diff --git a/src/CommandLine/Parser.php b/src/CommandLine/Parser.php
index 0c75125..33805bc 100644
--- a/src/CommandLine/Parser.php
+++ b/src/CommandLine/Parser.php
@@ -21,6 +21,7 @@ class Parser
 		Repeatable = 'repeatable',
 		Enum = 'enum',
 		RealPath = 'realpath',
+		Normalizer = 'normalizer',
 		Default = 'default';
 
 	/** @deprecated use Parser::Argument */
@@ -139,9 +140,11 @@ public function parse(?array $args = null): array
 				}
 			}
 
+			$this->checkArg($opt, $arg);
+
 			if (
 				!empty($opt[self::Enum])
-				&& !in_array($arg, $opt[self::Enum], true)
+				&& !in_array(is_array($arg) ? reset($arg) : $arg, $opt[self::Enum], true)
 				&& !(
 					$opt[self::Optional]
 					&& $arg === true
@@ -150,8 +153,6 @@ public function parse(?array $args = null): array
 				throw new \Exception("Value of option $name must be " . implode(', or ', $opt[self::Enum]) . '.');
 			}
 
-			$this->checkArg($opt, $arg);
-
 			if (empty($opt[self::Repeatable])) {
 				$params[$name] = $arg;
 			} else {
@@ -187,6 +188,10 @@ public function help(): void
 
 	public function checkArg(array $opt, &$arg): void
 	{
+		if (isset($opt[self::Normalizer])) {
+			$arg = $opt[self::Normalizer]($arg);
+		}
+
 		if (!empty($opt[self::RealPath])) {
 			$path = realpath($arg);
 			if ($path === false) {
diff --git a/tests/Parser.phpt b/tests/Parser.phpt
index 4031106..cb6a729 100644
--- a/tests/Parser.phpt
+++ b/tests/Parser.phpt
@@ -194,6 +194,36 @@ test('realpath', function () {
 
 
 
+test('normalizer', function () {
+	$cmd = new Parser('
+		-p param
+	', [
+		'-p' => [Parser::Normalizer => function ($arg) { return "$arg-normalized"; }],
+	]);
+
+	Assert::same(['-p' => 'val-normalized'], $cmd->parse(explode(' ', '-p val')));
+
+
+	$cmd = new Parser('
+		-p <a|b>
+	', [
+		'-p' => [Parser::Normalizer => function () { return 'a'; }],
+	]);
+
+	Assert::same(['-p' => 'a'], $cmd->parse(explode(' ', '-p xxx')));
+
+
+	$cmd = new Parser('
+		-p <a|b>
+	', [
+		'-p' => [Parser::Normalizer => function () { return ['a', 'foo']; }],
+	]);
+
+	Assert::same(['-p' => ['a', 'foo']], $cmd->parse(explode(' ', '-p xxx')));
+});
+
+
+
 test('positional arguments', function () {
 	$cmd = new Parser('', [
 		'pos' => [],