|comparable
      */
-    public function offsetExists($offset)
+    public function offsetExists(mixed $offset): bool
     {
         switch ($offset) {
             case 'raw':
             case 'comparable':
                 return true;
-            break;
+                break;
             default:
                 return false;
         }
@@ -128,15 +129,15 @@ public function offsetExists($offset)
      *
      * @param string $offset Key operator|leftSubTerm|rightSubTerm
      */
-    public function offsetGet($offset)
+    public function offsetGet(mixed $offset): mixed
     {
         switch ($offset) {
             case 'raw':
                 return $this->getRaw();
-            break;
+                break;
             case 'comparable':
                 return $this->getComparable();
-            break;
+                break;
             default:
                 throw new \UnexpectedValueException("Offset $offset does not exist.");
         }
@@ -149,7 +150,7 @@ public function offsetGet($offset)
      *
      * @param string $offset
      */
-    public function offsetSet($offset, $value)
+    public function offsetSet(mixed $offset, mixed $value): void
     {
         throw new \UnexpectedValueException("Offset $offset cannot be set.");
     }
@@ -159,7 +160,7 @@ public function offsetSet($offset, $value)
      *
      * @param string $offset
      */
-    public function offsetUnset($offset)
+    public function offsetUnset(mixed $offset): void
     {
         throw new \BadMethodCallException("Offset $offset can not be unset.");
     }
diff --git a/src/Field.php b/src/Field.php
index b27603d..dc428ca 100755
--- a/src/Field.php
+++ b/src/Field.php
@@ -1,4 +1,5 @@
 <?php
+
 /**
  * MARCspec is the specification of a reference, encoded as string, to a set of data
  * from within a MARC record.
@@ -63,22 +64,22 @@ public function __construct($fieldspec)
         // check string length
         if (3 > $specLength) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::FS.
-                InvalidMARCspecException::MINIMUM3,
+                InvalidMARCspecException::FS .
+                    InvalidMARCspecException::MINIMUM3,
                 $fieldspec
             );
         }
         if (preg_match('/\s/', $fieldspec)) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::FS.
-                InvalidMARCspecException::SPACE,
+                InvalidMARCspecException::FS .
+                    InvalidMARCspecException::SPACE,
                 $fieldspec
             );
         }
         if ($strpos = strpos($fieldspec, '{')) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::FS.
-                InvalidMARCspecException::DETECTEDSS,
+                InvalidMARCspecException::FS .
+                    InvalidMARCspecException::DETECTEDSS,
                 $fieldspec
             );
         }
@@ -89,16 +90,16 @@ public function __construct($fieldspec)
 
         if (array_key_exists('subfields', $parser->parsed)) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::FS.
-                InvalidMARCspecException::DETECTEDSF,
+                InvalidMARCspecException::FS .
+                    InvalidMARCspecException::DETECTEDSF,
                 $fieldspec
             );
         }
 
         if (array_key_exists('indicatorpos', $parser->parsed)) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::FS.
-                InvalidMARCspecException::DETECTEDIN,
+                InvalidMARCspecException::FS .
+                    InvalidMARCspecException::DETECTEDIN,
                 $fieldspec
             );
         }
@@ -146,8 +147,8 @@ private function validateTag($tag)
     {
         if (!preg_match('/[.0-9a-z]{3,3}|[.0-9A-Z]{3,3}/', $tag)) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::FS.
-                InvalidMARCspecException::FTAG,
+                InvalidMARCspecException::FS .
+                    InvalidMARCspecException::FTAG,
                 $tag
             );
         }
@@ -173,7 +174,7 @@ public function addSubSpec($SubSpec)
         } else {
             throw new \InvalidArgumentException('Param 1 must be instance of
                 SubSpecInterface or array with instances of SubSpecInterface. Got "'
-                .gettype($subSpec).'".');
+                . gettype($SubSpec) . '".');
         }
     }
 
@@ -188,7 +189,7 @@ public function getSubSpecs()
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize(): mixed
     {
         $_fieldSpec['tag'] = $this->getTag();
 
@@ -238,9 +239,9 @@ public function getBaseSpec()
         if ($indexStart === 0 && $indexEnd === '#') {
             // use abbreviation
         } else {
-            $fieldSpec .= '['.$indexStart;
+            $fieldSpec .= '[' . $indexStart;
             if ($indexStart !== $indexEnd) {
-                $fieldSpec .= '-'.$indexEnd;
+                $fieldSpec .= '-' . $indexEnd;
             }
             $fieldSpec .= ']';
         }
@@ -250,9 +251,9 @@ public function getBaseSpec()
             if ($charStart === 0 && $charEnd === '#') {
                 // use abbreviation
             } else {
-                $fieldSpec .= '/'.$charStart;
+                $fieldSpec .= '/' . $charStart;
                 if ($charEnd !== $charStart) {
-                    $fieldSpec .= '-'.$charEnd;
+                    $fieldSpec .= '-' . $charEnd;
                 }
             }
         }
@@ -273,9 +274,9 @@ public function __toString()
                     foreach ($subSpec as $orKey => $orSubSpec) {
                         $subSpec[$orKey] = $orSubSpec->__toString();
                     }
-                    $fieldSpec .= '{'.implode('|', $subSpec).'}';
+                    $fieldSpec .= '{' . implode('|', $subSpec) . '}';
                 } else {
-                    $fieldSpec .= '{'.$subSpec->__toString().'}';
+                    $fieldSpec .= '{' . $subSpec->__toString() . '}';
                 }
             }
         }
@@ -291,32 +292,33 @@ public function __toString()
      * @param string $offset Key indexStart|indexEnd|charStart|charEnd|charLength
      *                       |indicator1|indicator2|subSpecs
      */
-    public function offsetExists($offset)
+    public function offsetExists(mixed $offset): bool
     {
         switch ($offset) {
             case 'tag':
                 return isset($this->tag);
-            break;
+                break;
             case 'indexStart':
                 return isset($this->indexStart);
-            break;
+                break;
             case 'indexEnd':
                 return isset($this->indexEnd);
-            break;
+                break;
             case 'indexLength':
                 return !is_null($this->getIndexLength());
-            break;
+                break;
             case 'charStart':
                 return isset($this->charStart);
-            break;
+                break;
             case 'charEnd':
                 return isset($this->charEnd);
-            break;
+                break;
             case 'charLength':
                 return !is_null($this->getCharLength());
-            break;
-            case 'subSpecs': return (0 < count($this->subSpecs)) ? true : false;
-            break;
+                break;
+            case 'subSpecs':
+                return (0 < count($this->subSpecs)) ? true : false;
+                break;
             default:
                 return false;
         }
@@ -330,32 +332,33 @@ public function offsetExists($offset)
      * @param string $offset Key indexStart|indexEnd|charStart|charEnd|charLength
      *                       |indicator1|indicator2|subSpecs
      */
-    public function offsetGet($offset)
+    public function offsetGet(mixed $offset): mixed
     {
         switch ($offset) {
             case 'tag':
                 return $this->getTag();
-            break;
+                break;
             case 'indexStart':
                 return $this->getIndexStart();
-            break;
+                break;
             case 'indexEnd':
                 return $this->getIndexEnd();
-            break;
+                break;
             case 'indexLength':
                 return $this->getIndexLength();
-            break;
+                break;
             case 'charStart':
                 return $this->getCharStart();
-            break;
+                break;
             case 'charEnd':
                 return $this->getCharEnd();
-            break;
+                break;
             case 'charLength':
                 return $this->getCharLength();
-            break;
-            case 'subSpecs': return $this->getSubSpecs();
-            break;
+                break;
+            case 'subSpecs':
+                return $this->getSubSpecs();
+                break;
             default:
                 throw new \UnexpectedValueException("Offset $offset does not exist.");
         }
@@ -369,7 +372,7 @@ public function offsetGet($offset)
      * @param string $offset Key indexStart|indexEnd|charStart|charEnd|charLength
      *                       |indicator1|indicator2|subSpecs
      */
-    public function offsetSet($offset, $value)
+    public function offsetSet(mixed $offset, mixed $value): void
     {
         switch ($offset) {
             case 'indexStart':
@@ -386,7 +389,7 @@ public function offsetSet($offset, $value)
 
