diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 9589f57..ab64409 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -583,17 +583,24 @@ protected function postProcessExtension(array &$cache, $theme) {
     }
     $prefixes[] = $theme->getName();
 
-    // Collect all known hooks. Discovered functions must be based on a known
-    // hook.
-    $hooks = implode('|', array_keys($cache));
-
     // Collect all variable processor functions in the correct order.
     $processors = [];
+    $matches = [];
+    // Look for functions named according to the pattern and add them if they
+    // have matching hooks in the registry.
     foreach ($prefixes as $prefix) {
       // Grep only the functions which are within the prefix group.
       list($first_prefix,) = explode('_', $prefix, 2);
-      if (isset($grouped_functions[$first_prefix])) {
-        $processors += preg_grep("/^{$prefix}_preprocess_($hooks)(__)?/", $grouped_functions[$first_prefix]);
+      if (!isset($grouped_functions[$first_prefix])) {
+        continue;
+      }
+
+      foreach ($grouped_functions[$first_prefix] as $candidate) {
+        if (preg_match("/^{$prefix}_preprocess_(.*)(__)?/", $candidate, $matches)) {
+          if (isset($cache[$matches[1]])) {
+            $processors[] = $candidate;
+          }
+        }
       }
     }