I had a similar issue, but when I installed the plugin and tried to use it, it would always show the text "ERROR" below it but I think the regex match work.
This made the plugin unusable since people wouldn't know when/if they were typing in an error.
However, I was able to fix it by doing a fresh install of jenkins through a build after downloading version 2.3 github repo.(NOTE version 2.4 doesn't work)

wget https://github.com/jenkinsci/validating-string-parameter-plugin/archive/validating-string-parameter-2.3.tar.gz
#install maven
mvn -X package
#Uninstall the validating-string-parameter plugin + deleted the directory /var/lib/jenkins/plugins/validating-string-parameter
#Manually uploaded the 
validating-string-parameter.hpi 

Pretty crazy workaround since I literally used the same code as in the plugin downloaded from jenkins but it seems the custom build and upload worked for some reason. I didn't see anything different in the jelly conf or java files so it's strange but this works. Hope this helps someone out since this parameter plugin is pretty essential.

I'm still having the same problem now after the latest jenkins update.
I was able to pinpoint what was causing the "ERROR" message, it's the index.jelly file under

src/main/resources/hudson/plugins/validating_string_parameter/ValidatingStringParameterDefinition/index.jelly

<div name="parameter" description="${it.formattedDescription}">
                        <input type="hidden" name="name" value="${it.name}" />
                        <f:textbox name="value" value="${it.defaultValue}"
                                *checkUrl="'${it.rootUrl}/descriptor/hudson.plugins.validating_string_parameter.ValidatingStringParameterDefinition/validate?regex='+encodeURIComponent(&quot;${it.jsEncodedRegex}&quot;)+'&amp;failedValidationMessage='+encodeURIComponent(&quot;${it.failedValidationMessage}&quot;)+'&amp;value='+encodeURIComponent(this.value)"*/>
                </div>

There's something wrong with the checkUrl descriptor that errors out..wish it was easier to read so I can debug it. Removing it removes the ERROR, but probably removes the regex validating.

If anyone knows how to fix it, please let me know.

Awesome I figured it out!
Modified the code to use the resURL instead of the rootURL
rootURL - the Jenkins instance
resURL - static webapp resources such as JavaScript or HTML (cf. Jenkins.RESOURCE_PATH)

Not sure why resURL works while rootURL doesn't but it might be that rootURL doesn't have a proper context path in our setup or something so it might not be pointing to localhost:8080.

Changed checkUrl below, rebuild plugin in maven, installed manually and it worked.

checkUrl="'${it.resURL}/descriptor/hudson.plugins.validating_string_parameter.ValidatingStringParameterDefinition/validate?regex='+encodeURIComponent("${it.jsEncodedRegex}")+'&failedValidationMessage='+encodeURIComponent("${it.failedValidationMessage}")+'&value='+encodeURIComponent(this.value)"

slide_o_mix I see with below commit  you removed the code that lhschema mentions, what was the reason for removing, Leo's solution not work?

I still get this error when using this plugin in a Jenkins pipeline

https://github.com/jenkinsci/validating-string-parameter-plugin/commit/9caac7b713976af9363cd944f89cadd44fbb4652#diff-12cb1c400787ab164b54d0ef8d0cbda9