Complex sentence: In 1851 the four Sikh regiments and the Corps of Guides became part of the Punjab Irregular Force, men of the regiments of which (or their successors) are to this day known as "Piffers".
Split into simpler sentences: In 1851 the four Sikh regiments and the Corps of Guides became part of the Punjab Irregular Force. Men of these regiments (or their successors) are to this day known as "Piffers".