Error textfield flutter

Apps often require users to enter information into a text field.
For example, you might require users to log in with an email address
and password combination.

To make apps secure and easy to use, check whether the
information the user has provided is valid. If the user has correctly filled
out the form, process the information. If the user submits incorrect
information, display a friendly error message letting them know what went
wrong.

In this example, learn how to add validation to a form that has
a single text field using the following steps:

1. Create a Form with a GlobalKey.
2. Add a TextFormField with validation logic.
3. Create a button to validate and submit the form.

1. Create a Form with a GlobalKey

First, create a Form.
The Form widget acts as a container for grouping
and validating multiple form fields.

When creating the form, provide a GlobalKey.
This uniquely identifies the Form,
and allows validation of the form in a later step.

import 'package:flutter/material.dart';

// Define a custom Form widget.
class MyCustomForm extends StatefulWidget {
  const MyCustomForm({super.key});

  @override
  MyCustomFormState createState() {
    return MyCustomFormState();
  }
}

// Define a corresponding State class.
// This class holds data related to the form.
class MyCustomFormState extends State<MyCustomForm> {
  // Create a global key that uniquely identifies the Form widget
  // and allows validation of the form.
  //
  // Note: This is a `GlobalKey<FormState>`,
  // not a GlobalKey<MyCustomFormState>.
  final _formKey = GlobalKey<FormState>();

  @override
  Widget build(BuildContext context) {
    // Build a Form widget using the _formKey created above.
    return Form(
      key: _formKey,
      child: Column(
        children: <Widget>[
          // Add TextFormFields and ElevatedButton here.
        ],
      ),
    );
  }
}

2. Add a TextFormField with validation logic

Although the Form is in place,
it doesn’t have a way for users to enter text.
That’s the job of a TextFormField.
The TextFormField widget renders a material design text field
and can display validation errors when they occur.

Validate the input by providing a validator() function to the
TextFormField. If the user’s input isn’t valid,
the validator function returns a String containing
an error message.
If there are no errors, the validator must return null.

For this example, create a validator that ensures the
TextFormField isn’t empty. If it is empty,
return a friendly error message.

TextFormField(
  // The validator receives the text that the user has entered.
  validator: (value) {
    if (value == null || value.isEmpty) {
      return 'Please enter some text';
    }
    return null;
  },
),

3. Create a button to validate and submit the form

Now that you have a form with a text field,
provide a button that the user can tap to submit the information.

When the user attempts to submit the form, check if the form is valid.
If it is, display a success message.
If it isn’t (the text field has no content) display the error message.

ElevatedButton(
  onPressed: () {
    // Validate returns true if the form is valid, or false otherwise.
    if (_formKey.currentState!.validate()) {
      // If the form is valid, display a snackbar. In the real world,
      // you'd often call a server or save the information in a database.
      ScaffoldMessenger.of(context).showSnackBar(
        const SnackBar(content: Text('Processing Data')),
      );
    }
  },
  child: const Text('Submit'),
),

How does this work?

To validate the form, use the _formKey created in
step 1. You can use the _formKey.currentState()
method to access the FormState,
which is automatically created by Flutter when building a Form.

The FormState class contains the validate() method.
When the validate() method is called, it runs the validator()
function for each text field in the form.
If everything looks good, the validate() method returns true.
If any text field contains errors, the validate() method
rebuilds the form to display any error messages and returns false.

Interactive example

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    const appTitle = 'Form Validation Demo';

    return MaterialApp(
      title: appTitle,
      home: Scaffold(
        appBar: AppBar(
          title: const Text(appTitle),
        ),
        body: const MyCustomForm(),
      ),
    );
  }
}

// Create a Form widget.
class MyCustomForm extends StatefulWidget {
  const MyCustomForm({super.key});

  @override
  MyCustomFormState createState() {
    return MyCustomFormState();
  }
}

// Create a corresponding State class.
// This class holds data related to the form.
class MyCustomFormState extends State<MyCustomForm> {
  // Create a global key that uniquely identifies the Form widget
  // and allows validation of the form.
  //
  // Note: This is a GlobalKey<FormState>,
  // not a GlobalKey<MyCustomFormState>.
  final _formKey = GlobalKey<FormState>();

