Go: Replace String with Regular Expression Callback

Go has many tools for regular expressions available in the default regex package. However, let’s see if you can pick up the pattern…

• PHP: preg_replace_callback($pattern, $callback, $subject)
• Ruby: subject.gsub(pattern) {|match| callback}
• Python: re.sub(pattern, callback, subject)
• JavaScript: subject.replace(pattern, callback)
• Go: …oh noes!

All the individual mechanics that need to come together exist in the Go regexpackage but they need to be strung together:

import "regexp"
func ReplaceAllStringSubmatchFunc(re *regexp.Regexp, str string, repl func([]string) string) string {
    result := ""
    lastIndex := 0
    for _, v := range re.FindAllSubmatchIndex([]byte(str), -1) {
        groups := []string{}
        for i := 0; i < len(v); i += 2 {
            groups = append(groups, str[v[i]:v[i+1]])
        }
        result += str[lastIndex:v[0]] + repl(groups)
        lastIndex = v[1]
    }
    return result + str[lastIndex:]
}

And here is a working example:

func main() {
    str := "abc foo:bar def baz:qux ghi"
    re := regexp.MustCompile("([a-z]+):([a-z]+)")
    result := ReplaceAllStringSubmatchFunc(re, str, func(groups []string) string {
        return "(" + groups[1] + "->" + groups[2] + ")"
    })
    fmt.Printf("%s\n", result)
}
abc (foo->bar) def (baz->qui) ghi
abc (foo->bar) def (baz->qui) ghi

So finally, it can join the list!

• Go: ReplaceAllStringSubmatchFunc(pattern, subject, callback)

Originally published at http://elliot.land on March 12, 2016.