How to highlight only numbers and special characters inside Text in Flutter?

I'm trying to highlight any numbers & special characters like '%' from a String that comes dynamically from JSON. This is my current implementation but I can't understand how to change it dynamically.

                          RichText(
                                  overflow: TextOverflow.ellipsis,
                                  textAlign: TextAlign.center,
                                  maxLines: 4,
                                  text: TextSpan(
                                    children: <TextSpan>[
                                      TextSpan(
                                          text: 'Hey I\'m',
                                          style: TextStyle(
                                              color: kDarkBlue, fontSize: 21)),
                                      TextSpan(
                                          text: '1234 ',
                                          style: TextStyle(
                                              fontWeight: FontWeight.w800,
                                              fontSize: 24)),
                                      TextSpan(
                                          text: 'and',
                                          style: TextStyle(fontSize: 21)),
                                      TextSpan(
                                          text: '%',
                                          style: TextStyle(
                                              fontWeight: FontWeight.w800,
                                              fontSize: 24)),
                                    ],
                                  ),
                                ),

Final Output:

You can try this:

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  String str = "Hey I'm 1234 and %";
  int findLen(String word) {
    return word.replaceAll(new RegExp(r'[a-zA-Z]'), "").length;
  }

  var styleOne = TextStyle(color: Colors.black87, fontSize: 21);

  var styleTwo = TextStyle(
      color: Colors.black87, fontWeight: FontWeight.w800, fontSize: 24);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: RichText(
        overflow: TextOverflow.ellipsis,
        textAlign: TextAlign.center,
        maxLines: 4,
        text: TextSpan(
          children: str
              .split(" ")
              .map((word) => TextSpan(
                  text: word + " ",
                  style: findLen(word) != word.length ? styleOne : styleTwo))
              .toList(),
        ),
      ),
    );
  }
}

Working Demo: Codepen