MAGETWO-31904: There is no validation for 'Table prefix' on Step 2: Add a Database during instalation

- added format info to tooltip message
- fixed validation according to format

Author: Ivan Gavryshko

setup/src/Magento/Setup/Model/Installer.php

@@ -766,7 +766,7 @@ public function checkDatabaseConnection($dbName, $dbHost, $dbUser, $dbPass = '')
      */
     public function checkDatabaseTablePrefix($prefix)
     {
-        if ($prefix !== '' && !preg_match('/^([[:alnum:]]+)([[:alnum:]_]+)$/', $prefix)) {
+        if ($prefix !== '' && !preg_match('/^([a-zA-Z])([[:alnum:]]+)([[:alnum:]_]+)$/', $prefix)) {
             throw new \InvalidArgumentException('Please correct the table prefix format.');
         }
 

setup/view/magento/setup/add-database.phtml

@@ -149,7 +149,7 @@
         <label class="col-sm-4 control-label">Table prefix</label>
         <div class="col-sm-4">
             <input tooltip-placement="right"
-                   tooltip="Enter a tracking prefix to be used for database tables created for this Magento installation. (ex: 'mg1_' )"
+                   tooltip="Enter a tracking prefix to be used for database tables created for this Magento installation. (ex: 'mg1_' ). The table prefix should contain only letters (a-z), numbers (0-9) or underscores (_); the first character should be a letter."
                    tooltip-trigger="focus"
                    tooltip-append-to-body="true"
                    class="form-control"