replace all characters in string except last 4 characters - regex

Question

Using Go, how do I replace all characters in a string with "X" except the last 4 characters?
This works fine for php/javascript but not for golang as "?=" is not supported.
\w(?=\w{4,}$)
Tried this, but does not work. I couldn't find anything similar for golang
(\w)(?:\w{4,}$)
JavaScript working link
Go non-working link

Answer 1

A simple yet efficient solution that handles multi UTF-8-byte characters is to convert the string to []rune, overwrite runes with 'X' (except the last 4), then convert back to string.
func maskLeft(s string) string {
rs := []rune(s)
for i := 0; i < len(rs)-4; i++ {
rs[i] = 'X'
}
return string(rs)
}
Testing it:
fmt.Println(maskLeft("123"))
fmt.Println(maskLeft("123456"))
fmt.Println(maskLeft("1234世界"))
fmt.Println(maskLeft("世界3456"))
Output (try it on the Go Playground):
123
XX3456
XX34世界
XX3456
Also see related question: How to replace all characters in a string in golang

Answer 2

Let's say inputString is the string you want to mask all the characters of (except the last four).
First get the last four characters of the string:
last4 := string(inputString[len(inputString)-4:])
Then get a string of X's which is the same length as inputString, minus 4:
re := regexp.MustCompile("\w")
maskedPart := re.ReplaceAllString(inputString[0:len(inputString)-5], "X")
Then combine maskedPart and last4 to get your result:
maskedString := strings.Join([]string{maskedPart,last4},"")

Answer 3

Simpler approach without regex and looping
package main
import (
"fmt"
"strings"
)
func main() {
string := "thisisarandomstring"
head := string[:len(string)-4]
tail := string[len(string)-4:]
mask := strings.Repeat("x", len(head))
fmt.Printf("%v%v", mask, tail)
}
// Output:
// xxxxxxxxxxxxxxxring

Answer 4

Create a Regexp with
re := regexp.MustCompile("\w{4}$")
Let's say inputString is the string you want to remove the last four characters from. Use this code to return a copy of inputString without the last 4 characters:
re.ReplaceAllString(inputString, "")
Note: if it's possible that your input string could start out with less than four characters, and you still want those characters removed since they are at the end of the string, you should instead use:
re := regexp.MustCompile("\w{0,4}$")