How to remove redundant spaces/whitespace from a string in Golang?

strings.Fields() splits on any amount of white space, thus:

strings.Join(strings.Fields(strings.TrimSpace(s)), " ")

You can get quite far just using the strings package as strings.Fields does most of the work for you:

package main

import (
    "fmt"
    "strings"
)

func standardizeSpaces(s string) string {
    return strings.Join(strings.Fields(s), " ")
}

func main() {
    tests := []string{" Hello,   World  ! ", "Hello,\tWorld ! ", " \t\n\t Hello,\tWorld\n!\n\t"}
    for _, test := range tests {
        fmt.Println(standardizeSpaces(test))
    }
}
// "Hello, World !"
// "Hello, World !"
// "Hello, World !"

It seems that you might want to use both \s shorthand character class and \p{Zs} Unicode property to match Unicode spaces. However, both steps cannot be done with 1 regex replacement as you need two different replacements, and the ReplaceAllStringFunc only allows a whole match string as argument (I have no idea how to check which group matched).

Thus, I suggest using two regexps:

• ^[\s\p{Zs}]+|[\s\p{Zs}]+$ - to match all leading/trailing whitespace
• [\s\p{Zs}]{2,} - to match 2 or more whitespace symbols inside a string

Sample code:

package main

import (
    "fmt"
    "regexp"
)

func main() {
    input := "   Text   More here     "
    re_leadclose_whtsp := regexp.MustCompile(`^[\s\p{Zs}]+|[\s\p{Zs}]+$`)
    re_inside_whtsp := regexp.MustCompile(`[\s\p{Zs}]{2,}`)
    final := re_leadclose_whtsp.ReplaceAllString(input, "")
    final = re_inside_whtsp.ReplaceAllString(final, " ")
    fmt.Println(final)
}