What is the optimal wing sweep angle for a passenger airliner?
Generally, sweep makes the wing less efficient in creating lift and complicates structural design. Therefore, aircraft designers try to use as little sweep as they can get away with. However, sweep delays the onset of compressibility effects and allows jet aircraft to improve their maximum transport performance (the product of speed and payload).

Forward and backward sweep both help to fly faster, but backward sweep is preferred because it makes stalling less dangerous, improves directional stability and allows to make the wing lighter. The shortest take-off distance will be achieved with zero sweep - if it were not for the need to limit compressibility effects, sweep would only be used in tailless aircraft.

The main factor for choosing wing sweep is the desired cruise Mach number, and Mach 0.85 has become the de-facto standard. Other factors are:


Relative wing thickness: A thicker wing needs more sweep, because thickness adds its own compressibility effects.
Maximum local Mach number of the airfoil where recompression is shock-free: So-called supercritical airfoils help to reduce sweep, and older designs (before the A310) had generally more sweep because their critical Mach number would had been lower at equal sweep.


Regulations have nothing to do with it, only engineering determines the sweep angle. Since the design cruise Mach number is quasi-fixed at Mach 0.85 for long-range airliners, the sweep angle of different aircraft is very similar. Regional jets normally fly a little slower to improve fuel efficiency, and their sweep angle is less than that of their big, long-range cousins.