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How do I use python to zero pad an integer substring (not a whole string) within another string?

August 21, 2024

Say I have strings like (outputted from running glob.glob() on output from someone else’s code):

image-0.png
image-1.png
image-2.png
image-3.png
image-4.png
image-5.png
image-6.png
image-7.png
image-8.png
image-9.png
image-10.png
image-11.png

How do I left zero pad the integer substring within each string?

>Solution :

You can use regular expression to achieve your goal.

import re

files = [
    "image-0.png",
    "image-1.png",
    "image-2.png",
    "image-3.png",
    "image-4.png",
    "image-5.png",
    "image-6.png",
    "image-7.png",
    "image-8.png",
    "image-9.png",
    "image-10.png",
    "image-11.png"
]

def pad_image_filename(filename):
    return re.sub(r'(\d+)', lambda x: x.group(1).zfill(2), filename)

padded_files = [pad_image_filename(f) for f in files]

print(padded_files) # ['image-00.png', 'image-01.png', 'image-02.png', 'image-03.png', 'image-04.png', 'image-05.png', 'image-06.png', 'image-07.png', 'image-08.png', 'image-09.png', 'image-10.png', 'image-11.png']

r'(\d+)' matches matches 1 or more digits in the string. In this case, it matches 0, 1, 2, …, 11 in the file names.

group(1) returns the string matched by the first capturing group in the regular expression.

zfill(2) zero-pads the matched strings to 2 digits.

You can find the documentation of re.sub() here.