I've setup the plugin and I got no output on the validation. Looking at my browser's console I can see the following:

XMLHttpRequest cannot load http://server//descriptor/hudson.plugins.validating_string_paramete…0in%20the%20N.N.N%20format%20(e.g.%20%221.0.0%22%2C%20%221.2.10%22)&value=. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://server:8080' is therefore not allowed access.

Here is my job configuration:

Name: NewVersion
Regular expression: \d+\.\d+\.\d+
Failed Validation Message: The new version must in the N.N.N format (e.g. \"1.0.0\", \"1.2.10\")

Name: ReleaseVersion
Regular expression: \d+\.\d+\.\d+-SNAPSHOT
Failed Validation Message: The release version must in the N.N.N-SNAPSHOT format (e.g. \"1.0.1-SNAPSHOT\")