<cms:metadata name="mail" type="string">
     <label i18n="true">CONTENT_ARTICLE_AUTHOR_MAIL</label>
     <description i18n="true">CONTENT_ARTICLE_AUTHOR_MAIL_DESC</description>
     <validation>
         <regexp>^(\w+)([\-+.][\w]+)*@(\w[\-\w]*\.){1,5}([A-Za-z0-9][A-Za-z0-9]+)$</regexp>
         <invalidText i18n="true">PLUGINS_WEB_REGEXP_INVALID_MAIL</invalidText>
     </validation>
 </cms:metadata>

<regexp>^.+@.+$</regexp>