This is a reference for specifying versions of packages as used when interacting with Portage via emerge or /etc/portage. These are also known as "DEPEND atoms" in Portage documentation.
For the ebuild development perspective, see the devmanual.
Matches any version of a package.
Matches version and any revision.
Matches a version by the version range. Note that there's no "." before the "*".
Matches a version exactly.
Matches the specified version or any higher version.
Matches a version strictly later than specified.
Matches a version strictly older than specified.
Matches the specified version or any older version.
Matches package in the specified package SLOT. Note that there is no prefix.
Matches a package from a specific overlay. This can be combined with other specifiers. The official portage tree is ::gentoo.