How to fix E: Couldn't find any package by regex 'package-name' in APT

First, refresh apt's local cache of available packages:

sudo apt update

This downloads the latest package lists from all configured repositories. Without this, apt may be searching against outdated package information.

Search for the package to find its exact name in your repositories:

apt search package-name

Replace "package-name" with what you're looking for. This shows all packages matching your search term. For example:

apt search nodejs

Will show available Node.js related packages like "nodejs", "nodejs-dev", etc. Use the exact name shown in the search results.

If apt search doesn't find the package, it may be in a PPA or third-party repository. Add the necessary repository:

sudo add-apt-repository ppa:username/ppa-name
sudo apt update

For official packages, check the repository's documentation for installation instructions. For example, some projects provide their own .deb repositories:

sudo add-apt-repository "deb [arch=amd64] https://example.com/repo focal main"
sudo apt update

Then retry the installation with the correct package name.

Check your repository configuration:

cat /etc/apt/sources.list
ls /etc/apt/sources.list.d/

Ensure you have repository lines for your Ubuntu/Debian version. Common issues:

- Using repositories for wrong version (e.g., bionic sources on focal)
- Repositories are commented out (# at start of line)
- Non-existent or broken repository URLs

For Ubuntu, you can check your release version:

lsb_release -cs

Standard Ubuntu repositories include main, universe, restricted, and multiverse. If you need a package, ensure the appropriate repository is enabled in sources.list.

Once you've found the correct package name, install it:

sudo apt install package-name

For example:

sudo apt install curl

If you're unsure about package naming, install without wildcards first. Using glob patterns like apt install package-* is generally not recommended—apt install expects exact package names or uses apt-file for searching packaged files (not package names).