Regex for testing that a docker image name is prefixed with a registry?

Question

I'm looking for a regex that will test whether a docker image is prefixed with a registry.

Here are what it should match:

docker.io/library/busybox
docker.io/library/busybox:latest
docker.io/busybox

and what it shouldn't:

busybox
busybox:latest
library/busybox
library/busybox:latest

So you require that a `/` appears before a `:`? Or no `:` at all? — Evan Knowles, Nov 25 '20 at 11:49
To see if a pattern is "reasonable" we need a set of its requirements. What are they? — Wiktor Stribiżew, Nov 25 '20 at 11:51
Should it match `docker.io/library/busybox` (which in practice is identical to `busybox:latest`)? What problems are you having with the regex in the question? — David Maze, Nov 25 '20 at 12:01

Cyrille Pontvieux · Accepted Answer · 2020-11-25T12:21:48.717

Use [^/]+ instead of .* as * means 0 or more whereas + means 1 or more repetition

And [^/] means any character which is not a slash.

Also why do you need the parenthesis for? These are useful to extract groups when it matches. If you only need a boolean result (match or not) you could drop them from the expression.

You edited the question so here is my edited answer:

[^/]+\.[^/.]+/([^/.]+/)?[^/.]+(:.+)?

Explanation:

[^/]: any character not a slash
[^/]+: any string not containing a slash
\.: a dot (escape do not mean any character)
[^/.]+: any string not containing a slash nor a dot
[^/]+\.[^/.]+/: any string separated by a dot and ending with a slash (typically docker.io).
()? means this could occur 0 or 1 time (optional)
(:.+)?: an optional string consisting of a colon followed by any string but not empty.

Thanks, can you break this down for me? – rmf Nov 25 '20 at 12:17 — rmf, Nov 25 '20 at 12:17

Regex for testing that a docker image name is prefixed with a registry?

1 Answers1