Parse gitlab merge request ID from git commit message in Jenkins pipeline

gistfile1.txt

def getMergeRequestId() {
  sh "git show -s --format='%B' > commitmsg"
  msg = readFile 'commitmsg'; msg = msg.trim()
  def mr_id_match = msg =~ /!(\d+)/
  if ( mr_id_match ) {
    mr_id = mr_id_match[0][1]
    echo "Found merge request ID $mr_id"
  } else { mr_id = null }
  mr_id_match = null // have to null this or we'll get NotSerializable errors from Jenkins
  env.mergeRequestId = mr_id // set this so it can be used in the shell
  return mr_id
}