No need to use difficult regex, we can use functionality from JSON.stringify(object, undefined, 2) to get beautifully rendered strings from JSON.

var obj={"glossary":{"title":"example glossary","GlossDiv":{"title":"S","GlossList":{"GlossEntry":{"ID":"SGML","SortAs":"SGML","GlossTerm":"Standard Generalized Markup Language","Acronym":"SGML","Abbrev":"ISO 8879:1986","GlossDef":{"para":"A meta-markup language, used to create markup languages such as DocBook.","GlossSeeAlso":["GML","XML"]},"GlossSee":"markup"}}}}}


var pretty = JSON.stringify(obj, undefined, 2);

var ugly = document.getElementById('myTextArea').value; document.getElementById('myTextArea').value = pretty;

The syntax highlighting is tough but check out this fiddle for pretty printing a json object entered in a text area. Do note that the JSON has to be valid for this to work. (Use the dev console to catch errors.) Check jsLint for valid json.

The HTML:

<textarea id="myTextArea" cols=50 rows=10></textarea>
<button onclick="prettyPrint()">Pretty Print</button>

The js:

function prettyPrint() {
    var ugly = document.getElementById('myTextArea').value;
    var obj = JSON.parse(ugly);
    var pretty = JSON.stringify(obj, undefined, 4);
    document.getElementById('myTextArea').value = pretty;
}

First try simple input like: {"a":"hello","b":123}

Simple pretty printing of JSON can be done rather easily. Try this js code: (jsFiddle here)

// arbitrary js object:
var myJsObj = {a:'foo', 'b':'bar', c:[false,2,null, 'null']};

// using JSON.stringify pretty print capability:
var str = JSON.stringify(myJsObj, undefined, 4);

// display pretty printed object in text area:
document.getElementById('myTextArea').innerHTML = str;

For this HTML:

<textarea id="myTextArea" cols=50 rows=25></textarea>

And check out JSON.stringify documentation.

It's pretty hard in a semantic container to format text to looks like json but you can use a react package like react-json-pretty or you can make a JSON.stringify(json,undefined,2) and pass it in a semantic TextArea that you add into your container and "play" with css to make looks better.
Here an example with both solution:

import React from "react";
import JSONPretty from "react-json-pretty";
import JSONPrettyMon from "react-json-pretty/dist/monikai";
import { Container, TextArea } from "semantic-ui-react";

const ContainerExampleContainer = () => {
  return (
    <>
      <Container>
        <h2>WITH STYLE</h2>
        <JSONPretty id="json-pretty" data={json} theme={JSONPrettyMon} />
        <h2>WITHOUT STYLE</h2>
        <JSONPretty id="json-pretty" data={json} />
      </Container>
      <Container>
        <h2>In a TextArea </h2>
        <TextArea
          style={{
            border: "none",
            cursor: "text",
            width: "100%"
          }}
          value={JSON.stringify(json, undefined, 2)}
          placeholder="json here"
          rows={25}
          disabled
        />
      </Container>
    </>
  );
};
const json = {
  policies: {
    ExtensionSettings: {
      "*": {
        blocked_install_message: "Custom error message.",
        install_sources: ["about:addons", "https://addons.mozilla.org/"],
        installation_mode: "allowed",
        allowed_types: ["extension"]
      },
      "{d634138d-c276-4fc8-924b-40a0ea21d284}": {
        installation_mode: "force_installed",
        install_url:
          "https://addons.cdn.mozilla.net/user-media/addons/950528/1password_password_manager-1.23.1-fx.xpi?filehash=sha256%3A47e9e98f1072d93d595002dc8c221e5cca17e091b3431563a8e3e2be575c5cc1"
      }
    }
  }
};

export default ContainerExampleContainer;

***UPDATE***

 <Container
        as="textarea"
        style={{
          border: "none",
          cursor: "text",
          width: "100%"
        }}
        rows={25}
        value={JSON.stringify(json, undefined, 2)}
        disabled
      ></Container>