def overplot_ds9reg(filename, ax):
    """Overplot a ds9 region file.

    Parameters
    ----------
    filename : str
        File name of the ds9 region file.
    ax : matplotlib axes instance
        Matplotlib axes instance.

    """
    with open(filename) as f:
        file_content = f.read().splitlines()
    first_line = file_content[0]
    if '# Region file format: DS9' not in first_line:
        raise ValueError('Unrecognized ds9 region file format')
    for line in file_content:
        if line[0:4] == 'line':
            line_fields = line.split()
            x1 = float(line_fields[1])
            y1 = float(line_fields[2])
            x2 = float(line_fields[3])
            y2 = float(line_fields[4])
            if 'color' in line:
                i = line.find('color=')
                color = line[i + 6:i + 13]
            else:
                color = 'green'
            ax.plot([x1, x2], [y1, y2], '-', color=color)
        elif line[0:4] == 'text':
            line_fields = line.split()
            x0 = float(line_fields[1])
            y0 = float(line_fields[2])
            text = line_fields[3][1:-1]
            if 'color' in line:
                i = line.find('color=')
                color = line[i + 6:i + 13]
            else:
                color = 'green'
            ax.text(x0, y0, text, fontsize=8, bbox=dict(boxstyle=
                'round,pad=0.1', fc='white', ec='grey'), color=color,
                fontweight='bold', backgroundcolor='white', ha='center')
        else:
            pass