             case 'indexLength':
                 throw new \UnexpectedValueException('indexLength is always calculated.');
-            break;
+                break;
 
             case 'charStart':
                 $this->setCharStartEnd($value);
@@ -402,10 +405,11 @@ public function offsetSet($offset, $value)
 
             case 'charLength':
                 throw new \UnexpectedValueException('CharLength is always calculated.');
-            break;
+                break;
 
-            case 'subSpecs': $this->addSubSpec($value);
-            break;
+            case 'subSpecs':
+                $this->addSubSpec($value);
+                break;
 
             default:
                 throw new \UnexpectedValueException("Offset $offset cannot be set.");
@@ -415,7 +419,7 @@ public function offsetSet($offset, $value)
     /**
      * Access object like an associative array.
      */
-    public function offsetUnset($offset)
+    public function offsetUnset(mixed $offset): void
     {
         throw new \BadMethodCallException("Offset $offset can not be unset.");
     }
diff --git a/src/Indicator.php b/src/Indicator.php
index 89ebb73..600aa6e 100644
--- a/src/Indicator.php
+++ b/src/Indicator.php
@@ -1,4 +1,5 @@
 <?php
+
 /**
  * MARCspec is the specification of a reference, encoded as string, to a set of data
  * from within a MARC record.
@@ -36,15 +37,16 @@ class Indicator implements IndicatorInterface, \JsonSerializable, \ArrayAccess
     public function __construct($indicatorpos)
     {
         if (!is_string($indicatorpos) and !is_int($indicatorpos)) {
-            throw new \InvalidArgumentException('Method only accepts string as argument. '.
-                gettype($indicatorpos).' given.'
+            throw new \InvalidArgumentException(
+                'Method only accepts string as argument. ' .
+                    gettype($indicatorpos) . ' given.'
             );
         }
 
         if (strpos($indicatorpos, '{')) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::IN.
-                InvalidMARCspecException::DETECTEDSS,
+                InvalidMARCspecException::IN .
+                    InvalidMARCspecException::DETECTEDSS,
                 $indicatorpos
             );
         }
@@ -82,16 +84,18 @@ public function addSubSpec($SubSpec)
         } elseif (is_array($SubSpec)) {
             foreach ($SubSpec as $sub) {
                 if (!($sub instanceof SubSpecInterface)) {
-                    throw new \InvalidArgumentException('Values of array of subSpecs
+                    throw new \InvalidArgumentException(
+                        'Values of array of subSpecs
                         must be instances of SubSpecInterface.'
                     );
                 }
             }
             $this->subSpecs[] = $SubSpec;
         } else {
-            throw new \InvalidArgumentException('Param 1 must be instance of
+            throw new \InvalidArgumentException(
+                'Param 1 must be instance of
                 SubSpecInterface or array with instances of SubSpecInterface. Got "'
-                .gettype($subSpec).'".'
+                    . gettype($SubSpec) . '".'
             );
         }
     }
@@ -101,7 +105,7 @@ public function addSubSpec($SubSpec)
      */
     public function getBaseSpec()
     {
-        return '^'.$this->position;
+        return '^' . $this->position;
     }
 
     /**
@@ -117,9 +121,9 @@ public function __toString()
                     foreach ($subSpec as $orKey => $orSubSpec) {
                         $subSpec[$orKey] = $orSubSpec->__toString();
                     }
-                    $indicatorSpec .= '{'.implode('|', $subSpec).'}';
+                    $indicatorSpec .= '{' . implode('|', $subSpec) . '}';
                 } else {
-                    $indicatorSpec .= '{'.$subSpec->__toString().'}';
+                    $indicatorSpec .= '{' . $subSpec->__toString() . '}';
                 }
             }
         }
@@ -135,14 +139,17 @@ public function __toString()
      * @param string $offset Key indexStart|indexEnd|charStart|charEnd|charLength
      *                       |indicator1|indicator2|subSpecs
      */
-    public function offsetExists($offset)
+    public function offsetExists(mixed $offset): bool
     {
         switch ($offset) {
-            case 'position': return isset($this->position);
-            break;
-            case 'subSpecs': return (0 < count($this->subSpecs)) ? true : false;
-            break;
-            default: return false;
+            case 'position':
+                return isset($this->position);
+                break;
+            case 'subSpecs':
+                return (0 < count($this->subSpecs)) ? true : false;
+                break;
+            default:
+                return false;
         }
     }
 
@@ -154,14 +161,17 @@ public function offsetExists($offset)
      * @param string $offset Key indexStart|indexEnd|charStart|charEnd|charLength
      *                       |indicator1|indicator2|subSpecs
      */
-    public function offsetGet($offset)
+    public function offsetGet(mixed $offset): mixed
     {
         switch ($offset) {
-            case 'position': return $this->getPos();
-            break;
-            case 'subSpecs': return $this->getSubSpecs();
-            break;
-            default: throw new \UnexpectedValueException("Offset $offset does not exist.");
+            case 'position':
+                return $this->getPos();
+                break;
+            case 'subSpecs':
+                return $this->getSubSpecs();
+                break;
+            default:
+                throw new \UnexpectedValueException("Offset $offset does not exist.");
         }
     }
 
