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I'm trying to write a regex in Go to verify that a string only has alphanumerics, periods, and underscores. However, I'm running into an error that I haven't seen before and have been unsuccessful at Googling.

Here's the regex:

pattern = regexp.MustCompile(`^[A-Za-z0-9_\.]+`)
Here is the error:
const initializer regexp.MustCompile("^[A-Za-z0-9_\\.]+") is not a constant
What does "not a constant" mean and how do I fix this?

This happens when you're trying to assign to a constant that has a type that can't be constant (like for example, Regexp). Only basic types likes int, string, etc. can be constant. See here for more details.
Example:
pattern = regexp.MustCompile(`^[A-Za-z0-9_\.]+`)
// which translates to:
const pattern = regexp.MustCompile(`^[A-Za-z0-9_\.]+`)
You have to declare it as a var for it to work:
var pattern = regexp.MustCompile(`^[A-Za-z0-9_\.]+`)
In addition, I usually put a note to say that the variable is treated as a constant:
var /* const */ pattern = regexp.MustCompile(`^[A-Za-z0-9_\.]+`)
In Go declaration context, assignment with a simple = creates a constant, not a variable. (Outside of a declaration, it's an assignment to a variable that must already exist.)
But a constant initialization has to include only constants - not calls like regexp.MustCompile() - so pattern can't be a constant in this case, even if you don't plan on changing its value later. (In fact, even if you could somehow initialize it without calling anything, a Regexp can't be a constant in Go; only basic types can be.)
That means you need to make it a variable by either putting it inside a var statement or declaring it inside a function with := instead of =:
var (
   pattern = ...
var pattern = ...
func something() {
   pattern := ...
        
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