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Hi R626
I might be wrong but I'm afraid there is no native way to do it with custom cell formatting in excel
I see 3 options here
1-You may try using the formula given by RonRosenfeld above
2- Using a VBA macro
3- Using the FLASH FILL excel feature
Check this video (min: 3:35)
https://www.youtube.com/watch?v=5FSlXMzrHmQ
I hope this helps you and gives a solution to your problem
Do let me know if you need more help
Regards
Jeovany