Why is this javascript regex replace not replacing anything?

Question

I have been trying to do a simple replace using regex as see below. I was following this and this. But for some reason the code below just does not work. I honestly can't see what I did wrong.

  var equ = "77^7x";
  var base = "77";
  var exp = "7x";
  var output = equ;
  var replace = base + "^" + exp;
  var regex = new RegExp(replace, "gi");
  var newOutput = output.replace(regex, "Math.pow(" + base + "," + exp + ")");
  console.log(newOutput);

I don't think that it's a metacharacter. The problem is that the strings are different, so it doesn't match. The regex is is `/77^7x/gi`, whereas the string is `"Math.pow(77,7x)"`. — Endothermic_Dragon, Mar 23 '21 at 13:41
@Endothermic_Dragon I agree with Pointy. `/77^7x/` won't match, but `/77\^7x` will — evolutionxbox, Mar 23 '21 at 13:42
Oh wait, my mistake I forgot how to regex. However, the string does not match either. — Endothermic_Dragon, Mar 23 '21 at 13:47
@Endothermic_Dragon try `/77\^7x/.test("77^7x")` in your browser console — Pointy, Mar 23 '21 at 13:48
Yes, *that* will match, but the *string itself* is `"Math.pow(77,7x)"` and not `"77^7x"`. — Endothermic_Dragon, Mar 23 '21 at 13:51

score 1 · Accepted Answer · answered Mar 23 '21 at 13:49

^ is a regex metacharacter. You have to add a backslash. However, since you are converting from a string, you have to add 2 backslashes, one as the backslash, and another one to escape the backslash.

var equ = "77\^7x";
var base = "77";
var exp = "7x";
var output = equ;
var replace = base + "^" + exp;
var regex = new RegExp(replace, "gi");
var newOutput = output.replace(regex, "Math.pow(" + base + "," + exp + ")");
console.log("Regex: %o", regex);
console.log("String searched: Math.pow(" + base + "," + exp + ")");
console.log("Output: ", newOutput);

console.log("Does the backslash at the ^ make a difference?")

var replace = base + "\\^" + exp;
var regex = new RegExp(replace, "gi");
var newOutput = output.replace(regex, "Math.pow(" + base + "," + exp + ")");
console.log("Regex: %o", regex);
console.log("String searched: Math.pow(" + base + "," + exp + ")");
console.log("Output: ", newOutput);

As you can see, the regex is not present inside the string "Math.pow(77,7x)", so it does not match or replace anything.

Darius · Answer 2 · 2021-03-23T15:19:22.917

1

JSFiddle: https://jsfiddle.net/DariusM/1zy3sf9h/2/

As someone else mentioned, the ^ character is a special character in Regexp, therefore it needs to be escaped. Use:

var regex = new RegExp(base + "\\^" + exp, "gi");

edited Mar 23 '21 at 15:19

answered Mar 23 '21 at 13:51

Darius

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Why is this javascript regex replace not replacing anything?

2 Answers2