  @override
  Widget build(BuildContext context) {
    // Build a Form widget using the _formKey created above.
    return Form(
      key: _formKey,
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: [
          TextFormField(
            // The validator receives the text that the user has entered.
            validator: (value) {
              if (value == null || value.isEmpty) {
                return 'Please enter some text';
              }
              return null;
            },
          ),
          Padding(
            padding: const EdgeInsets.symmetric(vertical: 16.0),
            child: ElevatedButton(
              onPressed: () {
                // Validate returns true if the form is valid, or false otherwise.
                if (_formKey.currentState!.validate()) {
                  // If the form is valid, display a snackbar. In the real world,
                  // you'd often call a server or save the information in a database.
                  ScaffoldMessenger.of(context).showSnackBar(
                    const SnackBar(content: Text('Processing Data')),
                  );
                }
              },
              child: const Text('Submit'),
            ),
          ),
        ],
      ),
    );
  }
}

TextField Widget is used to get data from users and perform the desired operation…So, in this article, we will go through how to do Textfield Validation In Flutter.

A Minimal Example of what you want:

class MyHomePage extends StatefulWidget {
  @override
  MyHomePageState createState() {
    return new MyHomePageState();
  }
}

class MyHomePageState extends State<MyHomePage> {
  final _text = TextEditingController();
  bool _validate = false;

  @override
  void dispose() {
    _text.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('TextField Demo'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text('Error Showed if Field is Empty on Submit button Pressed'),
            TextField(
              controller: _text,
              decoration: InputDecoration(
                labelText: 'Enter the Value',
                errorText: _validate ? 'Value Can't Be Empty' : null,
              ),
            ),
            ElevatedButton(
              onPressed: () {
                setState(() {
                  _text.text.isEmpty ? _validate = true : _validate = false;
                });
              },
              child: Text('Submit'),
              style: ElevatedButton.styleFrom(
                primary: Colors.blueAccent,//button background color
                onPrimary: Colors.white,//button text color
                shape: RoundedRectangleBorder(
                  borderRadius: BorderRadius.circular(25),
                )
              )
            )
          ],
        ),
      ),
    );
  }
}

TextFormField is also a better option for validation. Flutter handles error text itself, so we don’t require to use variable _validate. It will check at runtime whether you satisfied the condition or not.

final confirmPassword = TextFormField(
  controller: widget.confirmPasswordController,
  obscureText: true,
  decoration: InputDecoration(
    prefixIcon: Icon(Icons.lock_open, color: Colors.grey),
    hintText: 'Confirm Password',
    errorText: validatePassword(widget.confirmPasswordController.text),
    contentPadding: EdgeInsets.fromLTRB(20, 10, 20, 10),
  ),
);

String? validatePassword(String value) {
  if (value == null || !(value.length > 5) && value.isNotEmpty) {
    return "Password should contain more than 5 characters";
  }
  return null;
}

Note: The user must add at least one character to get this error message. Using a TextFormField with a validator.

class MyHomePageState extends State<MyHomePage> {
  final _formKey = GlobalKey<FormState>();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('TextFormField validator'),
      ),
      body: Form(
        key: _formKey,
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            TextFormField(
              decoration: InputDecoration(
                hintText: 'Enter text',
              ),
              textAlign: TextAlign.center,
              validator: (text) {
                if (text == null || text.isEmpty) {
                  return 'Text is empty';
                }
                return null;
              },
            ),
            ElevatedButton(
              onPressed: () {
                if (_formKey.currentState!.validate()) {
                  // TODO submit
                }
              },
              child: Text('Submit'),
            )
          ],
        ),
      ),
    );
  }
}

