On the Dancer IRC channel, somebody was asking how to set the route matching a prefix exactly. The naive approach would be to do

prefix '/foo';

get '' => sub {
    return "matches /foo";
};

but, alas, it doesn’t work. Why? Because Dancer doesn’t accept empty routes, so '' is not acceptable. Now, that’s a behavior that could be seen as buggy and should get fixed shortly (a ticket is open for it). But for the moment, let’s assume we’re stuck with it. So, what can one do? Well, at least two options are open to us.

The first is simply to declare the root route outside of the prefix scope:

get '/foo' => sub {
    return "matches /foo";
};


prefix '/foo';

# all other routes go here

It works, but… meh. It also breaks the prefix encapsulation. Which is nothing terrible, but, really, meh.

The second option is sneakier. It consists in remembering that Dancer routes can also be defined as regular expressions:

prefix '/foo';

get qr/$/ => sub {
    return "matches /foo";
};

# all other routes go here

And, sure enough, magic happens and it works. If we run the application with debugging logging, we can also see the resulting route regular expression that Dancer made of it:

[12951]  core @0.000423> [hit #1]Trying to match 'GET /foo' against /(?^:^(?^:/foo(?^:$))$)/ (generated from '(?^:$)')