@@ -173,19 +183,21 @@ public function offsetGet($offset)
      * @param string $offset Key indexStart|indexEnd|charStart|charEnd|charLength
      *                       |indicator1|indicator2|subSpecs
      */
-    public function offsetSet($offset, $value)
+    public function offsetSet(mixed $offset, mixed $value): void
     {
         switch ($offset) {
-            case 'subSpecs': $this->addSubSpec($value);
-            break;
-            default: throw new \UnexpectedValueException("Offset $offset cannot be set.");
+            case 'subSpecs':
+                $this->addSubSpec($value);
+                break;
+            default:
+                throw new \UnexpectedValueException("Offset $offset cannot be set.");
         }
     }
 
     /**
      * Access object like an associative array.
      */
-    public function offsetUnset($offset)
+    public function offsetUnset(mixed $offset): void
     {
         throw new \BadMethodCallException("Offset $offset can not be unset.");
     }
@@ -193,7 +205,7 @@ public function offsetUnset($offset)
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize(): mixed
     {
         $_indicatorSpec['position'] = $this->getPos();
 
diff --git a/src/MARCspec.php b/src/MARCspec.php
index 546daf3..8e97cb8 100644
--- a/src/MARCspec.php
+++ b/src/MARCspec.php
@@ -1,4 +1,5 @@
 <?php
+
 /**
  * MARCspec is the specification of a reference, encoded as string, to a set of data
  * from within a MARC record.
@@ -50,16 +51,16 @@ public function __construct($spec)
             // check string length
             if (3 > $specLength) {
                 throw new InvalidMARCspecException(
-                    InvalidMARCspecException::MS.
-                    InvalidMARCspecException::LENGTH.
-                    InvalidMARCspecException::MINIMUM3,
+                    InvalidMARCspecException::MS .
+                        InvalidMARCspecException::LENGTH .
+                        InvalidMARCspecException::MINIMUM3,
                     $spec
                 );
             }
             if (preg_match('/\s/', $spec)) {
                 throw new InvalidMARCspecException(
-                    InvalidMARCspecException::MS.
-                    InvalidMARCspecException::SPACE,
+                    InvalidMARCspecException::MS .
+                        InvalidMARCspecException::SPACE,
                     $spec
                 );
             }
@@ -149,7 +150,7 @@ public function getSubfield($arg)
     {
         if (1 < strlen($arg)) {
             throw new \UnexpectedValueException(
-                'Method only allows argument to be 1 character long. Got '.strlen($arg)
+                'Method only allows argument to be 1 character long. Got ' . strlen($arg)
             );
         }
 
@@ -181,16 +182,16 @@ public function setIndicator(IndicatorInterface $indicator)
     {
         if (!is_null($this->getSubfields())) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::MS.
-                InvalidMARCspecException::SFEX,
+                InvalidMARCspecException::MS .
+                    InvalidMARCspecException::SFEX,
                 $this->__toString()
             );
         }
 
         if (!is_null($this->field->getCharStart())) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::MS.
-                InvalidMARCspecException::CSEX,
+                InvalidMARCspecException::MS .
+                    InvalidMARCspecException::CSEX,
                 $this->__toString()
             );
         }
@@ -259,16 +260,16 @@ public function addSubfields($subfields)
     {
         if (!is_null($this->getIndicator())) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::MS.
-                InvalidMARCspecException::INEX,
+                InvalidMARCspecException::MS .
+                    InvalidMARCspecException::INEX,
                 $this->__toString()
             );
         }
 
         if (!is_null($this->field->getCharStart())) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::MS.
-                InvalidMARCspecException::CSEX,
+                InvalidMARCspecException::MS .
+                    InvalidMARCspecException::CSEX,
                 $this->__toString()
             );
         }
@@ -279,16 +280,16 @@ public function addSubfields($subfields)
             $this->checkIfString($subfields);
             if (2 > strlen($subfields)) {
                 throw new InvalidMARCspecException(
-                    InvalidMARCspecException::SF.
-                    InvalidMARCspecException::LENGTH.
-                    InvalidSubfieldspecException::MINIMUM2,
+                    InvalidMARCspecException::SF .
+                        InvalidMARCspecException::LENGTH .
+                        InvalidSubfieldspecException::MINIMUM2,
                     $subfields
                 );
             }
             if ('$' !== $subfields[0]) {
                 throw new InvalidMARCspecException(
-                    InvalidMARCspecException::SF.
-                    InvalidSubfieldspecException::PREFIX,
+                    InvalidMARCspecException::SF .
+                        InvalidSubfieldspecException::PREFIX,
                     $subfields
                 );
             }
@@ -318,37 +319,37 @@ private function handleSubfieldRanges($arg)
     {
         if (strlen($arg) < 3) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::SF.
-                InvalidMARCspecException::LENGTH3,
+                InvalidMARCspecException::SF .
+                    InvalidMARCspecException::LENGTH3,
                 $arg
             );
         }
 
         if (preg_match('/[a-z]/', $arg[0]) && !preg_match('/[a-z]/', $arg[2])) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::SF.
-                InvalidMARCspecException::RANGE,
+                InvalidMARCspecException::SF .
+                    InvalidMARCspecException::RANGE,
                 $arg
             );
         }
         if (preg_match('/[A-Z]/', $arg[0]) && !preg_match('/[A-Z]/', $arg[2])) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::SF.
-                InvalidMARCspecException::RANGE,
+                InvalidMARCspecException::SF .
+                    InvalidMARCspecException::RANGE,
                 $arg
             );
         }
         if (preg_match('/[0-9]/', $arg[0]) && !preg_match('/[0-9]/', $arg[2])) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::SF.
-                InvalidMARCspecException::RANGE,
+                InvalidMARCspecException::SF .
+                    InvalidMARCspecException::RANGE,
                 $arg
             );
         }
         if ($arg[0] > $arg[2]) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::SF.
-                InvalidMARCspecException::RANGE,
+                InvalidMARCspecException::SF .
+                    InvalidMARCspecException::RANGE,
                 $arg
             );
         }
@@ -368,8 +369,8 @@ private function handleSubfieldRanges($arg)
     private function checkIfString($arg)
     {
         if (!is_string($arg)) {
-            throw new \InvalidArgumentException('Method only accepts string as argument. '.
-                gettype($arg).' given.');
+            throw new \InvalidArgumentException('Method only accepts string as argument. ' .
+                gettype($arg) . ' given.');
         }
     }
 