You can do something like this

class _validateTextField extends State<validateTextField> {
  TextEditingController userNameController = TextEditingController();
  bool userNameValidate = false, isUserNameValidate = false;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
          child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                TextField(
                  controller: userNameController,
                  decoration: InputDecoration(
                      labelText: 'Enter Username',
                      errorText: isUserNameValidate ? 'Please enter a Username' : null
                  ),
                ),
                SizedBox(height: 50,),
                OutlinedButton(
                  onPressed: () {
                    validateTextField(userNameController.text);
                  },
                  child: Text('Validate'),
                  style: OutlinedButton.styleFrom(
                    side: BorderSide(
                      width: 2,
                      color: Colors.blue,
                      style: BorderStyle.solid,
                    ),
                  ),
                ),
              ]
          )
      ),
    );
  }
}

Create Function – This function will validate whether the value you entered is validate or not. And call it at the click of a submit or validate button.

bool validateTextField(String userInput) {
  if (userInput.isEmpty) {
    setState(() {
      isUserNameValidate = true;
    });
    return false;
  }
  setState(() {
    isUserNameValidate = false;
  });
  return true;
}

The above code will give us output like the below:

• If you use TextFormField then you could easily implement ‘Error below your text fields’.
• You can do this without using _validate or any other flags.
• In this example, I have used the validator method of the TextFormField Widget. This makes the work a lot easier and readable at the same time.
• I also used FormState to make the work easier.

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

class MyApp extends StatelessWidget {
  final _form = GlobalKey<FormState>(); //for storing form state.

//saving form after validation  
void _saveForm() {
    final isValid = _form.currentState!.validate();
    if (!isValid) {
      return;
    }
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Form(
          key: _form, //assigning key to form

          child: ListView(
            children: <Widget>[

              TextFormField(
                decoration: InputDecoration(labelText: 'Full Name'),
                validator: (text) {
                  if (text == null || !(text.length > 5) && text.isNotEmpty) {
                    return "Enter valid name of more then 5 characters!";
                  }
                  return null;
                },
              ),

              TextFormField(
                decoration: InputDecoration(labelText: 'Email'),
                validator: (text) {
                  if (text == null || !(text.contains('@')) && text.isNotEmpty) {
                    return "Enter a valid email address!";
                  }
                  return null;
                },
              ),

              ElevatedButton(
                child: Text('Submit'),
                onPressed: () => _saveForm(),
              )
            ],
          ),
        ),
      ),
    );
  }
}

Stay connected for more articles like flutter textfield validation without form, password validation in flutter, flutter form( autovalidate), flutter regex validation, flutter form validation bloc, flutter registration form, flutter form builder, textformfield flutter, etc.

Conclusion:

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Flutter has many different types of built-in input widgets like DatePicker, Checkbox, Slider, Radio Button, Dropdown Button amongst others for the time being we’re going to focus on the text.

Everybody has used a TextField to get information from the user. TextField is simple widgets, simply means a way of capturing a collection of data from the end-user.

As you can see we have a simple text field widget. By default when you add a TextField you will get the material styling.

Input Decoration

This allows to added behavior object without affecting the object itself. It will add visual enhancements to your TextFormField without actually changing the text because the TextFormField itself is pretty basic.

TextFormField(decoration: const InputDecoration(
                        icon: const Icon(Icons.person),
                        hintText: 'Enter your name',
                        labelText: 'Name'),
                        ....)

Input widgets can accept a number of properties. You can change the fill color, content patent, content padding, border ever border focus. You could change how that site with a disabled counter text.

The InputDeclaration does have some limitations.  Label and Error messages are limited to text only. If you want a label to then that’s possible but if you want to add a star for required or if you want to add an exclamation mark if your message is invalid then it’s not possible.

As many of you know that to make apps secure you want to make sure that the information that users enter is valid and you want to display a nice friendly message as you can see.

Flutter Form Framework

You can probably come up with your own validation framework but Flutter has a set of form widgets that manages validation as along with other few features as well.

There are two types of widgets Form and FormField<T>. This represents Form in the widget tree and a Form will have FormField descendants that could be the string, int or a custom class.

The Form widgets maintain the states of the FormField and it stores data such as the current value and the validity.

Each FormField can handle its own validation but Form itself can coordinate and at once validates every FormField widgets. It can traverse down the tree, find every descendent of FormField and validate each one in turn.

