Laravel Validation Error - format is invalid

The following rules are used to validate an item code before it can be persisted to the database.

• code must have five digits
• must start with 5
• must be unique within the codes table
• The first three characters of a code must be the same as the first three characters of the parent code

For validation I am using the following form request rules

class StoreItemCodeRequest extends FormRequest
{

    public function authorize()
    {
        return true;
    }

    public function rules()
    {

        Validator::extend('in_category', function ($attribute, $value, $parameters, $validator){
            $category = $this->get('parent_code');
            return substr($value, 0, 3) === substr($category, 0, 3);
        });

       return [
            'code' => 'required|http://regex:/^5(\d{4})$/|in_category|unique:exp_codes,code,' . $this->get('id'),
            'name'    => 'required|max:50|unique:exp_codes,name,' . $this->get('id'),
            'parent_code' => 'exists:exp_categories,code'
      ];
}

}

Inserting a new code works without problems.

However, trying to update an existing code displays The code format is invalid. error against the code input field.

I am using Laravel 5.3 and would appreciate if someone can point the cause of the error that is preventing updating.

via Chebli Mohamed