Validate a URL, without requiring it, using a Regular Expression in Flutter

Question

I'm creating a form in Flutter and want the following functionality:

If the user enters a URL, I want to ensure it is valid with a regular expression.
If the user leaves the field blank, I do not want to return an error message.

The following Reg Exp validator performs this way when I perform these four steps:

If I hot reload and enter a valid URL, it accepts it.
If I change the input field to a new non-valid URL, it still accepts it.
If I hot reload and enter an invalid URL, it does not accept it.
If I change the URL to a valid URL, it still does not accept it.

It's as if it only runs the validator once and then any subsequent entry is not checked again. I do have a field that uses value.isEmpty as a validator and it does work as expected by checking the input each time I click my button with _formKey.currentState.save();

child: TextFormField(
              decoration: InputDecoration(
                border: OutlineInputBorder(),
                labelText: 'Related URL',
              ),
              textInputAction: TextInputAction.next,
              keyboardType: TextInputType.url,
              focusNode: _relatedUrlFocusNode,
              onFieldSubmitted: (_) {
                FocusScope.of(context).requestFocus(_notesFocusNode);
              },
              //this doesn't work
              validator: (String value) {
                if (RegExp(r"[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?")
                        .hasMatch(value)) {
                  return 'Please enter a valid URL';
                }
                return null;
              },
              onSaved: (String value) {
                relatedUrl = value;
              },
            ),

...other fields...

ElevatedButton(
            child: Text('ADD & ACTIVATE'),
            onPressed: () {
              if (!_formKey.currentState.validate()) {
                return;
              }
              ScaffoldMessenger.of(context)
                  .showSnackBar(SnackBar(content: Text('Processing Data')));
              _formKey.currentState.save();
            },
          )

How do I ensure it performs the validator each time I click submit?

Thierry · Accepted Answer · 2021-02-16 12:11:54Z

You have two problems with your code:

Your check should be if (!RegExp(r"...").hasMatch(value)) {} instead of if (RegExp(r"...").hasMatch(value)) {}
Your Regular Expression is not correct. You may check here. (Dart RegExp are the same as Javascript RegExp)

Solution with validators package

Instead of a RegExp, you can use the validators package.

Full source code for easy copy-paste:

import 'package:flutter/material.dart';
import 'package:flutter_hooks/flutter_hooks.dart';
import 'package:validators/validators.dart';

void main() {
  runApp(
    MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'URL Validation Demo',
      home: HomePage(),
    ),
  );
}

class HomePage extends HookWidget {
  @override
  Widget build(BuildContext context) {
    final _formKey = useState(GlobalKey<FormState>());
    final _focusNode = useFocusNode();
    return Scaffold(
      body: Container(
        padding: EdgeInsets.all(8.0),
        alignment: Alignment.center,
        child: Form(
          key: _formKey.value,
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              TextFormField(
                decoration: InputDecoration(
                  border: OutlineInputBorder(),
                  labelText: 'URL',
                ),
                validator: (value) {
                  if (!isURL(value)) {
                    return 'Please enter a valid URL';
                  }
                  return null;
                },
              ),
              const SizedBox(height: 24.0),
              ElevatedButton(
                child: Text('VALIDATE'),
                onPressed: () {
                  if (_formKey.value.currentState.validate()) {
                    ScaffoldMessenger.of(context).showSnackBar(
                        SnackBar(content: Text('Processing Data')));
                  }
                },
              ),
            ],
          ),
        ),
      ),
    );
  }
}

Thanks Thierry! Good catch on the ! (not). I was playing around trying to fix it before I asked the question and had taken it out to inverse the logic (hence I was able to get an initial expected validation).

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Validate a URL, without requiring it, using a Regular Expression in Flutter

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Solution with validators package

Full source code for easy copy-paste:

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Solution with validators package

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