diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index ab64409..38ab0b7 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -340,9 +340,10 @@ protected function build() {
       $this->processExtension($cache, $this->theme->getEngine(), 'theme_engine', $this->theme->getName(), $this->theme->getPath());
     }
 
-    // Finally, hooks provided by the theme itself.
+    // Hooks provided by the theme itself.
     $this->processExtension($cache, $this->theme->getName(), 'theme', $this->theme->getName(), $this->theme->getPath());
 
+    // Discover and add all preprocess functions for theme hook suggestions.
     $this->postProcessExtension($cache, $this->theme);
 
     // Let modules and themes alter the registry.
@@ -562,14 +563,16 @@ protected function processExtension(array &$cache, $name, $type, $theme, $path)
   }
 
   /**
-   * This completes the theme registry adding missing functions and hooks.
+   * This completes the theme registry adding discovered functions and hooks.
    *
    * @param array $cache
-   *  @see ::processExtension()
-   * @param object $theme
+   *   The theme registry.
+   * @param \Drupal\Core\Theme\ActiveTheme $theme
    *   Current active theme.
+   *
+   * @see ::processExtension()
    */
-  protected function postProcessExtension(array &$cache, $theme) {
+  protected function postProcessExtension(array &$cache, ActiveTheme $theme) {
     $grouped_functions = drupal_group_functions_by_prefix();
 
     // Gather prefixes. This will be used to limit the found functions to the
@@ -594,24 +597,24 @@ protected function postProcessExtension(array &$cache, $theme) {
       if (!isset($grouped_functions[$first_prefix])) {
         continue;
       }
-
+      // Add the function and the name of the associated theme hook to the list
+      // of processors if a matching base hook is found.
       foreach ($grouped_functions[$first_prefix] as $candidate) {
-        if (preg_match("/^{$prefix}_preprocess_(.*)(__)?/", $candidate, $matches)) {
-          if (isset($cache[$matches[1]])) {
-            $processors[] = $candidate;
+        if (preg_match("/^{$prefix}_preprocess_(((?:[^_]++|_(?!_))+)__.*)/", $candidate, $matches)) {
+          if (isset($cache[$matches[2]])) {
+            $processors[$candidate] = $matches[1];
           }
         }
       }
     }
 
     // Add missing variable processors. This is needed for hooks that do not
-    // explictly register the hook. For example, when a theme contains a
+    // explicitly register the hook. For example, when a theme contains a
     // variable process function but it does not implement a template, it will
     // go missing. This will add the expected function. It also allows modules
     // or themes to have a variable process function based on a pattern even if
     // the hook does not exist.
-    foreach ($processors as $processor) {
-      $hook = substr($processor, strpos($processor, 'preprocess_') + strlen('preprocess_'));
+    foreach ($processors as $processor => $hook) {
       if (isset($cache[$hook]['preprocess functions']) && !in_array($hook, $cache[$hook]['preprocess functions'])) {
         // Add missing processor to existing hook.
         $cache[$hook]['preprocess functions'][] = $processor;