Regex "is not a constant" compilation error

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_\.]+`)

The error is pretty clear. If you are trying to do this globally...

Don't do:

const pattern = regexp.MustCompile(`^[A-Za-z0-9_\.]+`)

Instead do:

var pattern = regexp.MustCompile(`^[A-Za-z0-9_\.]+`)

Or if you really want the pattern in a constant:

const pattern = `^[A-Za-z0-9_\.]+`
var alphaNum = regexp.MustCompile(pattern)