The pom.xml file provided for versions 5.0 and 5.1 is incorrect. It specifies the GAV parameters for the regexp library directly within the <dependencies></dependencies> tags, but the GAV tags should be one layer deeper nexted within the <dependency></dependency> tags. The Bugzilla system correctly guessed my OS and platform, but this will produce at least a warning for Maven2 builds on any platform.
Code no longer uses regexp library