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(
        body: Center(
          child: Padding(
            padding: const EdgeInsets.symmetric(horizontal: 20.0),
            child: TextField(
              inputFormatters: [DecimalTextInputFormatter(decimalRange: 2),],
            ),
          ),
        ),
      ),
    );
  }
}

class DecimalTextInputFormatter extends TextInputFormatter {
  DecimalTextInputFormatter({int decimalRange})
      : assert(decimalRange == null || decimalRange >= 0,
            'DecimalTextInputFormatter declaretion error') {
        
    String dp = (decimalRange != null && decimalRange > 0)
        ? "([.][0-9]{0,$decimalRange}){0,1}"
        : "";
    String num = "[0-9]*$dp";

    _exp = new RegExp("^($num){0,1}\$");
  }

  RegExp _exp;

  @override
  TextEditingValue formatEditUpdate(
    TextEditingValue oldValue,
    TextEditingValue newValue,
  ) {
    print('newValue: $newValue');
    print('oldValue: $newValue');
    
    if (_exp.hasMatch(newValue.text)) {
      return newValue;
    }
    return oldValue;
  }
}