zfill prints extra et the end of a var

[tester_V]

Greetings to you all!
I'm trying to replace single digits at the beginning of each string in a files
The integers are always in the range of 1-12.
Here is a test file:

1,xxx,ddd-02,cc1,
2,xxx,ddd-02,cc1,
3,xxx,ddd-02,cc1,
10,xxx,ddd-02,cc1,
11,xxx,ddd-02,cc1
12,xxx,ddd-02,cc1,

Here is a snippet:

with open(fls) as blt_orig :
    for el in blt_orig :
        el.strip()
        num,*gb = el.split(",")
        new_n = num.zfill(2)     # <-- Adding a zero to the number      
        nln =re.sub("\d|\d{2}",new_n,el,count=1)  # <-- replacing the digits in the string with the new one  
        print(f"{nln}")

And a strange printout:

01,xxx,ddd-02,cc1,

02,xxx,ddd-02,cc1,

03,xxx,ddd-02,cc1,

100,xxx,ddd-02,cc1,

111,xxx,ddd-02,cc1

122,xxx,ddd-02,cc1,

For some reason, it prints extra digits to the 10/11 and 12.
Thank you.