Improve the installation of -dev and -BETA versions

src/Symfony/Installer/NewCommand.php

@@ -105,19 +105,14 @@ protected function execute(InputInterface $input, OutputInterface $output)
     protected function checkSymfonyVersionIsInstallable()
     {
         // 'latest' is a special version name that refers to the latest stable version
-        // available at the moment of installing Symfony
-        if ('latest' === $this->version) {
-            return $this;
-        }
-
         // 'lts' is a special version name that refers to the current long term support version
-        if ('lts' === $this->version) {
+        if (in_array($this->version, array('latest', 'lts'))) {
             return $this;
         }
 
         // validate semver syntax
-        if (!preg_match('/^2\.\d(?:\.\d{1,2})?$/', $this->version)) {
-            throw new \RuntimeException('The Symfony version should be 2.N or 2.N.M, where N = 0..9 and M = 0..99');
+        if (!preg_match('/^2\.\d(?:\.\d{1,2})?(-(dev|BETA\d*))?$/i', $this->version)) {
+            throw new \RuntimeException('The Symfony version should be 2.N or 2.N.M, where N = 0..9 and M = 0..99. The special "-dev" or "-BETA" suffixes are also supported.');
         }
 
         if (preg_match('/^2\.\d$/', $this->version)) {
@@ -175,6 +170,23 @@ protected function checkSymfonyVersionIsInstallable()
             ));
         }
 
+        // "-dev" versions are not supported because Symfony doesn't provide packages for them
+        if (preg_match('/^.*\-dev$/i', $this->version)) {
+            throw new \RuntimeException(sprintf(
+                "The selected version (%s) cannot be installed because it hasn't\n".
+                "been published as a package yet. Read the following article for\n".
+                "an alternative installation method:\n\n".
+                "> How to Install or Upgrade to the Latest, Unreleased Symfony Version\n".
+                '> http://symfony.com/doc/current/cookbook/install/unstable_versions.html',
+                $this->version
+            ));
+        }
+
+        // warn the user when downloading an unstable version
+        if (preg_match('/^.*\-BETA\d*$/i', $this->version)) {
+            $this->output->writeln("\n <bg=red> WARNING </> You are downloading an unstable Symfony version.");
+        }
+
         return $this;
     }