[3.1] Minor preg refactoring and added more nesting tests

src/Loader.php

@@ -4,6 +4,7 @@
 
 use Dotenv\Environment\FactoryInterface;
 use Dotenv\Exception\InvalidPathException;
+use Dotenv\Regex\Regex;
 use PhpOption\Option;
 
 /**
@@ -170,29 +171,21 @@ private function processEntries(array $entries)
      */
     private function resolveNestedVariables($value = null)
     {
-        if ($value === null || strpos($value, '$') === false) {
-            return $value;
-        }
-
-        return preg_replace_callback(
-            '/\${([a-zA-Z0-9_.]+)}/',
-            [$this, 'nestedVariableResolver'],
-            $value
-        );
-    }
-
-    /**
-     * Resolve the matched variable to its value, if possible.
-     *
-     * @param string[] $matches
-     *
-     * @return string
-     */
-    private function nestedVariableResolver(array $matches)
-    {
-        $nested = $this->getEnvironmentVariable($matches[1]);
-
-        return $nested === null ? $matches[0] : $nested;
+        return Option::fromValue($value)
+            ->filter(function ($str) {
+                return strpos($str, '$') !== false;
+            })
+            ->flatMap(function ($str) {
+                return Regex::replaceCallback(
+                    '/\${([a-zA-Z0-9_.]+)}/',
+                    function (array $matches) {
+                        return Option::fromValue($this->getEnvironmentVariable($matches[1]))
+                            ->getOrElse($matches[0]);
+                    },
+                    $str
+                )->success();
+            })
+            ->getOrElse($value);        
     }
 
     /**

src/Parser.php

@@ -155,7 +155,7 @@ private static function processQuotedValue($value)
             $quote
         );
 
-        return Regex::pregReplace($pattern, '$1', $value)
+        return Regex::replace($pattern, '$1', $value)
             ->mapSuccess(function ($str) use ($quote) {
                 return str_replace('\\\\', '\\', str_replace("\\$quote", $quote, $str));
             })

src/Regex/Regex.php

@@ -7,17 +7,48 @@
 class Regex
 {
     /**
-     * Perform a preg replace, failing with an exception.
+     * Perform a preg replace, wrapping up the result.
      *
      * @param string $pattern
-     * @param string $repalcement
+     * @param string $replacement
      * @param string $subject
      *
      * @return \Dotenv\Regex\Result
      */
-    public static function pregReplace($pattern, $replacement, $subject)
+    public static function replace($pattern, $replacement, $subject)
     {
-        $result = (string) @preg_replace($pattern, $replacement, $subject);
+        return self::pregAndWrap(function ($subject) use ($pattern, $replacement) {
+            return preg_replace($pattern, $replacement, $subject);
+        }, $subject);
+    }
+
+    /**
+     * Perform a preg replace callback, wrapping up the result.
+     *
+     * @param string   $pattern
+     * @param callable $callback
+     * @param string   $subject
+     *
+     * @return \Dotenv\Regex\Result
+     */
+    public static function replaceCallback($pattern, callable $callback, $subject)
+    {
+        return self::pregAndWrap(function ($subject) use ($pattern, $callback) {
+            return preg_replace_callback($pattern, $callback, $subject);
+        }, $subject);
+    }
+
+    /**
+     * Perform a preg operation, wrapping up the result.
+     *
+     * @param callable $operation
+     * @param string   $subject
+     *
+     * @return \Dotenv\Regex\Result
+     */
+    private static function pregAndWrap(callable $operation, $subject)
+    {
+        $result = (string) @$operation($subject);
 
         if (($e = preg_last_error()) !== PREG_NO_ERROR) {
             return Error::create(self::lookupError($e));

tests/Dotenv/DotenvTest.php

@@ -151,6 +151,9 @@ public function testDotenvNestedEnvironmentVars()
         $this->assertSame('$NVAR1 {NVAR2}', $_ENV['NVAR5']); // not resolved
         $this->assertSame('Special Value', $_ENV['N.VAR6']); // new '.' (dot) in var name
         $this->assertSame('Special Value', $_ENV['NVAR7']);  // nested '.' (dot) variable
+        $this->assertSame('', $_ENV['NVAR8']);
+        $this->assertSame('', $_ENV['NVAR9']);  // nested variable is empty string
+        $this->assertSame('${NVAR888}', $_ENV['NVAR10']);  // nested variable is not set
     }
 
     /**

tests/fixtures/env/nested.env

@@ -5,3 +5,6 @@ NVAR4="${NVAR1} ${NVAR2}"
 NVAR5="$NVAR1 {NVAR2}"
 N.VAR6="Special Value"
 NVAR7="${N.VAR6}"
+NVAR8=""
+NVAR9="${NVAR8}"
+NVAR10="${NVAR888}"