Return the corenlp version pointed at by CORENLP_HOME, or None

def _get_corenlp_version():
    "Return the corenlp version pointed at by CORENLP_HOME, or None"
    corenlp_home = os.environ.get("CORENLP_HOME")
    if corenlp_home:
        for fn in os.listdir(corenlp_home):
            m = re.match("stanford-corenlp-([\d.]+)-models.jar", fn)
            if m:
                return m.group(1)