nflz - Numbered Filenames With Leading Zeros - CLI + Library
nflz is a CLI-Tool + library that helps you to add leading zeros to numbered filenames in ascending order.
Build: 
What it does
Content of some directory:
paris (1).png => paris (01).png
paris (2).png => paris (02).png
...
paris (12).png => paris (12).png
...
paris (n).png => n digits => indicator for how many zeros to add
Note
NFLZ can't guarantee you 100% safety. Always make a backup first (: But at my best will this should work when no catastrophic failure is happening.
Install / use
Rust library
Cargo.toml:
nflz = "<latest-version>"
CLI tool
$ cargo install nflz
How it works
It either works in pwd (present working dir) or in the directory passed as the first argument.
$ nflz
$ nflz <absolute or relative path to dir>
nflz asks you for confirmation before it does any changes to your file system!
Background
If you select multiple files in Windows Explorer and rename them to the same name, Windows automatically numbers all files for you in parentheses. The downside is that there are no leading zeros. Other programs than Windows, e.g. Google Drive, can't order the files properly without the leading zeros. Here comes my CLI/lib into the game!
Example output
NFLZ: using dir: "./test"
NFLZ: skipping file 'invalid (100) (19231).jpg' because: The filename 'invalid (100) (19231).jpg' must include exactly one numbered group.
NFLZ: Files that don't need a renaming:
paris (734).jpg
NFLZ would rename files:
paris (1).jpg => paris (001).jpg
paris (2).jpg => paris (002).jpg
paris (3).jpg => paris (003).jpg
paris (4).jpg => paris (004).jpg
paris (5).jpg => paris (005).jpg
paris (6).jpg => paris (006).jpg
paris (7).jpg => paris (007).jpg
paris (8).jpg => paris (008).jpg
paris (9).jpg => paris (009).jpg
paris (10).jpg => paris (010).jpg
Please confirm with 'y' or abort with 'n'
NFLZ can't guarantee you 100% safety. Always make a backup first (:
But at my best will this should work when no catastrophic failure is happening.