Slim/FastRoute support the idea of an unlimited capture, for example:

// Greedy variant.

/path/to/{somevalue:.*}

// Non-greedy variant.

/path/to/{somevalue:.*?}

This will match

In these cases the path_parameter has an is_required() method which checks whether the parameter is required in the path pattern for the route, however it is not aware of the unlimited captures. We also need to update the specification to be aware of these placeholders (or rather strip them out).

To add to this explanation, here's an excerpt from a discussion wtih David:
We support templates at location such as the following:

core/modal

core/notification

core/navbar

core/local/modal/alert

core/local/toast/notification

When we request those from the new API we use the URI:

/templates/{themename}/{component}/{identifier}

So those examples would be:

https://example.com/moodle/rest/api/v2/core/templates/boost/core/modal

https://example.com/moodle/rest/api/v2/core/templates/boost/core/notification

https://example.com/moodle/rest/api/v2/core/templates/boost/core/navbar

https://example.com/moodle/rest/api/v2/core/templates/boost/core/local/modal/alert

https://example.com/moodle/rest/api/v2/core/templates/boost/core/local/toast/notification

However, the way that the URI is passed that `

/([^/]*/?)$

That is look for a leading slash (`/`) followed by any character up to an optional slash and the end of the URI (`([^/]*/?)$`)).

So the first three of those work because the `identifier` components (`modal`, `notification`, and `navbar`) are the end of the URI.

But the last two have identifiers of:

local/modal/alert

local/toast/notification

They match the `local/` part, but then they have content after them, and the match fails.

What this change is doing is to switch to what FastRoute and Slim call an 'Unlimited capture'... that is that they keep capturing the identifier, including any other slashes and content to the end of the URI. We do that by usngn the `{identifier:.*}` or `{identifer:.*?}` notation.

So they'll match `local/modal/alert` and `local/toast/notification` as the identifiers.

The difference between the two notations is that the first one is 'greedy' and the second is 'non-greedy'. Basically that means that if we had another Route looking like:

/templates/{themename}/{component}/{identifier}

/DELETE

Then that route would match before the other variant.