FormField is a wrapper that adds validation to an existing input widget. It can also add saving and reset functionality. It’s uniform and consistent across all form widgets. 

Validating TextFormField

Now I’m just going to explain a little bit more about validating TextFormFields. 

1.Create a Form with a global key.

final _formKey = GlobalKey<FormState>();
...

body: Center(
          child: Form(
              key: _formKey,
              autovalidate: false,
              ....

In the case of a TextFormField the first thing you would do is create a Form and create a GlobalKey of the type FormField state. Then you will add that key to your Form

2. Add a FormField with validation logic

TextFormField( .....
                  validator: (value) {
                      if (value.isEmpty) {
                        return 'Please enter name';
                      }
                      return null;
                    },

It’s simple as you see there we want to make sure that username is required. Make sure that there is a value if not then the field is invalid and we will turn the message otherwise we return null indicating that is valid.

3. Create a button to validate and submit of the Form

Finally, we can call the validate function and then we submit the data.

onPressed: () {
                        if (_formKey.currentState.validate()) {
                          print("Process data");
                          ...
                        } else {
                          print('Error');
                        }
                      },
                      child: Text('Create'),
                    ),

How validation work

It records the valid function of every single FormField. Every FormField has it’s own validate function and within that validate function it will invoke the validator. The error message is displayed in the TextField. If FormFields is invalid this function will return false if all FormField is valid then it will return true and you can go ahead and process and submit the data.

String Length Validation

Simply add this code to any TextFormField to limit the length of characters to the passed in value.  In our case, we will limit the name to 20 characters.

TextFormField(
       ......
[new LengthLimitingTextInputFormatter(20)],)

Phone Number Validation

A keyboard type of phone makes easier for a user to enter a phone number.  For our validation, we’ll use a standard 10-digit Indian phone format.

TextFormField(
                    ...
                    keyboardType: TextInputType.phone,
                    inputFormatters: [
                      new WhitelistingTextInputFormatter(
                          new RegExp(r'^[0-9]*$')),
                      new LengthLimitingTextInputFormatter(10)
                    ],
                    ...
                  ),

What the above pattern does is essentially create a whitelist of characters we want to allow the user to enter.  If a character doesn’t match this whitelist, it is discarded and not allowed to be entered. This keeps the user from entering anything other digits.

final RegExp phoneRegex = new RegExp(r'^[6-9]d{9}$');
TextFormField(
                  ....
                    validator: (value) {
                      if (!phoneRegex.hasMatch(value)) {
                        return 'Please enter valid phone number';
                      }
                      return null;
                    },
                    ...
                  ),

In addition to whitelisting characters, we need to tell the user when they have an issue with their entry.

Email Address Validation

The email address enter by the user will be validated against this regular expression and if it doesn’t pass, then the user will see a message informing them to correct their entry.

final RegExp emailRegex = new RegExp(
      r"^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-][email protected][a-zA-Z0-9](?:[a-zA-Z0-9-]{0,253}[a-zA-Z0-9])?(?:.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,253}[a-zA-Z0-9])?)*$");


 TextFormField(
                    ...
                    keyboardType: TextInputType.emailAddress,
                    validator: (value) {
                      if (!emailRegex.hasMatch(value)) {
                        return 'Please enter valid email';
                      }
                      return null;
                    },
                    ...
                  ),

Password Cross Input Validation

The confirm password field needs to know the value of the password field in order to check that the passwords don’t match. 

final _passwordFieldKey = GlobalKey<FormFieldState<String>>();
TextFormField(
                    key: _passwordFieldKey,
                    obscureText: true,
                    ....
             ),
TextFormField(
                    obscureText: true,
                    decoration: const InputDecoration(                    
                        labelText: 'Conform Password'),
                    validator: (value) {
                      if (value != _passwordFieldKey.currentState.value) {
                        return 'Password do not match';
                      }
                      return null;
                    },
                  ),

We can create a password field kay. When we pass that key into our TextFormField we can get access to the state of the password field and then get the value and then here we can do our check and then return the desired error message.

