travel_would_cause_regency_trigger

travel_would_cause_regency_trigger = {
	trigger_if = {
		limit = { exists = current_travel_plan }
		OR = {
			# Travelling one-way.
			AND = {
				exists = current_travel_plan
				# Make sure that we _aren't_ going bilaterally.
				NOT = { current_travel_plan.final_destination_province = current_travel_plan.departure_location }
				# Then check that we're actually going outside of the realm.
				current_travel_plan.final_destination_province.county.holder ?= {
					NOR = {
						any_liege_or_above = { this = $REALM_HOLDER$ }
						this = $REALM_HOLDER$
					}
				}
			}
			# Travelling in a circle.
			AND = {
				exists = current_travel_plan
				# Make sure that we _are_ going bilaterally.
				current_travel_plan.final_destination_province = current_travel_plan.departure_location
				# Check the next destination province instead.
				## We put a break in to check if there _is_ a county because you might be travelling through the ocean.
				current_travel_plan.next_destination_province.county ?= {
					holder = {
						NOR = {
							any_liege_or_above = { this = $REALM_HOLDER$ }
							this = $REALM_HOLDER$
						}
					}
				}
			}
		}
	}
}

As written in game/common/scripted_triggers/00_diarchy_scripted_triggers.txt.