@@ -390,40 +391,40 @@ public static function validatePos($pos)
 
         if (1 > $posLength) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::PR.
-                InvalidMARCspecException::PR1,
+                InvalidMARCspecException::PR .
+                    InvalidMARCspecException::PR1,
                 $pos
             );
         }
 
         if (preg_match('/[^0-9\-#]/', $pos)) { // alphabetic characters etc. are not valid
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::PR.
-                InvalidMARCspecException::PR2,
+                InvalidMARCspecException::PR .
+                    InvalidMARCspecException::PR2,
                 $pos
             );
         }
 
         if (strpos($pos, '-') === $posLength - 1) { // something like 123- is not valid
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::PR.
-                InvalidMARCspecException::PR3,
+                InvalidMARCspecException::PR .
+                    InvalidMARCspecException::PR3,
                 $pos
             );
         }
 
         if (0 === strpos($pos, '-')) { // something like -123 ist not valid
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::PR.
-                InvalidMARCspecException::PR4,
+                InvalidMARCspecException::PR .
+                    InvalidMARCspecException::PR4,
                 $pos
             );
         }
 
         if (strpos($pos, '-') !== strrpos($pos, '-')) { // only one - is allowed
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::PR.
-                InvalidMARCspecException::PR5,
+                InvalidMARCspecException::PR .
+                    InvalidMARCspecException::PR5,
                 $pos
             );
         }
@@ -432,8 +433,8 @@ public static function validatePos($pos)
 
         if (2 < count($_pos)) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::PR.
-                InvalidMARCspecException::PR6,
+                InvalidMARCspecException::PR .
+                    InvalidMARCspecException::PR6,
                 $pos
             );
         }
@@ -462,7 +463,7 @@ private function createSubSpec($_subTerms, $Subfield = null)
         $fieldContext = $this->field->getBaseSpec();
         $context = $fieldContext;
         if (!is_null($Subfield)) {
-            $context = $fieldContext.$Subfield->getBaseSpec();
+            $context = $fieldContext . $Subfield->getBaseSpec();
             $subfieldContext = $Subfield->getBaseSpec();
         }
 
@@ -472,8 +473,8 @@ private function createSubSpec($_subTerms, $Subfield = null)
             } else {
                 if (strpos('[/_$', $subTerm[0]) && is_null($context)) {
                     throw new InvalidMARCspecException(
-                        InvalidMARCspecException::SS.
-                        InvalidMARCspecException::MISSINGFIELD,
+                        InvalidMARCspecException::SS .
+                            InvalidMARCspecException::MISSINGFIELD,
                         $subTerm
                     );
                 }
@@ -482,35 +483,35 @@ private function createSubSpec($_subTerms, $Subfield = null)
                     case '_':
                         if ($refPos = strpos($context, $subTerm[0])) {
                             if ('$' !== substr($context, $refPos - 1, 1)) {
-                                return new MARCspec(substr($context, 0, $refPos).$subTerm);
+                                return new MARCspec(substr($context, 0, $refPos) . $subTerm);
                             }
                         }
 
-                        return new MARCspec($fieldContext.$subTerm);
+                        return new MARCspec($fieldContext . $subTerm);
 
                     case '$':
-                        return new MARCspec($fieldContext.$subTerm);
+                        return new MARCspec($fieldContext . $subTerm);
 
                     case '/':
                         $refPos = strrpos($context, $subTerm[0]);
                         if ($refPos) {
                             if ('$' !== substr($context, $refPos - 1, 1)) {
-                                return new MARCspec(substr($context, 0, $refPos).$subTerm);
+                                return new MARCspec(substr($context, 0, $refPos) . $subTerm);
                             }
                         }
 
-                        return new MARCspec($context.$subTerm);
+                        return new MARCspec($context . $subTerm);
 
                     case '[':
                         $refPos = strrpos($context, $subTerm[0]);
                         if ($refPos) {
                             if ('$' !== substr($context, $refPos - 1, 1)) {
-                                return new MARCspec(substr($context, 0, $refPos).$subTerm);
+                                return new MARCspec(substr($context, 0, $refPos) . $subTerm);
                             } else {
-                                return new MARCspec($context.$subTerm);
+                                return new MARCspec($context . $subTerm);
                             }
                         } else {
-                            return new MARCspec(substr($context, 0, 3).$subTerm);
+                            return new MARCspec(substr($context, 0, 3) . $subTerm);
                         }
 
                     default:
@@ -535,7 +536,7 @@ private function createSubSpec($_subTerms, $Subfield = null)
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize(): mixed
     {
         $_marcSpec['field'] = $this->field->jsonSerialize();
 
@@ -574,16 +575,18 @@ public function __toString()
      *
      * @param string $offset Key field|subfield
      */
-    public function offsetExists($offset)
+    public function offsetExists(mixed $offset): bool
     {
         switch ($offset) {
             case 'field':
                 return isset($this->field);
-            break;
-            case 'indicator': return isset($this->indicator);
-            break;
-            case 'subfields': return (0 < count($this->subfields)) ? true : false;
-            break;
+                break;
+            case 'indicator':
+                return isset($this->indicator);
+                break;
+            case 'subfields':
+                return (0 < count($this->subfields)) ? true : false;
+                break;
             default:
                 return false;
         }
@@ -596,16 +599,18 @@ public function offsetExists($offset)
      *
      * @param string $offset Key field|subfield
      */
-    public function offsetGet($offset)
+    public function offsetGet(mixed $offset): mixed
     {
         switch ($offset) {
             case 'field':
                 return $this->getField();
-            break;
-            case 'indicator': return $this->getIndicator();
-            break;
-            case 'subfields': return $this->getSubfields();
-            break;
+                break;
+            case 'indicator':
+                return $this->getIndicator();
+                break;
+            case 'subfields':
+                return $this->getSubfields();
+                break;
             default:
                 return $this->getSubfield($offset);
         }
@@ -618,14 +623,17 @@ public function offsetGet($offset)
      *
      * @param string $offset Key subfield
      */
-    public function offsetSet($offset, $value)
+    public function offsetSet(mixed $offset, mixed $value): void
     {
         switch ($offset) {
-            case 'indicator': $this->addIndicator($value);
-            break;
-            case 'subfields': $this->addSubfields($value);
-            break;
-            default: throw new \UnexpectedValueException("Offset $offset cannot be set.");
+            case 'indicator':
+                $this->addIndicator($value);
+                break;
+            case 'subfields':
+                $this->addSubfields($value);
+                break;
+            default:
+                throw new \UnexpectedValueException("Offset $offset cannot be set.");
         }
     }
 
@@ -634,12 +642,12 @@ public function offsetSet($offset, $value)
      *
      * @param string $offset
      */
-    public function offsetUnset($offset)
+    public function offsetUnset(mixed $offset): void
     {
         throw new \BadMethodCallException("Offset $offset can not be unset.");
     }
 
