RC versions don't need a number

src/Composer/Package/BasePackage.php

@@ -149,7 +149,7 @@ abstract class BasePackage implements PackageInterface
      */
     public static function parseVersion($version)
     {
-        if (!preg_match('#^v?(\d+)(\.\d+)?(\.\d+)?-?((?:beta|RC\d+|alpha)\d*)?-?(dev)?$#i', $version, $matches)) {
+        if (!preg_match('#^v?(\d+)(\.\d+)?(\.\d+)?-?((?:beta|RC|alpha)\d*)?-?(dev)?$#i', $version, $matches)) {
             throw new \UnexpectedValueException('Invalid version string '.$version);
         }