Passgen generates random sequences from a regex-like pattern. It can be used to generate secure passphrases. It is available for download as a command-line utility. For a quick overview of how it works, see usage.
Passgen uses the strong system randomness generator.
Passgen uses a custom language to be able to generate any kinds of sequences.
Passgen comes with many useful and secure presets out-of-the box.
Open Source (MIT)
Passgen is licensed as MIT and is available on GitLab.
If you want to try it out, take a look at the releases section where you can find signed tarballs and packages for your platform operating system.
For documentation on the syntax, see grammar.
If you want to help develop it, see development, where you can find resources and links.