Minimum A/C Voltage to drive 12V regulator
Your transformer and two half bridges will give you about 

+/-(\$\sqrt{2}\times\$14)-1~= 19VDC from 14+14VAC RMS.

The 1V is to account for the diode drop. 

That gives you a bit of margin for ripple on the capacitor and mains voltage tolerance.

Note that the transformer voltage may be significantly higher with a light load compared to full rated load for the transformer- not a problem with max input voltage of the regulators, but power dissipation could become an issue in some cases.