Add leading 0 stripped from FIPS code in CSV files

regex.txt

This is a problem when number string fields are encoded as numbers.
Regex expression in VS Code where the FIPS code column is surrounding by two string fields.
step 1:
find pattern breaks search into two subexpressions
(",)([0-9]{4},")
step 2:
replace pattern inserts 0 in front of the second subexpression
$10$2

To wrap FIPS codes in quotes, find the comma before and after the 5-digit number. Insert quote.

Search pattern to find the comma after a 5-digit string (in a CSV whose first column contains a FIPS code) 
(^"[0-9]{5})(,)

Replace pattern using subexpressions
$1"$2

Select everythign between two commas
(?<=,)[^,]+(?=,)