If my regex is:

^URL: (\S)+( .*)?$

And my replacement text is:

<a href="\1">My URL\2</a>

It is possible that the regex can succeed and the matcher.group(2) will be null. If that happens the description setter fails like this:

ERROR: Publisher hudson.plugins.descriptionsetter.DescriptionSetterPublisher aborted due to exception
java.lang.NullPointerException
at java.lang.String.replace(String.java:2208)
at hudson.plugins.descriptionsetter.DescriptionSetterPublisher.getExpandedDescription(DescriptionSetterPublisher.java:158)
at hudson.plugins.descriptionsetter.DescriptionSetterPublisher.perform(DescriptionSetterPublisher.java:80)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:644)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:623)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:601)
at hudson.model.Build$RunnerImpl.post2(Build.java:159)
at hudson.model.AbstractBuild$AbstractRunner.post(AbstractBuild.java:570)
at hudson.model.Run.run(Run.java:1386)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:145)

This can probably be fixed by changing:

result = result.replace("
" + i, matcher.group)

to

result = result.replace("
" + i, matcher.group == null ? '' : matcher.group)

But I haven't actually checked it.