5 Ways to Validate Phone Numbers in Flutter

Validating phone numbers in a Flutter app is a key step for ensuring data quality and successful user communication. We explore two common approaches, complete with code snippets, look at their pitfalls, and see how Abstract API provides a more robust solution.

How to Implement Phone Number Validation in Flutter

Explore different methods to validate phone numbers within your Flutter application. Each approach offers a distinct way to check user input, from simple pattern matches to more sophisticated library-based checks.

Validation with Regular Expressions

This strategy uses a regular expression, or "regex", to check the phone number's format. The check happens directly inside the validator of a "TextFormField" or "Form" widget. It is a quick, offline way to perform a basic sanity filter on the input.

For production applications, you would maintain a map of different regex patterns. Each pattern would correspond to a specific country, identified by its ISO code. The app would then select the correct pattern based on the user's locale or a country picker.

The provided code demonstrates this for United States numbers. It defines a regex pattern for the North American Numbering Plan. The "validator" function first removes all non-digit characters and then checks if the remaining string matches the pattern.

final _usPattern = RegExp(r'^(?:\+1)?[2-9]\d{2}[2-9](?:\d{6})$');

TextFormField(
  keyboardType: TextInputType.phone,
  validator: (v) {
    if (v == null || !_usPattern.hasMatch(v.replaceAll(RegExp(r'\D'), ''))) {
      return 'Invalid phone number';
    }
    return null;
  },
);

In-App Validation with a Library

This method uses a Flutter package like "flutter_libphonenumber" or "phone_number". These packages are Dart wrappers for Google’s powerful "libphonenumber" C++ library, compiled for Android and iOS. The library handles complex parsing, format validation, and even detects the phone number type.

Setup requires an initialization step to load the necessary metadata for different regions. This is usually done once when the app starts. The code shows how to initialize the library and then validate a number.

The "validate" function attempts to parse the raw phone number string with its ISO country code. A successful parse that returns a valid type confirms the number's format is correct. The package also offers other features, like a text formatter for input fields. You can find more details on pub.dev.

import 'package:flutter_libphonenumber/flutter_libphonenumber.dart';

Future<void> initPhone() async {
  await FlutterLibphonenumber().init();     // loads metadata
}

Future<bool> validate(String raw, String iso) async {
  try {
    final parsed = await FlutterLibphonenumber()
        .parse(raw, region: iso);           // throws on failure
    return parsed['type'] != null;          // extra checks possible
  } catch (_) {
    return false;
  }
}

Challenges of Phone Number Validation in Flutter

While regex and library-based validation seem straightforward, they introduce significant maintenance and reliability issues. These methods often fail to account for the dynamic nature of global phone number systems.

• Numbering plans change without notice. A country might add digits to all its numbers overnight. Your regex patterns or library metadata become instantly obsolete, which causes the validator to reject valid new numbers.
• Library-based methods use a fixed snapshot of phone number data. Ambiguous rules, like mobile prefixes in Argentina or Mexico, remain incorrect until you ship a new app version with an updated package.
• The library approach depends on native plugins. A simple Flutter SDK upgrade can cause build failures or runtime exceptions. This may force the validator to incorrectly return false for every phone number input.
• Real-time validation inside a TextFormField can degrade the user experience. Without careful implementation, every keystroke triggers a rebuild. This causes the input field to flicker and the cursor to jump unexpectedly.