-    public function getIterator()
+    public function getIterator(): \Traversable
     {
         return new SpecIterator(['field' => $this->field, 'subfields' => $this->subfields, 'indicator' => $this->indicator]);
     }
diff --git a/src/MARCspecParser.php b/src/MARCspecParser.php
index 49fcb60..6dd704b 100755
--- a/src/MARCspecParser.php
+++ b/src/MARCspecParser.php
@@ -1,4 +1,5 @@
 <?php
+
 /**
  * MARCspec is the specification of a reference, encoded as string, to a set of data
  * from within a MARC record.
@@ -34,9 +35,14 @@ class MARCspecParser
      */
     protected $CHARPOS;
     /**
-     * @var string Regex for indicator position
+     * @var string Regex for indicator position fragment
+     */
+    protected $INDICATORPOS;
+
+    /**
+     * @var string Regex for subspecs fragment
      */
-    protected $INDICATOR;
+    protected $SUBSPECS;
     /**
      * @var string Regex for field subspecs
      */
@@ -125,10 +131,10 @@ public function __construct($spec = null)
      */
     public function parse($marcspec)
     {
-        if (!preg_match_all('/'.$this->MARCSPEC.'/', $marcspec, $this->parsed, PREG_SET_ORDER)) {
+        if (!preg_match_all('/' . $this->MARCSPEC . '/', $marcspec, $this->parsed, PREG_SET_ORDER)) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::FS.
-                InvalidMARCspecException::MISSINGFIELD,
+                InvalidMARCspecException::FS .
+                    InvalidMARCspecException::MISSINGFIELD,
                 $marcspec
             );
         }
@@ -137,16 +143,16 @@ public function parse($marcspec)
 
         if (!array_key_exists('field', $this->parsed)) { // TODO: check if 'tag' is the required key
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::FS.
-                InvalidMARCspecException::FTAG,
+                InvalidMARCspecException::FS .
+                    InvalidMARCspecException::FTAG,
                 $marcspec
             );
         }
 
         if (strlen($this->parsed[0]) !== strlen($marcspec)) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::FS.
-                InvalidMARCspecException::USELESS,
+                InvalidMARCspecException::FS .
+                    InvalidMARCspecException::USELESS,
                 $marcspec
             );
         }
@@ -154,16 +160,16 @@ public function parse($marcspec)
         if (array_key_exists('charpos', $this->parsed)) {
             if (array_key_exists('indicatorpos', $this->parsed)) {
                 throw new InvalidMARCspecException(
-                    InvalidMARCspecException::FS.
-                    InvalidMARCspecException::CHARORIND,
+                    InvalidMARCspecException::FS .
+                        InvalidMARCspecException::CHARORIND,
                     $marcspec
                 );
             }
 
             if (array_key_exists('subfields', $this->parsed)) {
                 throw new InvalidMARCspecException(
-                    InvalidMARCspecException::FS.
-                    InvalidMARCspecException::CHARORSF,
+                    InvalidMARCspecException::FS .
+                        InvalidMARCspecException::CHARORSF,
                     $marcspec
                 );
             }
@@ -172,8 +178,8 @@ public function parse($marcspec)
         if (array_key_exists('subfields', $this->parsed)) {
             if (array_key_exists('indicatorpos', $this->parsed)) {
                 throw new InvalidMARCspecException(
-                    InvalidMARCspecException::FS.
-                    InvalidMARCspecException::INDORSF,
+                    InvalidMARCspecException::FS .
+                        InvalidMARCspecException::INDORSF,
                     $marcspec
                 );
             }
@@ -204,10 +210,10 @@ public function parse($marcspec)
      */
     public function parseSubfields($subfieldspec)
     {
-        if (!preg_match_all('/'.$this->SUBFIELD.'/', $subfieldspec, $_subfieldMatches, PREG_SET_ORDER)) {
+        if (!preg_match_all('/' . $this->SUBFIELD . '/', $subfieldspec, $_subfieldMatches, PREG_SET_ORDER)) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::SF.
-                InvalidMARCspecException::SFCHAR,
+                InvalidMARCspecException::SF .
+                    InvalidMARCspecException::SFCHAR,
                 $subfieldspec
             );
         }
@@ -250,8 +256,8 @@ function (&$_subfield) use (&$test) {
 
         if ($test !== $subfieldspec) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::SF.
-                InvalidMARCspecException::USELESS,
+                InvalidMARCspecException::SF .
+                    InvalidMARCspecException::USELESS,
                 $subfieldspec
             );
         }
@@ -270,24 +276,24 @@ public function subfieldToArray($subfieldspec)
     {
         if (!$_sf = $this->parseSubfields($subfieldspec)) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::SF.
-                InvalidMARCspecException::UNKNOWN,
+                InvalidMARCspecException::SF .
+                    InvalidMARCspecException::UNKNOWN,
                 $subfieldspec
             );
         }
 
         if (1 < count($_sf)) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::SF.
-                InvalidMARCspecException::MULTISF,
+                InvalidMARCspecException::SF .
+                    InvalidMARCspecException::MULTISF,
                 $subfieldspec
             );
         }
 
         if ($_sf[0]['subfield'] !== $subfieldspec) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::SF.
-                InvalidMARCspecException::USELESS,
+                InvalidMARCspecException::SF .
+                    InvalidMARCspecException::USELESS,
                 $subfieldspec
             );
         }
@@ -305,10 +311,10 @@ public function subfieldToArray($subfieldspec)
     private function matchSubSpecs($subSpecs)
     {
         $_subSpecs = [];
-        if (!preg_match_all('/'.$this->SUBSPEC.'/U', $subSpecs, $_subSpecMatches, PREG_SET_ORDER)) {
+        if (!preg_match_all('/' . $this->SUBSPEC . '/U', $subSpecs, $_subSpecMatches, PREG_SET_ORDER)) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::SS.
-                InvalidMARCspecException::UNKNOWN,
+                InvalidMARCspecException::SS .
+                    InvalidMARCspecException::UNKNOWN,
                 $subSpecs
             );
         }
@@ -318,8 +324,8 @@ private function matchSubSpecs($subSpecs)
                 $_subSpecs[$key] = preg_split('/(?<!\\\)\|/', $_subSpecMatch[1], -1, PREG_SPLIT_NO_EMPTY);
             } else {
                 throw new InvalidMARCspecException(
-                    InvalidMARCspecException::SS.
-                    InvalidMARCspecException::UNKNOWN,
+                    InvalidMARCspecException::SS .
+                        InvalidMARCspecException::UNKNOWN,
                     $subSpecs
                 );
             }
@@ -339,20 +345,20 @@ private function matchSubTerms($subSpec)
     {
         if (preg_match('/(?<![\\\\\$])[\{\}]/', $subSpec, $_error, PREG_OFFSET_CAPTURE)) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::SS.
-                InvalidMARCspecException::ESCAPE,
+                InvalidMARCspecException::SS .
+                    InvalidMARCspecException::ESCAPE,
                 $subSpec
             );
         }
 
