Use existing regexp validator

app/models/application_setting.rb

@@ -257,7 +257,7 @@ class ApplicationSetting < ApplicationRecord
 
   validates :snippet_size_limit, numericality: { only_integer: true, greater_than: 0 }
 
-  validate :email_restrictions_regex_valid?
+  validates :email_restrictions, untrusted_regexp: true
 
   SUPPORTED_KEY_TYPES.each do |type|
     validates :"#{type}_key_restriction", presence: true, key_restriction: { type: type }
@@ -411,14 +411,6 @@ class ApplicationSetting < ApplicationRecord
     recaptcha_enabled || login_recaptcha_protection_enabled
   end
 
-  def email_restrictions_regex_valid?
-    return if email_restrictions.blank?
-
-    Gitlab::UntrustedRegexp.new(email_restrictions)
-  rescue RegexpError
-    errors.add(:email_restrictions, _('is not a valid regular expression'))
-  end
-
   private
 
   def parsed_grafana_url

locale/gitlab.pot

@@ -23945,9 +23945,6 @@ msgstr ""
 msgid "is not a valid X509 certificate."
 msgstr ""
 
-msgid "is not a valid regular expression"
-msgstr ""
-
 msgid "is not allowed for sign-up"
 msgstr ""
 

spec/models/application_setting_spec.rb

@@ -715,7 +715,7 @@ describe ApplicationSetting do
         subject.email_restrictions = '+'
 
         expect(subject).not_to be_valid
-        expect(subject.errors.messages[:email_restrictions].first).to eq(_('is not a valid regular expression'))
+        expect(subject.errors.messages[:email_restrictions].first).to eq(_('not valid RE2 syntax: no argument for repetition operator: +'))
       end
     end