Regular customers would be those customers who shop in a fixed cycle - basis i.e daily/weekly/monthly.
A supermarket offers a wide variety of food and household products, hence a very diverse group of people come to shop.
We can estimate the footfall of an average sized supermarket. Typically supermarket keeps a count of people entering and exting.
Further, the conversion rate i.e actual number of people buying / No. of people that entered the supermarket, is typically between 20% - 40% ( i.e the balance 80% to 60% people are leaving without buying - lost opportunity) This rate can be considered higher like 70% or 80% by considering the competition or location or Industry standard if available.
So, Conversion rate X No. of people in the super market = Actual customers
Out of this actual customers, customers who would buy on regular cycles ( mentioned above) can be taken as 90%.
Therefore, Regular customers = 90% X Actual Customers
Why 90%? because given the diverse choice in a supermarket, a customer was able to spend his quality time to go through aisles and then actually decide on what to purchase, shows the likeability of that customer to return in future to make either a same purchase or supplementary or a complimentary purchase.