-        if (preg_match_all('/'.$this->SUBTERMS.'/', $subSpec, $_subTermMatches, PREG_SET_ORDER)) {
+        if (preg_match_all('/' . $this->SUBTERMS . '/', $subSpec, $_subTermMatches, PREG_SET_ORDER)) {
             if (empty($_subTermMatches[0]['operator'])) {
                 $_subTermMatches[0]['operator'] = '?';
             }
             if (!$_subTermMatches[0]['rightsubterm']) {
                 throw new InvalidMARCspecException(
-                    InvalidMARCspecException::SS.
-                    InvalidMARCspecException::MISSINGRIGHT,
+                    InvalidMARCspecException::SS .
+                        InvalidMARCspecException::MISSINGRIGHT,
                     $subSpec
                 );
             }
@@ -360,8 +366,8 @@ private function matchSubTerms($subSpec)
             return array_filter($_subTermMatches[0], 'strlen');
         } else {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::SS.
-                InvalidMARCspecException::UNKNOWN,
+                InvalidMARCspecException::SS .
+                    InvalidMARCspecException::UNKNOWN,
                 $subSpec
             );
         }
@@ -374,19 +380,19 @@ private function setConstants()
     {
         $this->FIELDTAG = '(?<tag>(?:[a-z0-9\.]{3,3}|[A-Z0-9\.]{3,3}|[0-9\.]{3,3}))';
         $this->POSITIONORRANGE = '(?:(?:(?:[0-9]+|#)\-(?:[0-9]+|#))|(?:[0-9]+|#))';
-        $this->INDEX = '(?:\[(?<index>'.$this->POSITIONORRANGE.')\])?';
-        $this->CHARPOS = '(?:\/(?<charpos>'.$this->POSITIONORRANGE.'))?';
+        $this->INDEX = '(?:\[(?<index>' . $this->POSITIONORRANGE . ')\])?';
+        $this->CHARPOS = '(?:\/(?<charpos>' . $this->POSITIONORRANGE . '))?';
         $this->INDICATORPOS = '(?:\^(?<indicatorpos>[12]))';
         $this->SUBSPECS = '(?<subspecs>(?:\{.+?(?<!(?<!(\$|\\\))(\$|\\\))\})+)?';
         $this->SUBFIELDS = '(?<subfields>\$.+)';
-        $this->FIELD = '(?<field>'.$this->FIELDTAG.$this->INDEX.')';
-        $this->MARCSPEC = '^'.$this->FIELD.'(?:'.$this->SUBFIELDS.'|(?:'.$this->INDICATORPOS.'|'.$this->CHARPOS.')'.$this->SUBSPECS.')$';
+        $this->FIELD = '(?<field>' . $this->FIELDTAG . $this->INDEX . ')';
+        $this->MARCSPEC = '^' . $this->FIELD . '(?:' . $this->SUBFIELDS . '|(?:' . $this->INDICATORPOS . '|' . $this->CHARPOS . ')' . $this->SUBSPECS . ')$';
         $this->SUBFIELDTAGRANGE = '(?<subfieldtagrange>(?:[0-9a-z]\-[0-9a-z]))';
         $this->SUBFIELDTAG = '(?<subfieldtag>[\!-\?\[-\{\}-~])';
-        $this->SUBFIELD = '(?<subfield>\$(?:'.$this->SUBFIELDTAGRANGE.'|'.$this->SUBFIELDTAG.')'.$this->INDEX.$this->CHARPOS.$this->SUBSPECS.')';
+        $this->SUBFIELD = '(?<subfield>\$(?:' . $this->SUBFIELDTAGRANGE . '|' . $this->SUBFIELDTAG . ')' . $this->INDEX . $this->CHARPOS . $this->SUBSPECS . ')';
         $this->LEFTSUBTERM = '^(?<leftsubterm>(?:\\\(?:(?<=\\\)[\!\=\~\?]|[^\!\=\~\?])+)|(?:(?<=\$)[\!\=\~\?]|[^\!\=\~\?])+)?';
         $this->OPERATOR = '(?<operator>\!\=|\!\~|\=|\~|\!|\?)';
-        $this->SUBTERMS = '(?:'.$this->LEFTSUBTERM.$this->OPERATOR.')?(?<rightsubterm>.+)$';
+        $this->SUBTERMS = '(?:' . $this->LEFTSUBTERM . $this->OPERATOR . ')?(?<rightsubterm>.+)$';
         $this->SUBSPEC = '(?:\{(.+)\})';
     }
 }
diff --git a/src/PositionOrRange.php b/src/PositionOrRange.php
index 5dc3f75..4fd53c8 100755
--- a/src/PositionOrRange.php
+++ b/src/PositionOrRange.php
@@ -1,4 +1,5 @@
 <?php
+
 /**
  * MARCspec is the specification of a reference, encoded as string, to a set of data
  * from within a MARC record.
@@ -144,8 +145,8 @@ private function getLength($type = true)
 
         if (1 > $length) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::PR.
-                InvalidMARCspecException::NEGATIVE
+                InvalidMARCspecException::PR .
+                    InvalidMARCspecException::NEGATIVE
             );
         }
 
@@ -174,13 +175,15 @@ private function validateStartEnd($start, $end)
             $_startEnd[0] = '#';
         } else {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::PR.
-                InvalidMARCspecException::PR7,
+                InvalidMARCspecException::PR .
+                    InvalidMARCspecException::PR7,
                 $start
             );
         }
 
-        if (preg_match('/[0-9#]/', $end)) {
+        if (null === $end) {
+            $_startEnd[1] = null;
+        } elseif (preg_match('/[0-9#]/', $end)) {
             if ('#' === $end) {
                 $_startEnd[1] = '#';
             } elseif (preg_match('/[0-9]/', $end)) {
@@ -188,16 +191,16 @@ private function validateStartEnd($start, $end)
 
                 if ($_startEnd[1] < $_startEnd[0]) {
                     throw new InvalidMARCspecException(
-                        InvalidMARCspecException::PR.
-                        InvalidMARCspecException::PR8,
-                        $start.'-'.$end
+                        InvalidMARCspecException::PR .
+                            InvalidMARCspecException::PR8,
+                        $start . '-' . $end
                     );
                 }
             } else {
                 throw new InvalidMARCspecException(
-                    InvalidMARCspecException::PR.
-                    InvalidMARCspecException::PR8,
-                    $start.'-'.$end
+                    InvalidMARCspecException::PR .
+                        InvalidMARCspecException::PR8,
+                    $start . '-' . $end
                 );
             }
         } else {
@@ -258,7 +261,7 @@ protected function checkIfString($arg)
     {
         if (!is_string($arg)) {
             throw new \InvalidArgumentException('Method only accepts string as argument. '
-                .gettype($arg).' given.');
+                . gettype($arg) . ' given.');
         }
     }
 } // EOC
diff --git a/src/SpecIterator.php b/src/SpecIterator.php
index a185fad..a423fe0 100755
--- a/src/SpecIterator.php
+++ b/src/SpecIterator.php
@@ -1,4 +1,5 @@
 <?php
+
 /**
  * MARCspec is the specification of a reference, encoded as string, to a set of data
  * from within a MARC record.
@@ -21,36 +22,28 @@ public function __construct($array)
         }
     }
 
-    public function rewind()
+    public function rewind(): void
     {
         reset($this->var);
     }
 