The Save Function

Another feature of the FormField is save() function. Save function will allow you just to save all the value at once if you want to save the values on submission. It’s pretty simple you have an save() function, in this case, we’re just assigning the value to state variable.

TextFormField(
                    ...
                    onSaved: (value) {
                      user.phoneNo = value;
                    },
                  ),

Like validate() what save will do is it will go through every FormField and invoke save functional every form filter to invoke the save function of every FormField.

Auto Validation

Use the auto validate property if you want to validate on changed. A lot of the time when you have a form you want the validation. What you can do is that once you press submit for the first time you can set auto validate to true so that as soon as you correct your mistake the other message goes away.

Download this project from GitHub

If you are building the sign-in or sign-up page in Android application, then Obviously you need to validate the Text field. otherwise, it can be null or empty. This will cause save the null values into the database. Then further make the application less impact. So to overcome this problem we need to prior check that there can not be a empty text field. So that null value can not be saved into the database. A sample video is given below to get an idea about what we are going to do in this article.

Step By Step Implementation

Step 1: Create a New Project in Android Studio

To set up Flutter Development on Android Studio please refer to Android Studio Setup for Flutter Development, and then create a new project in Android Studio please refer to Creating a Simple Application in Flutter.

Step 2: Import the material package

Adding material package that gives us the important functions and calls the runApp method in the main function that will call our application.

import 'package:flutter/material.dart';
void main() {
   runApp(RunMyApp());
}

Step 3: Creating stateful Widget

Now we have to make a stateful widget and then return the materialApp widget which allows us the set the title and theme and many more or default values. 

Step 4: Create the text editing controller

Create the text editing controller to retrieve the text from the Text field. You can refer to this article to Retrieve the text from the text field.

final _text = TextEditingController();  //texteditingcontroller
bool _validate = false;  //variable to store the bool value

Step 5: Check the text field is empty or not while pressing the button.

ElevatedButton(
                 onPressed: () {
                   setState(() {
                     _text.text.isEmpty ? _validate = true : _validate = false;
                   });
                 },
                 child: Text('Submit'),
               )

Here , _text.text.isEmpty  we are checking using the text editing controller. If empty validate variable will assign with false otherwise assign with true. And finally according to the value of validate we can raise the error in the text field as shown in the output video.

Final Code

Output

In this example, we are going to show you the easiest way to validate TextFiled or TextFormField in Flutter App. Validation is very important to retrieve correct data from users. See the example below to validate email, full name, phone number, credit card number, URL, and many more.

To validate the correctness of data, you can use Regular Expression (Regex) with the pattern.

RegExp(r'^[a-z A-Z]+$')

This pattern allows upper/lower case alphabets and space.

RegExp(r'^[w-.][email protected]([w-]+.)+[w-]{2,4}$')

This regex checks email with domain extension length. Allows 2 to 4 domain extension lengths. For example, .com has 3 characters, .np as 2 characters, .store as 4 characters. 

RegExp(r'^[+]*[(]{0,1}[0-9]{1,4}[)]{0,1}[-s./0-9]*$')

This pattern allows «+» single sign at first.

if(fname.text.length > 10){
    //length of textfield with controller "name" has more than 10 characters.
}

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget{
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Home(),
    );
  }
}

class Home extends StatefulWidget{
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  final formKey = GlobalKey<FormState>(); //key for form

  @override
  Widget build(BuildContext context) {
    return  Scaffold(
          appBar: AppBar( 
             title: Text("Form Validation"),
             backgroundColor: Colors.indigoAccent,
          ),
          body: Container(
            padding: EdgeInsets.all(20),
            child: Form( 
              key: formKey, //key for form
              child:Column(children: [
                     
                     TextFormField(
                        decoration: InputDecoration( 
                          labelText: 'Enter Name'
                        ),
                        validator: (value){
                          if(value.isEmpty || !RegExp(r'^[a-z A-Z]+$').hasMatch(value)){
                              //allow upper and lower case alphabets and space
                              return "Enter Correct Name";
                          }else{
                             return null;
                          }
                        },
                     ),

