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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def extract_pdbid(string):
 """Use regular expressions to get a PDB ID from a string"""

    p = re.compile("[0-9][0-9a-z]{3}")
    m = p.search(string.lower())
    try:
        return m.group()
    except AttributeError:
        return "UnknownProtein"