Search and Replace Strings in Golang – Top 5 Examples

Go has a powerful standard library that makes string manipulation easy right out of the box. One of the functions I use most often is the strings package’s Replace() function. strings.Replace() returns a copy of its input string after replacing all instances of the given substring with a new one.

strings.Replace() signature

func Replace(s, old, new string, n int) string

Notes

  • s is the original string that contains the substrings to be replaced.
  • old is the substring you want to be replaced.
  • new is the substring that will be swapped out for old.
  • n limits the number of replacements. If you want to replace them all, just set n to -1, or use the more explicit ReplaceAll function.

Before we get too far, let me mention that I recently launched Go Mastery, a hands-on Golang course. Take a peek at it if you have a second, now let’s get back to the string manipulation guide.

Example #1 – Replacing delimiters

Let’s say you have some comma-separated-values, CSVs. Perhaps you want to separate each word with a space instead of a comma. This can be useful if you need to make your delimiters consistent so you can later split the string into a slice.

package main

import (
    "fmt"
    "strings"
)

func main() {
    fmt.Println(strings.Replace("apple,banana,orange,pear", ",", " ", -1))
    // prints "apple banana orange pear"
}

Example #2 – Only replace some strings

It can be useful to only print the replace the first n instances of a word. For example, let’s say we had some text containing dialogue, like in a movie script. If you want to change the delimiter between the speaker and there lines to be a dash instead of a colon, but don’t want to replace any colons in the dialogue, you can set n=1.

package main

import (
    "fmt"
    "strings"
)

func main() {
    fmt.Println(strings.Replace("Lane: 'The box said price:1'", ":", " -", 1))
    // prints "Lane - 'The box said price:1'"
}

Example #3 – Remove all instances of a string

Sometimes you just want to strip out specific characters. For example, you may want to remove all periods. To do so, you can simply replace all periods with an empty string.

package main

import (
    "fmt"
    "strings"
)

func main() {
    fmt.Println(strings.Replace("123.456.789.0", ".", "", -1))
    // prints "1234567890"
}

Example #4 – Replacing in multiple strings (high performance)

If you need to perform the same replacements on many different documents, it can make sense to initialize a Replacer, which is much faster when used repeatedly. It’s faster is because it builds a trie structure under the hood that it keeps in memory, and that structure can be used repeatedly.

package main

import (
    "fmt"
    "strings"
)

func main() {
    replacer := strings.NewReplacer(",", ":", "!", "?")
    fmt.Println(replacer.Replace("hello,there!good,reader!"))
    fmt.Println(replacer.Replace("glad,to!have,you!"))
    fmt.Println(replacer.Replace("bye,now!thank,you!"))
    // prints
    // hello:there?good:reader?
    // glad:to?have:you?
    // bye:now?thank:you?
}

NewReplacer() takes a list of old-new string pairs, so you can use it to perform many different replacement operations.

func NewReplacer(oldnew ...string) *Replacer

Example #5 – Complicated Replacements with Regex

We’re shifting packages entirely now, and will be using the standard library’s regexp package. This package exposes a ReplaceAllString() function that lets us do more complicated replacements using a standard regex. This may be useful if you need to do some dynamic replacements, or are fluent in regular expressions.

func (re *Regexp) ReplaceAllString(src, repl string) string
package main

import (
    "fmt"
    "regexp"
)

func main() {
    re := regexp.MustCompile(`r.t`)
    fmt.Println(re.ReplaceAllString("rat cat rot dog", "ram"))
    // prints "ram cat ram dog"

    re = regexp.MustCompile(`-.*-`)
    fmt.Println(re.ReplaceAllString("-rasjdkajnsdt-hello world", ""))
    // prints "hello world"
}

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