Fix HH\Lib\Regex\Regex references (hhvm#149)

Summary:
Autoimports are not enabled/configured by default, so Regex\ is interpreted relative to the current namespace, which is HH\Lib\Regex, leading to HH\Lib\Regex\Regex

Prefering namespace\Exception over just Exception for clarity=

Pull Request resolved: hhvm#149

Test Plan: hh_client on open source checkout, automated tests

Reviewed By: kmeht

Differential Revision: D25562298

Pulled By: fredemmott

fbshipit-source-id: c98709cb594da85847e1bf2afdf4683db5d12729

src/regex/regex.php

@@ -108,7 +108,7 @@ function replace(
     list ($result, $error) =
       _Private\_Regex\replace($haystack, $pattern, $replacement);
     if ($error is nonnull) {
-      throw new Regex\Exception($pattern, $error);
+      throw new namespace\Exception($pattern, $error);
     }
     return $result as nonnull;
   }
@@ -118,7 +118,7 @@ function replace(
   list ($result, $error) =
     _Private\_Regex\replace($haystack2, $pattern, $replacement);
   if ($error is nonnull) {
-    throw new Regex\Exception($pattern, $error);
+    throw new namespace\Exception($pattern, $error);
   }
   return $haystack1 . $result;
 }