Remove all duplicate whitespace

space := regexp.MustCompile(`\s+`)
s := space.ReplaceAllString("Hello  \t \n world!", " ")
fmt.Printf("%q", s) // "Hello world!"

\s+ is a regular expression:

• the character class \s matches a space, tab, new line, carriage return or form feed,
• and + says “one or more of those”.

In other words, the code will replace each whitespace substring with a single space character.

Trim leading and trailing space

To trim leading and trailing whitespace, use the strings.TrimSpace function.

Further reading

Regular expressions is a gentle introduction to the regexp package with cheat sheet and plenty of examples.