JENKINS-6158 did its job, and still works, and there is no regression, in the sense that the build process does indeed NOT start if the validation via regex fails.
The reason why
JENKINS-23944 or Jenkins-23951 are much needed features is because before the build starts, the Jenkins Job may be set to do various things: e.g. inject variables, fetch a git clone from a remote repository, prepare an environment for build, run a groovy script, ...
All of the above mentioned things will lead to a large amount of wasted time (and consequently resources) which get allocated just to end with an obvious message of "Build Failed" due to invalid input parameters.
Instead it would be much better if this plug-in prevented any user or an upstream job from triggering the build at all.