What is the ruling of one's fasting if he unintentionally touches a non-Mahram (permanently unmarriageable) woman? Does this invalidate his fasting or not?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and messenger.

The mere touching of a women does not nullify one's fasting unless it is associated with sexual secretions. If the touching results in the discharge of seminal fluid or pre-ejaculatory fluid, and it is intentionally done, then fasting is invalidated, as agreed upon by Muslim scholars. If the touching happens by mistake, then fasting remains valid. However, there is a well-known difference in scholars' opinions over the validity of fasting if the touching is intentionally done and causes discharge of pre-ejaculatory fluid.      

After all, the questioner has to know that a Muslim man is required to stay away from situations in which he may be liable to touch non-Mahram women, given the strict threat against this action that is most emphasized in Ramadan in particular.

Allah Knows best.