def scan(self, data, dlen=None):
        '''
        Scan a data block for matching signatures.

        @data - A string of data to scan.
        @dlen - If specified, signatures at offsets larger than dlen will be ignored.

        Returns a list of SignatureResult objects.
        '''
        results = []
        matched_offsets = set()

        # Since data can potentially be quite a large string, make it available to other
        # methods via a class attribute so that it doesn't need to be passed around to
        # different methods over and over again.
        self.data = data

        # If dlen wasn't specified, search all of self.data
        if dlen is None:
            dlen = len(data)

        for signature in self.signatures:
            # Use regex to search the data block for potential signature
            # matches (fast)
            for match in signature.regex.finditer(data):
                # Take the offset of the start of the signature into account
                offset = match.start() - signature.offset

                # Signatures are ordered based on the length of their magic bytes (largest first).
                # If this offset has already been matched to a previous signature, ignore it unless
                # self.show_invalid has been specified. Also ignore obviously invalid offsets (<0)
                # as well as those outside the specified self.data range (dlen).
                if (offset not in matched_offsets or self.show_invalid) and offset >= 0 and offset < dlen:
                # if offset >= 0 and offset < dlen:
                    # Analyze the data at this offset using the current
                    # signature rule
                    tags = self._analyze(signature, offset)

                    # Generate a SignatureResult object and append it to the results list if the
                    # signature is valid, or if invalid results were requested.
                    if (not tags['invalid'] or self.show_invalid) and not self._filtered(tags['description']):
                        # Only display results with the 'once' tag once.
                        if tags['once']:
                            if signature.title in self.display_once:
                                continue
                            else:
                                self.display_once.add(signature.title)

                        # Append the result to the results list
                        results.append(SignatureResult(**tags))

                        # Add this offset to the matched_offsets set, so that it can be ignored by
                        # subsequent loops.
                        matched_offsets.add(offset)

        # Sort results by offset
        results.sort(key=lambda x: x.offset, reverse=False)

        return results