def _sequence(self, i):
        """Handle character group."""

        result = ['[']
        end_range = 0
        escape_hyphen = -1
        removed = False
        last_posix = False

        c = next(i)
        if c in ('!', '^'):
            # Handle negate char
            result.append('^')
            c = next(i)
        if c == '[':
            last_posix = self._handle_posix(i, result, 0)
            if not last_posix:
                result.append(re.escape(c))
            c = next(i)
        elif c in ('-', ']'):
            result.append(re.escape(c))
            c = next(i)

        while c != ']':
            if c == '-':
                if last_posix:
                    result.append('\\' + c)
                    last_posix = False
                elif i.index - 1 > escape_hyphen:
                    # Found a range delimiter.
                    # Mark the next two characters as needing to be escaped if hyphens.
                    # The next character would be the end char range (s-e),
                    # and the one after that would be the potential start char range
                    # of a new range (s-es-e), so neither can be legitimate range delimiters.
                    result.append(c)
                    escape_hyphen = i.index + 1
                    end_range = i.index
                elif end_range and i.index - 1 >= end_range:
                    if self._sequence_range_check(result, '\\' + c):
                        removed = True
                    end_range = 0
                else:
                    result.append('\\' + c)
                c = next(i)
                continue
            last_posix = False

            if c == '[':
                last_posix = self._handle_posix(i, result, end_range)
                if last_posix:
                    c = next(i)
                    continue

            if c == '\\':
                # Handle escapes
                subindex = i.index
                try:
                    value = self._references(i, True)
                except PathNameException:
                    raise StopIteration
                except StopIteration:
                    i.rewind(i.index - subindex)
                    value = r'\\'
            elif c == '/':
                if self.pathname:
                    raise StopIteration
                value = c
            elif c in SET_OPERATORS:
                # Escape &, |, and ~ to avoid &&, ||, and ~~
                value = '\\' + c
            else:
                # Anything else
                value = c

            if end_range and i.index - 1 >= end_range:
                if self._sequence_range_check(result, value):
                    removed = True
                end_range = 0
            else:
                result.append(value)

            c = next(i)

        result.append(']')
        # Bad range removed.
        if removed:
            value = "".join(result)
            if value == '[]':
                # We specified some ranges, but they are all
                # out of reach.  Create an impossible sequence to match.
                result = ['[^%s]' % ('\x00-\xff' if self.is_bytes else uniprops.UNICODE_RANGE)]
            elif value == '[^]':
                # We specified some range, but hey are all
                # out of reach. Since this is exclusive
                # that means we can match *anything*.
                result = ['[%s]' % ('\x00-\xff' if self.is_bytes else uniprops.UNICODE_RANGE)]
            else:
                result = [value]

        if self.pathname or self.after_start:
            return self._restrict_sequence() + ''.join(result)

        return ''.join(result)