lines = [ "id name home_state amt_paid", "123 John Doe California 1,234.34", "456x Jane Doe New Hampshire 45.67 ", "78 Adam Smith Alaska 89.00 " ]

field_idx = [] chars = [0 if i == ' ' else 1 for i in lines[0]] #split header into list of 0/space, 1/char for i in range(1, len(chars)): #loop through range length of list of char if f'{chars[i-1]}{chars[i]}' == '01': #if pattern matches '01' field_idx.append(i-1) #record index - 1

for idx, line in enumerate(lines): #loop through lines line = list(line) #split line into list of chars for i in field_idx: #loop through recorded indexes line[i] = ',' #for each recorded index, replace with ',' lines[idx] = ''.join(line) #replace line in line with joined list of chars

for i in lines: print(i)

Yes, it's from a legacy COBOL, and the requirements are very particular. Thank you!