$re = '/(?im)([a-z0-9\-\_.]+)?(\/[a-z0-9\-.]+)?(:[\w\.\-\_]+)?$/';
$str = 'gitea/gitea:1.13.7
kiwigrid/k8s-sidecar:0.1.209
registry1.dso.mil/ironbank/opensource/docker/registry-v2:2.7.1
registry.opensource.zalan.do/acid/logical-backup:v1.7.0
docker.io/dpage/pgadmin4:5.5
registry1.dso.mil/ironbank/opensource/minio/minio:RELEASE.2021-08-31T05-46-54Z
ghcr.io/stefanprodan/podinfo:6.0.0
ghcr.io/stefanprodan/podinfo:6.0.0-v2
k3d-cluster.localhost/defenseunicorns/zarf-agent
localhost:5000/some-user/some-cool-image
localhost:5000/someuser/some-cool-image:tag
127.0.0.1:443/some-user/some-cool-image:tag
127.0.0.1/some_user/some-cool-image:tag
https://127.0.0.1:443/some-user/some-cool-image:tag
https://127.0.0.1/some-user/some-cool-image:tag
defenseunicorns/zarf-game:multi-tile-dark
/defenseunicorns/zarf-game:multi-tile-dark
paas.docker-registry:443/ironbank/opensource/goharbor/chartmuseum:v2.5.1
paas-docker.registry:443/ironbank/opensource/goharbor/chartmuseum:v2.5.1
paas-docker-registry:443/ironbank/opensource/goharbor/chartmuseum:v2.5.1
awesome.nexus.com/repository/docker/ironbank/opensource/goharbor/chartmuseum:v2.5.1';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php