warning: preg_match() expects parameter 2 to be string

I have installed drupal core with a database prefix on mysql database, installed the latest dev of l10n_update, enabled locale + admin menu + devel. Dowloaded the translations, installed an old version of a module, updated the translation, downloaded the latest version of this module, updated the translation again. And still got no error.

@grootte: Please describe all steps to reproduce your error.

This happened once to me too. The problem, I guess, comes from the fact that $db_prefix is not always a string, but it could be an array too. If you need (as I do) to share some tables among different sites, you would use something like this in you settings.php:

$db_prefix = array(
  'default' => '',
  'users' => 'shared_tables.',
  'sessions' => 'shared_tables.',
  'authmap' => 'shared_tables.',
  'role' => 'shared_tables.',
  'users_roles' => 'shared_tables.',
);

Of course, this will cause problems if passed to preg_match() as second parameter.
I'm not 100% absolutely sure this is the cause of that issue, but there's a good 95% of choice that this is causing some problems.. If you do not need to run simpletest in a multi-prefix database setup, I guess you'd better check that is_array($_GLOBALS['db_prefix']) too before using preg_match..

I can not find any simpletest which uses an array of prefixes, therefore a simple check could solve this issue.

Anyone interested in getting this fixed? Please test and review the patch.

Drupal core has the same handling, I'd just look there (it had a similar bug for some time too :).

Indeed, this is what core has in Drupal 6:

$ grep -r "simpletest" *
includes/common.inc:  if (is_string($db_prefix) && preg_match("/^simpletest\d+$/", $db_prefix, $matches)) {

Updated patch attached and committed.