-    public function current()
+    public function current(): mixed
     {
-        $var = current($this->var);
-
-        return $var;
+        return current($this->var);
     }
 
-    public function key()
+    public function key(): mixed
     {
-        $var = key($this->var);
-
-        return $var;
+        return key($this->var);
     }
 
-    public function next()
+    public function next(): void
     {
-        $var = next($this->var);
-
-        return $var;
+        next($this->var);
     }
 
-    public function valid()
+    public function valid(): bool
     {
-        $var = $this->current() !== false;
-
-        return $var;
+        return key($this->var) !== null;
     }
 }
diff --git a/src/SubSpec.php b/src/SubSpec.php
index 047b96f..612f49c 100755
--- a/src/SubSpec.php
+++ b/src/SubSpec.php
@@ -1,4 +1,5 @@
 <?php
+
 /**
  * MARCspec is the specification of a reference, encoded as string, to a set of data
  * from within a MARC record.
@@ -40,8 +41,10 @@ class SubSpec implements SubSpecInterface, \JsonSerializable, \ArrayAccess, \Ite
      */
     public function __construct($leftSubTerm, $operator, $rightSubTerm)
     {
-        if ($leftSubTerm instanceof MARCspecInterface
-            || $leftSubTerm instanceof ComparisonStringInterface) {
+        if (
+            $leftSubTerm instanceof MARCspecInterface
+            || $leftSubTerm instanceof ComparisonStringInterface
+        ) {
             $this->leftSubTerm = $leftSubTerm;
         } else {
             throw new \InvalidArgumentException(
@@ -50,14 +53,16 @@ public function __construct($leftSubTerm, $operator, $rightSubTerm)
             );
         }
 
-        if ($rightSubTerm instanceof MARCspecInterface
-            || $rightSubTerm instanceof ComparisonStringInterface) {
+        if (
+            $rightSubTerm instanceof MARCspecInterface
+            || $rightSubTerm instanceof ComparisonStringInterface
+        ) {
             $this->rightSubTerm = $rightSubTerm;
         } else {
             throw new \InvalidArgumentException(
                 'Argument 3 must be instance of CK\MARCspec\MARCspecInterface or 
                 CK\MARCspec\ComparisonStringInterface. Got '
-                .gettype($rightSubTerm)
+                    . gettype($rightSubTerm)
             );
         }
 
@@ -73,8 +78,8 @@ private function setOperator($operator)
     {
         if (!in_array($operator, ['=', '!=', '~', '!~', '!', '?'], true)) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::SS.
-                InvalidMARCspecException::OPERATOR,
+                InvalidMARCspecException::SS .
+                    InvalidMARCspecException::OPERATOR,
                 $operator
             );
         }
@@ -112,7 +117,7 @@ public function __toString()
     {
         switch ($this->operator) {
             case '!':
-                $subSpecString = $this->operator."$this->rightSubTerm";
+                $subSpecString = $this->operator . "$this->rightSubTerm";
                 break;
 
             case '?':
@@ -120,7 +125,7 @@ public function __toString()
                 break;
 
             default:
-                $subSpecString = "$this->leftSubTerm".$this->operator."$this->rightSubTerm";
+                $subSpecString = "$this->leftSubTerm" . $this->operator . "$this->rightSubTerm";
         }
 
         return $subSpecString;
@@ -129,7 +134,7 @@ public function __toString()
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize(): mixed
     {
         if (!is_null($this->leftSubTerm)) {
             $_subSpec['leftSubTerm'] = $this->leftSubTerm->jsonSerialize();
@@ -147,7 +152,7 @@ public function jsonSerialize()
      *
      * @param string $offset Key operator|leftSubTerm|rightSubTerm
      */
-    public function offsetExists($offset)
+    public function offsetExists(mixed $offset): bool
     {
         switch ($offset) {
             case 'operator':
@@ -168,7 +173,7 @@ public function offsetExists($offset)
      *
      * @param string $offset Key operator|leftSubTerm|rightSubTerm
      */
-    public function offsetGet($offset)
+    public function offsetGet(mixed $offset): mixed
     {
         switch ($offset) {
             case 'operator':
@@ -195,7 +200,7 @@ public function offsetGet($offset)
      *
      * @param string $offset
      */
-    public function offsetSet($offset, $value)
+    public function offsetSet(mixed $offset, mixed $value): void
     {
         throw new \UnexpectedValueException("Offset $offset cannot be set.");
     }
@@ -205,12 +210,12 @@ public function offsetSet($offset, $value)
      *
      * @param string $offset
      */
-    public function offsetUnset($offset)
+    public function offsetUnset(mixed $offset): void
     {
         throw new \BadMethodCallException("Offset $offset can not be unset.");
     }
 
-    public function getIterator()
+    public function getIterator(): \Traversable
     {
         return new SpecIterator(
             [
diff --git a/src/Subfield.php b/src/Subfield.php
index 43934a1..4f2d449 100755
--- a/src/Subfield.php
+++ b/src/Subfield.php
@@ -1,4 +1,5 @@
 <?php
+
 /**
  * MARCspec is the specification of a reference, encoded as string, to a set of data
  * from within a MARC record.
@@ -56,35 +57,36 @@ class Subfield extends PositionOrRange implements SubfieldInterface, \JsonSerial
     public function __construct($subfieldspec)
     {
         if (!is_string($subfieldspec)) {
-            throw new \InvalidArgumentException('Method only accepts string as argument. '.
-            gettype($subfieldspec).' given.'
+            throw new \InvalidArgumentException(
+                'Method only accepts string as argument. ' .
+                    gettype($subfieldspec) . ' given.'
             );
         }
 
         if (!is_string($subfieldspec)) {
-            throw new \InvalidArgumentException('Method only accepts string as argument. '.
-            gettype($subfieldspec).' given.');
+            throw new \InvalidArgumentException('Method only accepts string as argument. ' .
+                gettype($subfieldspec) . ' given.');
         }
 
         $argLength = strlen($subfieldspec);
 
         if (0 === $argLength) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::SF.
-                InvalidMARCspecException::MISSINGTAG,
+                InvalidMARCspecException::SF .
+                    InvalidMARCspecException::MISSINGTAG,
                 $subfieldspec
             );
         }
 
         if (1 == $argLength) {
-            $subfieldspec = '$'.$subfieldspec;
+            $subfieldspec = '$' . $subfieldspec;
         }
 
         if (1 < $argLength) {
             if ('-' == $subfieldspec[1]) { // assuming subfield range
                 throw new InvalidMARCspecException(
-                    InvalidMARCspecException::SF.
-                    InvalidMARCspecException::SFRANGE,
+                    InvalidMARCspecException::SF .
+                        InvalidMARCspecException::SFRANGE,
                     $subfieldspec
                 );
             }
@@ -92,8 +94,8 @@ public function __construct($subfieldspec)
 
         if (preg_match('/\{.*\}$/', $subfieldspec)) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::SF.
-                InvalidMARCspecException::DETECTEDSS,
+                InvalidMARCspecException::SF .
+                    InvalidMARCspecException::DETECTEDSS,
                 $subfieldspec
             );
         }
@@ -104,9 +106,9 @@ public function __construct($subfieldspec)
 
         if (!preg_match('/^[\!-\?\[-\{\}-~]$/', $subfield['subfieldtag'])) {
             throw new InvalidMARCspecException(
-                InvalidMARCspecException::SF.
-                InvalidMARCspecException::SFCHAR,
-                $arg
+                InvalidMARCspecException::SF .
+                    InvalidMARCspecException::SFCHAR,
+                $subfieldspec
             );
         }
 
