diff --git a/src/TwigExtension.php b/src/TwigExtension.php
index 9016ec4..676310f 100644
--- a/src/TwigExtension.php
+++ b/src/TwigExtension.php
@@ -384,7 +384,12 @@ class TwigExtension extends \Twig_Extension {
    *   The new text if matches are found, otherwise unchanged text.
    */
   public function pregReplaceFilter($text, $pattern, $replacement) {
-    return preg_replace("/$pattern/", $replacement, $text);
+    // BC layer. Before version 8.x-1.8 the pattern was without delimiters.
+    // @todo Remove this in Drupal 9.
+    if (strpos($pattern, '/') !== 0) {
+      return preg_replace("/$pattern/", $replacement, $text);
+    }
+    return preg_replace($pattern, $replacement, $text);
   }
 
   /**
diff --git a/tests/src/Functional/TwigTweakTest.php b/tests/src/Functional/TwigTweakTest.php
index 432a9d8..3108f5c 100644
--- a/tests/src/Functional/TwigTweakTest.php
+++ b/tests/src/Functional/TwigTweakTest.php
@@ -148,7 +148,11 @@ class TwigTweakTest extends BrowserTestBase {
     $this->assertByXpath($xpath);
 
     // Test preg replacement.
-    $xpath = '//div[@class = "tt-preg-replace" and text() = "foo-bar"]';
+    $xpath = '//div[@class = "tt-preg-replace" and text() = "FOO-bar"]';
+    $this->assertByXpath($xpath);
+
+    // Test preg replacement (legacy).
+    $xpath = '//div[@class = "tt-preg-replace-legacy" and text() = "foo-bar"]';
     $this->assertByXpath($xpath);
 
     // Test image style.
diff --git a/tests/twig_tweak_test/templates/twig-tweak-test.html.twig b/tests/twig_tweak_test/templates/twig-tweak-test.html.twig
index 59985e7..bf08269 100644
--- a/tests/twig_tweak_test/templates/twig-tweak-test.html.twig
+++ b/tests/twig_tweak_test/templates/twig-tweak-test.html.twig
@@ -36,7 +36,8 @@
   <div class="tt-title">{{ drupal_title() }}</div>
   <div class="tt-url">{{ drupal_url('node/1', {absolute: true}) }}</div>
   <div class="tt-token-replace">{{ 'Site name: [site:name]' | token_replace }}</div>
-  <div class="tt-preg-replace">{{ 'foo' | preg_replace('(foo)', '$1-bar') }}</div>
+  <div class="tt-preg-replace">{{ 'FOO' | preg_replace('/(foo)/i', '$1-bar') }}</div>
+  <div class="tt-preg-replace-legacy">{{ 'foo' | preg_replace('(foo)', '$1-bar') }}</div>
   <div class="tt-image-style">{{ 'public://images/ocean.jpg' | image_style('thumbnail') }}</div>
   <div class="tt-transliterate">{{ 'Привет!' | transliterate('ru') }}</div>
   <div class="tt-check-markup">{{ '<b>bold</b> <strong>strong</strong>' | check_markup('twig_tweak_test') }}</div>