Fix issue #17140 match only APP_KEY in replace

Changed str_replace to preg_replace to ensure no clashes with other
environment variables

src/Illuminate/Foundation/Console/KeyGenerateCommand.php

@@ -51,8 +51,8 @@ public function fire()
      */
     protected function setKeyInEnvironmentFile($key)
     {
-        file_put_contents($this->laravel->environmentFilePath(), str_replace(
-            'APP_KEY='.$this->laravel['config']['app.key'],
+        file_put_contents($this->laravel->environmentFilePath(), preg_replace(
+            $this->getAppKeyPattern(),
             'APP_KEY='.$key,
             file_get_contents($this->laravel->environmentFilePath())
         ));
@@ -69,4 +69,16 @@ protected function generateRandomKey()
             $this->laravel['config']['app.cipher'] == 'AES-128-CBC' ? 16 : 32
         ));
     }
+
+    /**
+     * Get a regex pattern that will match env APP_KEY with any random key
+     *
+     * @return string
+     */
+    protected function getAppKeyPattern()
+    {
+        $escapedKey = preg_quote('='.$this->laravel['config']['app.key'], '/');
+
+        return "/^APP_KEY$escapedKey/m";
+    }
 }