Files/Validate Plugin

This plugin will become part of EPrints 3.1, possibly even a 3.0.x

It allows you to specify validation errors in the workflow.

 <component type="Validate">
   <epc:if test="!is_set(creators) and !is_set(editors)">
      <error><span class="ep_problem_field:creators">Creators</span> or <span class="ep_problem_field:editors">Editors</span> must be set.</error>
   <epc:if>
   <epc:if test="length(subjects) gt 5">
      <error>You have selected <epc:print expr="length(subjects)" /> <span class="ep_problem_field:subjects">subjects</span>, but there is a maximum of 5 allowed.</error>
   <epc:if>
 </component>

Don't put the if's inside the (error) blocks as they will still cause a validation failure even if they contain no text.