def _split_extensions(stac_extensions):
    """Split extensions into common_extensions and custom_extensions"""

    common_extensions = []
    custom_extensions = []
    for ext in stac_extensions:
        # Custom extensions are a URI
        if ext.endswith('.json') or '/' in ext:
            custom_extensions.append(ext)
        else:
            common_extensions.append(ext)

    return (common_extensions, custom_extensions)