Sorry for my ignorance, but wtf is a regen
Modern diesels to cope with ever more stringent Euro emission laws, now have to be fitted with DPF's (diesel particulate filters). These filter out all the harmful soot deposits from the exhaust system, which is why modern diesels don't leave a trail of black smoke when they accelerate.
Trouble is, to effectively burn these soot by products, the DPF has to be at a critical operating temperature band (which is very high). If you use the car everyday for reasonable journeys, then the DPF will attain temperature and the soot deposits will be burnt off. The DPF can self regulate itself and constantly measures the soot level contained within in. It can only hold a finite number of grams worth of soot, and when it feels its building up, the ECU will instigate a regeneration (or regen) cycle. This can with be passive (I.e unnoticed to you, and taken care of by the ECU during the normal driving of the car), or forced or active.
If however you do constant short journeys, the car is unable to burn the soot using the passive regen method, as the DPF temperature and length of journey isn't sufficient to allow a passive regen. In these instances where the soot begins to build up, the car can at any time instigate an active regen. With an active regen, changes in the timing and fuelling characteristics are deliberately altered to raise the exhaust gas temperatures, thereby heating up the DPF. However this method will have an impact on fuel economy and will raise the revs and you normally smell the increase in temperature from the engine bay. If the car still can't perform even an active regen (journeys are far too short), then the ECU can switch in a dash warning light and it then becomes a dealer matter to force an active regen in their service bay. Note that this isn't covered under warranty and would be a chargeable exercise.
That's the reason dealers are supposed to ask what sort of mileage the car will be doing before selling someone a Diesel, as constant too short a journey (I.e. just the school run for instance), will block the DPF very quickly and even a dealer active regen might not be able to recover it resulting in a new DPF at a cost of well into 4 figures.
Hope this helps.