    This is __strong__ emphasis

    This is also **strong** emphasis

    This is __multiple words__ strong emphasis

    This is also **multiple words** strong emphasis

    This is __ not valid __

    **strong** emphasis could be the first word in the line

    __strong__ emphasis could be the first word in the line

    Strong **emphasis
    could span** multiple lines

    Strong __emphasis
    could span__ multiple lines

    Strong __emphasis `could` contain__ inline code

    Strong __emphasis *could* contain__ weak emphasis

    The following is not a strong emphasis __
    that span multiple lines
    __ because the start delimiter must be followed by something (not the end of line)

    The following is not a strong emphasis **
    that span multiple lines
    ** because the start delimiter must be followed by something (not the end of line)

    The usage of alternate delimiters must be idempotent, so all __foo__ **foo** __**foo**__ **__foo__** must be "foo" highlighted as bold

    The same delimiter could not be used twice ****foo*** and ____foo____ must be normal

    __foo____bar__ must be "foo" highlighted as bold followed by "{UNDERSCORE}{UNDERSCORE}bar{UNDERSCORE}{UNDERSCORE}" highlighted as normal

    __foo__**bar** must be "foo" highlighted as bold followed by "bar" highlighted as bold

    __foo__b must be "foo" highlighted as bold followed by "b" highlighted as normal

    __foo__bar__baz__ must be "foo" highlighted as bold followed by "bar{UNDERSCORE}baz{UNDERSCORE}" highlighted as normal

    __foo__bar __baz__ must be "foo" and "baz" highlighted as bold and "bar" highlighted as normal

    Four underscore ____ or four asterisks **** must not be highlighted without something in between

    Spaces do not count for something so __ __ and ** ** must be normal

    FOO__BAR must not be normal

    FOO__BAR and BAR__FOO must be normal

    Unfortunately __FOO__BAR doesn't work but escaping \__FOO__BAR is always possible