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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 pattern(token):
 """ Convert an hexadecimal search pattern into a POSIX regular expression. For example, the following pattern:: "B8 0? ?0 ?? ??" Would match the following data:: "B8 0D F0 AD BA" # mov eax, 0xBAADF00D @type token: str @param token: String to parse. @rtype: str @return: Parsed string value. """

        token = ''.join([ c for c in token if c == '?' or c.isalnum() ])
        if len(token) % 2 != 0:
            raise ValueError("Missing characters in hex data")
        regexp = ''
        for i in compat.xrange(0, len(token), 2):
            x = token[i:i+2]
            if x == '??':
                regexp += '.'
            elif x[0] == '?':
                f = '\\x%%.1x%s' % x[1]
                x = ''.join([ f % c for c in compat.xrange(0, 0x10) ])
                regexp = '%s[%s]' % (regexp, x)
            elif x[1] == '?':
                f = '\\x%s%%.1x' % x[0]
                x = ''.join([ f % c for c in compat.xrange(0, 0x10) ])
                regexp = '%s[%s]' % (regexp, x)
            else:
                regexp = '%s\\x%s' % (regexp, x)
        return regexp