函数/结构/接口名称中允许使用哪些字符？

I am new to go and have started playing around with A Tour of Go. I noticed one peculiarity namely that I am allowed to name a function _ but that function can not be called:

import "fmt"

type sel struct {
    s string
}

func _(s string) sel {
    return sel{s}
}

func main() {
    fmt.Println("Hello")
    _("foo") // <-- does not compile
}

If I comment the entire _("foo") line then the program compiles.

My question is what characters are allowed in function names? Is it only alphanumeric characters or can I use $ for instance?

Are the rules for naming other things e.g. structs, interfaces etc. the same as those for functions?

The spec says that func, var or const name must begin with (unicode_letter or _), and can end with many (unicode_letter, unicode_digit or _).

unicode_letter can be a Chinese, or Hebrew letter if you'd like it to.

From the spec

The blank identifier, represented by the underscore character _, may be used in a declaration like any other identifier but the declaration does not introduce a new binding.

Which explains why the code was valid but you couldn't call the function called _

_ is used in Go when you want to assign a variable but ignore it. Calling a function _ does just the same - you defined it but the compiler will ignore it.