Replace variables "as words" without replacing the prefixes of other ones.

replace_variables.go

package main

import (
	"fmt"
)

func replaceByRange(s string, startIdx, endIdx int, new string) string {
	var res string

	var replaced bool
	for i := 0; i < len(s); i++ {
		if i < startIdx || i >= endIdx {
			res += string(s[i])
		} else if !replaced {
			res += new
			replaced = true
		}
	}

	return res
}

func replaceVariables(s, old, new string) string {
	if len(old) > len(s) {
		return s
	}

	const varSymbols = "abcdefghijklmnopqrstuvwxyz_0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"

	isVarSymbol := func(r byte) bool {
		for _, vs := range varSymbols {
			if vs == rune(r) {
				return true
			}
		}
		return false
	}

	var idxs [][2]int

	for i := 0; i < len(s); i++ {
		if i > len(s)-1 || i+len(old) > len(s) {
			break
		}

		window := s[i : i+len(old)]

		if window == old && ((i+len(old) < len(s) && !isVarSymbol(s[i+len(old)])) || i+len(old) == len(s)) {
			idxs = append(idxs, [2]int{i, i + len(old)})
		}

	}

	for i := len(idxs) - 1; i >= 0; i-- {
		s = replaceByRange(s, idxs[i][0], idxs[i][1], new)
	}

	return s
}

func main() {
	fmt.Println(replaceVariables("var+var1+var2", "var", "rrr"))
}