def filterit(s):
    """
    A functoin to filter out unnecessary characters from a sentece, helper for nBOW
    Args: 
        s = Sentence
    returns: filtered Sentence
    """
    s=s.lower()
    S=''
    for c in s:
        if c in ' abcdefghijklmnopqrstuvwxyz0123456789':
            if c.isdigit():
                c=' '+c+' '
            S+=c
    return S