Hallo Gabriel,

I don't know which equipement you want to connect. If a fixed voltage of 12V is needed with small tolerance then a switched regulator is neccessary. Because output voltage is comparable to input voltage classical pure step-down or step-up converter cannot be used, easily. When only small supply currents are needed (<100mA) I would start with Buck-Boost regulator arround LM1578A. Advantage is that a small inductor is sufficient.
If transformer selecting is not a problem for you, you can also try Flyback regulator arround LM2577-12, giving you some hundreds of mA.
You can also combine two regulator types in series: Step-up regulator (arround LM2577-ADJ, e.g.) from battery voltage to a voltage higher then highest battery voltage (e.g. 18V) and afterwards LM7812. Disadvanatge of such solution is rather high power dissipation of linear 12V regulator, when current drawn is high.
You can also change 12V regulator against step-down regulator, but big inrush currents of this one must be avoided at any case. Otherwise one or both regulators will not start to work. So, a version with softstart is highly recommended.
You can also proceed in another direction. 'Traco Power' has very nice DC-DC-converters. TED1212 e.g. converts everything between 9V...18V into 12V at 160mA. TEL 3-1212 does the same at 250mA.

If it is in your application only neccessary that voltage goes not beyond 12V you can take a low-drop regulator LM2941 or just LM7812 for only voltage limiting purpose. With correct decoupling arround these regulators voltage limiting function works very clean and smooth, without any spikes or dips. In any case same filtering as recommended in my last posting is highly recommended!
Keep in mind that LM2941 only accepts longer lasting overvoltages below 26V and that it is an adjustable regulator. So, don't forget Transzorb diode.
Have a look at datasheets. Chips are manufactured from National Semiconductors.

Finally, use of self constructed switching regulator needs some testing before use. They are a little bit more critical than 'simple' linear regulators.

Bye,
Kai