Form Builder Validators

Form Builder Validators set of validators for any FormField widget or widgets that extend the FormField class - e.g., TextFormField, DropdownFormField, et cetera. It provides standard ready-made validation rules and a way to compose new validation rules combining multiple rules, including custom ones.

Also included is the l10n / i18n of error text messages to multiple languages.

Pub Version GitHub Workflow Status Codecov CodeFactor Grade

Migrating from version 7 to 8

To migrate from v7 to v8, remove context as a parameter to validator functions. For example, FormBuilderValidators.required(context) becomes FormBuilderValidators.required() without context passed in.



  • Ready-made validation rules
  • Compose multiple reusable validation rules
  • Default error messages in multiple languages


This package comes with several most common FormFieldValidators such as required, numeric, mail, URL, min, max, minLength, maxLength, minWordsCount, maxWordsCount, IP, credit card, etc., with default errorText messages.

Available built-in validators include:

  • FormBuilderValidators.creditCard() - requires the field's value to be a valid credit card number.
  • - requires the field's value to be a valid date string.
  • - requires the field's value to be a valid email address.
  • FormBuilderValidators.equal() - requires the field's value to be equal to the provided object.
  • FormBuilderValidators.integer() - requires the field's value to be an integer.
  • FormBuilderValidators.ip() - requires the field's value to be a valid IP address.
  • FormBuilderValidators.match() - requires the field's value to match the provided regex pattern.
  • FormBuilderValidators.max() - requires the field's value to be less than or equal to the provided number.
  • FormBuilderValidators.maxLength() - requires the length of the field's value to be less than or equal to the provided maximum size.
  • FormBuilderValidators.maxWordsCount() - requires the word count of the field's value to be less than or equal to the provided maximum count.
  • FormBuilderValidators.min() - requires the field's value to be greater than or equal to the provided number.
  • FormBuilderValidators.minLength() - requires the length of the field's value to be greater than or equal to the provided minimum length.
  • FormBuilderValidators.minWordsCount() - requires the word count of the field's value to be greater than or equal to the provided minimum count.
  • FormBuilderValidators.equalLength() - requires the length of the field's value to be equal to the provided minimum length.
  • FormBuilderValidators.numeric() - requires the field's value to be a valid number.
  • FormBuilderValidators.required() - requires the field to have a non-empty value.
  • FormBuilderValidators.url() - requires the field's value to be a valid URL.

Supported languages

Validators support default errorText messages in these languages:

  • Albanian (al)
  • Arabic (ar)
  • Bangla (bn)
  • Bosnian (bs)
  • Catalan (ca)
  • Chinese Simplified (zh_Hans)
  • Chinese Traditional (zh_Hant)
  • Croatian (hr)
  • Czech (cs)
  • Dutch (nl)
  • English (en)
  • Estonian (et)
  • Farsi/Persian (fa)
  • French (fr)
  • German (de)
  • Greek (el)
  • Hungarian (hu)
  • Indonesian (id)
  • Italian (it)
  • Japanese (ja)
  • Korean (ko)
  • Lao (lo)
  • Malay (ms)
  • Mongolian (mn)
  • Polish (pl)
  • Portuguese (pt)
  • Romanian (ro)
  • Russian (ru)
  • Slovak (sk)
  • Slovenian (sl)
  • Spanish (es)
  • Swahili (sw)
  • Swedish (se)
  • Tamil(ta)
  • Thai (th)
  • Turkish (tr)
  • Ukrainian (uk)
  • Vietnamese (vi)

And you can still add your custom error messages.



The default error message is in English. To allow for localization of default error messages within your app, add FormBuilderLocalizations.delegate in the list of your app's localizationsDelegates.

return MaterialApp(
    supportedLocales: [
    localizationsDelegates: [

Basic use

    decoration: InputDecoration(labelText: 'Name'),
    autovalidateMode: AutovalidateMode.always,
    validator: FormBuilderValidators.required(),

See example tab or github code for more details

Specific uses

Composing multiple validators

The FormBuilderValidators class comes with a handy static function named compose(), which takes a list of FormFieldValidator functions. Composing allows you to create once and reuse validation rules across multiple fields, widgets, or apps.

On validation, each validator is run, and if any validator returns a non-null value (i.e., a String), validation fails, and the errorText for the field is set as the returned string.


    decoration: InputDecoration(labelText: 'Age'),
    keyboardType: TextInputType.number,
    autovalidateMode: AutovalidateMode.always,
    validator: FormBuilderValidators.compose([
        /// Makes this field required
        /// Ensures the value entered is numeric - with a custom error message
        FormBuilderValidators.numeric(errorText: 'La edad debe ser numérica.'),
        /// Sets a maximum value of 70
        /// Include your own custom `FormFieldValidator` function, if you want
        /// Ensures positive values only. We could also have used `FormBuilderValidators.min(0)` instead
        (val) {
            final number = int.tryParse(val);
            if (number == null) return null;
            if (number < 0) return 'We cannot have a negative age';
            return null;

Modify the default error message in a specific language

see override_form_builder_localizations_en for more detail.



You have some ways to contribute to this package.

  • Beginner: Reporting bugs or requesting new features
  • Intermediate: Answer questions, implement new features (from issues or not), and create pull requests
  • Advanced: Join organization like a member and help to code, manage issues, discuss new features, and other things

See the contribution file for more details

Add new supported language

We welcome efforts to internationalize/localize the package by translating the default validation errorText strings for built-in validation rules.

  1. Add ARB files

    Create one ARB file inside the lib/l10n folder for each locale you need to add support. Name the files in the following way: intl_<LOCALE_ISO_CODE>.arb. For example: intl_fr.arb or intl_fr_FR.arb.

  2. Translate the error messages

    Copy and paste the contents of intl_en.arb into your newly created ARB file. Then translate the error messages by overwriting the default messages.

  3. Generate localization code

    To generate boilerplate code for localization, run the generate command inside the package directory where pubspec.yaml file is located:

    flutter gen-l10n

    The command will automatically create/update files inside the lib/localization directory, including your newly added locale support.

  4. Update README

    Remember to update README, adding the new language (and language code) under Supported languages section in alphabetic order, so that everyone knows your new language is now supported!

  5. Submit PR

    Submit your PR and be of help to millions of developers all over the world!

Add new validator

  1. Add method to validators.dart with your Dart documentation
  2. Implement tests
  3. Add to validators with name and description
  4. Add message error translated on all languages (yes, all languages). To accomplish this need: a. Add property to all intl_*.arb files, in alphabetic order. b. Translate message in all languages. c. Run flutter gen-l10n command
  5. Submit PR

Questions and answers

You can ask questions or search for answers on Github discussion or on StackOverflow


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