@@ -154,7 +156,7 @@ public function addSubSpec($SubSpec)
         } else {
             throw new \InvalidArgumentException('Param 1 must be instance of 
                 SubSpecInterface or array with instances of SubSpecInterface. Got "'
-                .gettype($subSpec).'".');
+                . gettype($SubSpec) . '".');
         }
     }
 
@@ -169,7 +171,7 @@ public function getSubSpecs()
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize(): mixed
     {
         $_subfieldSpec['tag'] = $this->getTag();
 
@@ -211,16 +213,16 @@ public function jsonSerialize()
      */
     public function getBaseSpec()
     {
-        $subfieldSpec = '$'.$this->getTag();
+        $subfieldSpec = '$' . $this->getTag();
         $indexStart = $this->getIndexStart();
         $indexEnd = $this->getIndexEnd();
 
         if ($indexStart === 0 && $indexEnd === '#') {
             // use abbreviation
         } else {
-            $subfieldSpec .= '['.$indexStart;
+            $subfieldSpec .= '[' . $indexStart;
             if ($indexStart !== $indexEnd) {
-                $subfieldSpec .= '-'.$indexEnd;
+                $subfieldSpec .= '-' . $indexEnd;
             }
             $subfieldSpec .= ']';
         }
@@ -230,9 +232,9 @@ public function getBaseSpec()
             if ($charStart === 0 && $charEnd === '#') {
                 // use abbreviation
             } else {
-                $subfieldSpec .= '/'.$charStart;
+                $subfieldSpec .= '/' . $charStart;
                 if ($charEnd !== $charStart) {
-                    $subfieldSpec .= '-'.$charEnd;
+                    $subfieldSpec .= '-' . $charEnd;
                 }
             }
         }
@@ -253,9 +255,9 @@ public function __toString()
                     foreach ($subSpec as $orKey => $orSubSpec) {
                         $subSpec[$orKey] = $orSubSpec->__toString();
                     }
-                    $subfieldSpec .= '{'.implode('|', $subSpec).'}';
+                    $subfieldSpec .= '{' . implode('|', $subSpec) . '}';
                 } else {
-                    $subfieldSpec .= '{'.$subSpec->__toString().'}';
+                    $subfieldSpec .= '{' . $subSpec->__toString() . '}';
                 }
             }
         }
@@ -270,34 +272,35 @@ public function __toString()
      *
      * @param string $offset Key indexStart|indexEnd|charStart|charEnd|charLength|subSpecs
      */
-    public function offsetExists($offset)
+    public function offsetExists(mixed $offset): bool
     {
         switch ($offset) {
 
             case 'code':
-            case 'tag': return isset($this->tag);
-            break;
+            case 'tag':
+                return isset($this->tag);
+                break;
             case 'indexStart':
                 return isset($this->indexStart);
-            break;
+                break;
             case 'indexEnd':
                 return isset($this->indexEnd);
-            break;
+                break;
             case 'indexLength':
                 return !is_null($this->getIndexLength());
-            break;
+                break;
             case 'charStart':
                 return isset($this->charStart);
-            break;
+                break;
             case 'charEnd':
                 return isset($this->charEnd);
-            break;
+                break;
             case 'charLength':
                 return !is_null($this->getCharLength());
-            break;
+                break;
             case 'subSpecs':
                 return (0 < count($this->subSpecs)) ? true : false;
-            break;
+                break;
             default:
                 return false;
         }
@@ -310,35 +313,36 @@ public function offsetExists($offset)
      *
      * @param string $offset Key indexStart|indexEnd|charStart|charEnd|charLength|subSpecs
      */
-    public function offsetGet($offset)
+    public function offsetGet(mixed $offset): mixed
     {
         switch ($offset) {
             case 'tag':
-            case 'code': return $this->getTag();
-            break;
+            case 'code':
+                return $this->getTag();
+                break;
             case 'indexStart':
                 return $this->getIndexStart();
-            break;
+                break;
             case 'indexEnd':
                 return $this->getIndexEnd();
-            break;
+                break;
             case 'indexLength':
                 return $this->getIndexLength();
-            break;
+                break;
             case 'charStart':
                 return $this->getCharStart();
-            break;
+                break;
             case 'charEnd':
                 return $this->getCharEnd();
-            break;
+                break;
             case 'charLength':
                 return $this->getCharLength();
-            break;
+                break;
             case 'subSpecs':
                 return $this->getSubSpecs();
-            break;
+                break;
             default:
-                return;
+                return null;
         }
     }
 
@@ -349,12 +353,13 @@ public function offsetGet($offset)
      *
      * @param string $offset Key indexStart|indexEnd|charStart|charEnd|charLength|subSpecs
      */
-    public function offsetSet($offset, $value)
+    public function offsetSet(mixed $offset, mixed $value): void
     {
         switch ($offset) {
 
-            case 'indexStart': $this->setIndexStartEnd($value);
-            break;
+            case 'indexStart':
+                $this->setIndexStartEnd($value);
+                break;
 
             case 'indexEnd':
                 if (!isset($this['indexStart'])) {
@@ -366,7 +371,7 @@ public function offsetSet($offset, $value)
 
             case 'indexLength':
                 throw new \UnexpectedValueException('indexLength is always calculated.');
-            break;
+                break;
 
             case 'charStart':
                 $this->setCharStartEnd($value);
@@ -382,7 +387,7 @@ public function offsetSet($offset, $value)
 
             case 'charLength':
                 throw new \UnexpectedValueException('CharLength is always calculated.');
-            break;
+                break;
 
             case 'subSpecs':
                 $this->addSubSpec($value);
@@ -396,12 +401,12 @@ public function offsetSet($offset, $value)
     /**
      * Access object like an associative array.
      */
-    public function offsetUnset($offset)
+    public function offsetUnset(mixed $offset): void
     {
         throw new \BadMethodCallException("Offset $offset can not be unset.");
     }
 
-    public function getIterator()
+    public function getIterator(): \Traversable
     {
         foreach ($this as $key => $value) {
             $_[$key] = $value;