You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: interpolate.go
    path of file: ./repos/streamdal/apps/cli/vendor/github.com/alecthomas/kong
    
    the code of the file until where you have to start completion: package kong

import (
	"fmt"
	"regexp"
)

var interpolationRegex = regexp.MustCompile(`(\$\$)|((?:\${([[:alpha:]_][[:word:]]*))(?:=([^}]+))?})|(\$)|([^$]+)`)

// HasInterpolatedVar returns true if the variable "v" is interpolated in "s".
func HasInterpolatedVar(s string, v string) bool {
	matches := interpolationRegex.FindAllStringSubmatch(s, -1)
	for _, match := range matches {
		if name := match[3]; name == v {
			return true
		}
	}
	return false
}

// Interpolate v
    
func interpolate(s string, vars Vars, updatedVars map[string]string) (string, error) {
	out := ""
	matches := interpolationRegex.FindAllStringSubmatch(s, -1)
	if len(matches) == 0 {
		return s, nil
	}
	for key, val := range updatedVars {
		if vars[key] != val {
			vars = vars.CloneWith(updatedVars)
			break
		}
	}
	for _, match := range matches {
		if dollar := match[1]; dollar != "" {
			out += "$"
		} else if name := match[3]; name != "" {
			value, ok := vars[name]
			if !ok {
				// No default value.
				if match[4] == "" {
					return "", fmt.Errorf("undefined variable ${%s}", name)
				}
				value = match[4]
			}
			out += value
		} else {
			out += match[0]
		}
	}
	return out, nil
}