x <- "https://a...content-available-to-author-only...m.ar/MLA-723950998-peugeot-208-0km-16-active-plan-100-financiado-darc-_JM"
gsub('.*/(M\\w+)-(\\d+).*', '\\1\\2', x)
eCA8LSAiaHR0cHM6Ly9hLi4uY29udGVudC1hdmFpbGFibGUtdG8tYXV0aG9yLW9ubHkuLi5tLmFyL01MQS03MjM5NTA5OTgtcGV1Z2VvdC0yMDgtMGttLTE2LWFjdGl2ZS1wbGFuLTEwMC1maW5hbmNpYWRvLWRhcmMtX0pNIgpnc3ViKCcuKi8oTVxcdyspLShcXGQrKS4qJywgJ1xcMVxcMicsIHgp