It is about making sure that two different plugins do not try to define a DB table with the same name. The way to do that is to say that the tables for each plugin must start with the internal plugin name. In this case that is qtype_regex.
I know that, historically, question types have used tables with names starting question_, but we should change that. You will not that in Moodle 2.1, the essay question type uses a table called qtype_essay_options.
Some time, I will rename all the core question type tables to match the new naming scheme, but I am going to wait until they say something like "to upgrade to Moodle 2.3, you must already have Moodle 2.2", because that gives us a clean break in the upgrade process at which to make the change.
To make your plugin obey the rules, you should just have to:
- Search-and-replace question_regex -> qtype_regex_options where it is the table name.
- Write an upgrade step in db/upgrade.php to rename the existing table.
