Trigger jenkins job when push/commit on master

Question

I have created a webhook in gitlab to trigger jenkins job and i have selected push events. But problem is job is getting trigger even then there is push in feature branch and i want it to trigger only when push in master branch.

I am using generic webhook trigger plugin and used below code in jenkinsfile.

properties ([
    pipelineTriggers([
        [$class: 'GenericTrigger',
            genericVariables: [
                [key: 'ref', value: '$.ref']               
            ],            
            regexpFilterText: 'BRANCH: $ref END',
            regexpFilterExpression: "BRANCH: refs/heads/master END"
        ]
    ])
])

What i am doing wrong here as job is geeting trigger even for feature branches.