在TextFormField中禁用字母字符

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Disable Letters in TextFormField'),
        ),
        body: Center(
          child: MyForm(),
        ),
      ),
    );
  }
}

class MyForm extends StatefulWidget {
  @override
  _MyFormState createState() => _MyFormState();
}

class _MyFormState extends State<MyForm> {
  @override
  Widget build(BuildContext context) {
    return TextFormField(
      decoration: InputDecoration(
        labelText: 'Enter Numbers Only',
      ),
      inputFormatters: [
        FilteringTextInputFormatter.deny(RegExp(r'[a-zA-Z]')),
      ],
    );
  }
}