formrules

A comprehensive, zero-dependency, fail-fast, fluent validation library for Flutter TextFormField.

Why formrules?

1. Zero External Dependencies: Pure Dart. No intl, no bloat.
2. Fail-Fast: Stops checking the moment a rule fails, returning the exact error message.
3. Fluent Chainable API: Beautiful readable builder syntax.
4. Graceful Optionals: By design, all rules (except required) pass silently if the input is null or empty, making optional fields a breeze.
5. Highly Extensible: Easily add inline closures or full custom rule classes.

Screenshots

Rule Showcase Gallery

Comprehensive Registration Form Demo

Getting Started

Add the package to your pubspec.yaml:

dependencies:
  formrules: ^1.0.0

Basic Usage

Simply use FormRules.<rule>()...build() directly inside the validator parameter of a TextFormField.

TextFormField(
  decoration: const InputDecoration(labelText: 'Email Address'),
  validator: FormRules.required(message: 'Email is required')
      .email(message: 'Please enter a valid email address')
      .build(),
),

Note: The .build() call at the end of the chain is required. It converts the fluent builder into the String? Function(String?) closure expected by Flutter.

All 31 Built-in Rules

Rule	Description
required	Ensures the field is not null or empty.
email	Validates an email address.
minLength(n)	Validates a minimum length.
maxLength(n)	Validates a maximum length.
url	Validates a URL.
phone	Validates a phone number.
numeric	Validates that the input is numeric.
regex(pattern)	Validates against a custom regex pattern.
match(getValue)	Validates that the input matches another field.
noSpecialChars	Ensures there are no special characters.
alpha	Ensures the value contains only alphabetic characters.
alphaNumeric	Ensures the value contains only alphanumeric characters.
lowercase	Ensures the value is strictly lowercase.
uppercase	Ensures the value is strictly uppercase.
inList(values)	Ensures the value matches one of the items in the list.
notInList(values)	Ensures the value does NOT match any item in the list.
equals(value)	Ensures the value exactly equals a hardcoded string.
notEquals(value)	Ensures the value does NOT equal a hardcoded string.
contains(substring)	Ensures the value contains a specific substring.
startsWith(prefix)	Ensures the value starts with a specific prefix.
endsWith(suffix)	Ensures the value ends with a specific suffix.
ipAddress	Validates standard IPv4 and/or IPv6 addresses.
macAddress	Validates a standard MAC address.
creditCard	Validates credit card numbers (Luhn algorithm).
date	Validates that the input is a valid date (ISO-8601).
uuid	Validates a standard UUID.
hexColor	Validates a standard hex color string.
base64	Validates that the string is properly Base64 encoded.
json	Validates that the string is valid JSON.
slug	Validates a URL-friendly slug.
password	An all-in-one strong password validator rule.
custom(fn)	Add a custom inline validator.
addRule(rule)	Add a custom rule class.

Advanced Customization

Inline Custom Validation

If you need a quick custom check, use .custom():

TextFormField(
  validator: FormRules.required()
      .custom((value) {
        if (value != 'secret') return 'Wrong secret word!';
        return null; // pass
      })
      .build(),
)

Custom Rule Classes

For reusable custom logic, extend ValidationRule and add it via .addRule():

class AsyncLookaheadRule extends ValidationRule {
  @override
  String? validate(String? value) {
    if (value == null || value.isEmpty) return null;
    if (!value.startsWith('A')) return 'Must start with A';
    return null;
  }
}

// Usage:
TextFormField(
  validator: FormRules.required().addRule(AsyncLookaheadRule()).build(),
)

Global Localization

You can easily provide your own error messages globally for the entire app by overriding the default messages class:

class CustomMessages extends FormRulesMessagesEn {
  @override
  String get required => 'Este campo es obligatorio.';
  
  @override
  String get email => 'Correo electrónico inválido.';
}

void main() {
  FormRules.setMessages(CustomMessages());
  runApp(const MyApp());
}

Contributing

See our CONTRIBUTING.md for details on how to add new rules to this package. We strictly maintain 100% test coverage and zero dependencies.

License

This project is licensed under the MIT License - see the LICENSE file for details.