def bed_read_reg_str_into_dic(in_bed):
    """
    Read BED rows into dictionary of region strings.
    mapping: 'chr1,10,20,+' -> [reg_id].

    >>> test_bed = "test_data/test3.bed"
    >>> bed_read_reg_str_into_dic(test_bed)
    {'chr1,10,20,+': ['CLIP1'], 'chr1,30,45,-': ['CLIP2']}

    """
    reg_str_dic = {}
    with open(in_bed) as f:
        for line in f:
            cols = line.strip().split("\t")
            chr_id = cols[0]
            site_s = cols[1]
            site_e = cols[2]
            site_id = cols[3]
            site_pol = cols[5]
            reg_str = "%s,%s,%s,%s" %(chr_id, site_s, site_e, site_pol)
            if reg_str in reg_str_dic:
                reg_str_dic[reg_str].append(site_id)
            else:
                reg_str_dic[reg_str] = [site_id]
    assert reg_str_dic, "reg_str_dic empty"
    return reg_str_dic