substituting-variables.md

Substituting variables in strings

Problem

You need to implement a simple templating language to substitute variables in multiple documents.

Solution

Use the Replacer type from the strings package:

package main

import (
    "fmt"
    "strings"
)

func main() {
    macros := strings.NewReplacer(
        "$ABOUT", `<a href="/about.html">About Us</a>`,
        "$HOME", `<a href="/index.html">Home Page</a>`,
        "$SEARCH", `<a href="http://search.example.com/query.cgi">Search</a>`,
    )

    input := `
            <p>Navigation:
             <ul>
            <li>$HOME</li>
            <li>$SEARCH</li>
            <li>$ABOUT</li>
             </ul>
        </p>`

    fmt.Println(macros.Replace(input))
}

Discussion

This recipe shows how a Replacer could be used to implement a simple macro-based system for a website. The macros are held in a new Replacer created with the NewReplacer function. This function expects an even number of string arguments. Each pair represents a string to look for and a string to replace it with.

Performing the search and replace is simply done by calling the Replace method on the Replacer, passing in the input string. It returns a copy of the string with all the search strings substituted by their replacements.

The Replacer type is heavily optimized for repeated replacement operations. For small numbers of replacements it performs a fast linear search and replace but for longer lists it builds a trie index of replacements. Because of this, it's important to reuse Replacers whenever possible. If you have replacements that change frequently or must be computed then Replacer is not the best solution and you'd be better off directly using the Replace function in the strings package:

package main

import (
    "fmt"
    "strings"
    "time"
)

func main() {
    input := `<p>Published on $DATE.</p>`

    fmt.Println(strings.Replace(input, "$DATE", time.Now().Format("Jan 2, 2006"), -1))
}

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