diff --git a/src/LocalizedStrftimeFormatter.php b/src/LocalizedStrftimeFormatter.php
index 4e0f5e6..1cd61cc 100644
--- a/src/LocalizedStrftimeFormatter.php
+++ b/src/LocalizedStrftimeFormatter.php
@@ -152,24 +152,18 @@ private function strftime(string $format, DateTimeImmutable $timestamp, string $
 
 			$replace = $translationTable[$pattern];
 
-			if (is_callable($replace)) {
-				$result = $replace($timestamp, $pattern);
-			}
-			else {
+			if (is_string($replace)) {
 				$result = $timestamp->format($replace);
 			}
-
-			switch ($prefix) {
-				case '_':
-					// replace leading zeros with spaces but keep last char if also zero
-					return preg_replace('/\G0(?=.)/', ' ', $result);
-				case '#':
-				case '-':
-					// remove leading zeros but keep last char if also zero
-					return preg_replace('/^0+(?=.)/', '', $result);
+			else {
+				$result = $replace($timestamp, $pattern);
 			}
 
-			return $result;
+			return match ($prefix) {
+				'_' => (string)preg_replace('/\G0(?=.)/', ' ', $result),
+				'#', '-' => (string)preg_replace('/^0+(?=.)/', '', $result),
+				default => $result,
+			};
 		}, $format);
 
 		return str_replace('%%', '%', $out);
diff --git a/tests/LocalizedStrftimeFormatterTest.php b/tests/LocalizedStrftimeFormatterTest.php
new file mode 100644
index 0000000..3b980fc
--- /dev/null
+++ b/tests/LocalizedStrftimeFormatterTest.php
@@ -0,0 +1,38 @@
+<?php declare(strict_types=1);
+
+use PHPUnit\Framework\TestCase;
+use Sunkan\Dictus\LocalizedStrftimeFormatter;
+
+final class LocalizedStrftimeFormatterTest extends TestCase
+{
+	public function testReadableFormat(): void
+	{
+		$formatter = new LocalizedStrftimeFormatter('is_IS', '%A %c Test');
+
+		$date = new DateTimeImmutable('2023-08-21 16:26:14');
+
+		$this->assertSame('mánudagur 21. ágúst 2023 kl. 16:26 Test', $formatter->format($date));
+	}
+
+	public function testEnglishFormat(): void
+	{
+		$formatter = new LocalizedStrftimeFormatter('en_GB', '%A %c Test');
+
+		$date = new DateTimeImmutable('2023-08-21 16:26:14');
+
+		$this->assertSame('Monday 21 August 2023 at 16:26 Test', $formatter->format($date));
+	}
+
+	public function testGlobalFunctionCalls(): void
+	{
+		$formatter = new LocalizedStrftimeFormatter('is_IS', '%Y');
+
+		$date = new DateTimeImmutable('2023-08-21 16:26:14');
+
+		$this->assertSame('2023', $formatter->format($date));
+	}
+}
+
+function Y() {
+	throw new \RuntimeException('Should not be called');
+}