                     TextFormField(
                       decoration: InputDecoration( 
                          labelText: 'Enter Phone Number'
                        ),
                        validator: (value){
                          if(value.isEmpty || !RegExp(r'^[+]*[(]{0,1}[0-9]{1,4}[)]{0,1}[-s./0-9]*$').hasMatch(value)){
                              //  r'^[0-9]{10}$' pattern plain match number with length 10
                              return "Enter Correct Phone Number";
                          }else{
                             return null;
                          }
                        },
                     ),

                     TextFormField(
                        decoration: InputDecoration( 
                          labelText: 'Enter Email'
                        ),
                        validator: (value){
                          if(value.isEmpty || !RegExp(r'^[w-.][email protected]([w-]+.)+[w-]{2,4}$').hasMatch(value)){
                              return "Enter Correct Email Address";
                          }else{
                             return null;
                          }
                        },
                     ),
                     
                     ElevatedButton(
                      onPressed: (){
                          if(formKey.currentState.validate()){
                             //check if form data are valid, 
                             // your process task ahed if all data are valid
                          }
                      },
                      child: Text("Submit Data")
                    )

                ],
               ),
            ),
          )
       );
  }
}

class Home extends StatefulWidget{
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  TextEditingController fname = TextEditingController();
  bool texterror = false;

  @override
  void initState() {
    fname.text = "";
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return  Scaffold(
          appBar: AppBar( 
             title: Text("TextField Validation"),
             backgroundColor: Colors.indigoAccent,
          ),
          body: Container(
            padding: EdgeInsets.all(20),
            child: Column(children: [
                     TextField(
                       controller: fname,
                       decoration: InputDecoration( 
                         labelText: "Your Name",
                         errorText: texterror?"Enter Correct Name":null,
                       )
                     ),
                     
                     ElevatedButton(
                      onPressed: (){
                         if(fname.text.isEmpty || !RegExp(r'^[a-z A-Z]+$').hasMatch(fname.text)){
                              setState(() {
                                texterror = true;
                              });
                          }else{
                             setState(() {
                                texterror = false;
                              });
                          }
                      },
                      child: Text("Submit Data")
                    )

                ],
            )
          )
       );
  }
}

Furthermore, you can validate data easily without using Regex. For that,  add validators flutter package to your dependency by adding the following line in pubspec.yaml file:

dependencies:
  flutter:
    sdk: flutter
  validators: ^3.0.0

Import package to Script:

import 'package:validators/validators.dart';

if(isEmail(value)){
  //value is email
}

if(isURL(value)){
  //value is web URL link
}

if(isAlpha(value)){
  //value has alphabets only
}

if(isCreditCard(value)){
  //value is credit card number
}

if(isCreditCard(value)){
  //value is credit card number
}

• contains — check if the string contains the seed.
• equals — check if the string matches the comparison.
• isAfter — check if the string is a date that’s after the specified date.
• isAlphanumeric — check if the string str contains only letters and numbers.
• isBefore — check if the string is a date that’s before the specified date.
• isFloat — check if the string str is a float.
• isFQDN — check if the string str is a fully qualified domain name (e.g. domain.com).
• isHexadecimal — check if the string stris a hexadecimal number.
• isHexColor — check if the string str is a hexadecimal color.
• isInt — check if the string str is an integer.
• isIP — check if the string str is IP version 4 or 6.
• isISBN — check if the string is an ISBN (version 10 or 13).
• isJSON — check if the string is valid JSON.
• isLength(String str, int min, [int? max]) — check if the length of the string str falls in a range.
• isLowercase — check if the string str is lowercase.
• isMongoId — check if the string is a valid hex-encoded representation of a MongoDB ObjectId.
• isNull — check if the string str is null.
• isNumeric — check if the string str contains only numbers.
• isPostalCode — Check if the string is postal or zip code.
• isUppercase — check if the string str is uppercase.
• isUUID — check if the string is a UUID (version 3, 4 or 5).
• matches — check if string str matches the Regex pattern.

In this way, you can validate TextFormFiled or TextField data with Dart Regex and package in Flutter App.