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'll explore five common implementation methods with code snippets, examine their pitfalls, and see how Abstract API provides a more robust solution.

How to Implement Phone Number Validation in Flutter

Here are common methods to validate phone numbers in a Flutter application. Each approach offers a different way to check user input, from simple patterns to comprehensive library-based checks.

Regex-Only Validation

This strategy uses a country-specific regular expression inside the validator of a `TextFormField` for an offline, synchronous check. For production use, you can maintain a map of patterns keyed by ISO country code and select the appropriate one via locale detection or a country picker.

The code shows an example for US numbers. It defines a pattern for the North American Numbering Plan. The validator checks if the input, after non-digits are removed, matches the pattern and returns an error message on failure.

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 libphonenumber

This method uses a package like `flutter_libphonenumber`. It is a Dart binding for Google’s `libphonenumber` C++ library, compiled for Android and iOS. The library exposes functions to parse, detect types, and format numbers.

The setup requires an initialization step to load the necessary metadata for the library to function correctly.

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 a specific region ISO code.

A successful parse returns a map, and the code checks if a `type` key exists to confirm validity. The parse function throws an error on failure, which is caught to return false.

The package also offers a `LibPhonenumberTextFormatter` for text fields. You can find more information 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

These common validation methods introduce significant maintenance and user experience hurdles. They can cause validation failures, poor app performance, and a frustrating development cycle for your team.

• National numbering plans change often. A regex pattern or libphonenumber package with stale data will incorrectly reject valid numbers after a country updates its system, like when Gabon added a digit.
• The libphonenumber package uses a frozen data snapshot. This means complex rules, like mobile prefixes in Argentina and Mexico, remain incorrect or ambiguous until you release a new app update.
• Native plugin dependencies for libphonenumber create fragility. A simple Flutter upgrade can break the build or cause the validator to fail every input due to compatibility issues like a MissingPluginException.
• Real-time validation in a TextFormField causes performance problems. Each keystroke triggers a rebuild, which leads to UI flicker, cursor jumps, and a poor user experience without careful debouncing logic.