    This is _weak_ emphasis

    This is also *weak* emphasis

    This is _multiple words_ weak emphasis

    This is also *multiple words* weak emphasis

    This is _ not valid _ because the start delimiter must be followed by something (not a space)

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

    _weak_ emphasis could be the first word in the line

    Weak *emphasis
    could span* multiple lines

    Weak _emphasis
    could span_ multiple lines

    Weak _emphasis `could` contain_ inline code

    Weak _emphasis **could** contain_ strong emphasis

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

    The following is not a weak 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 italic

    The same delimiter could not be used twice **foo** and __foo__ must not be highlighted as italic

    _foo__bar_ must be "foo" highlighted as italic followed by "{UNDERSCORE}bar{UNDERSCORE}" highlighted as normal

    _foo_*bar* must be "foo" highlighted as italic followed by "bar" highlighted as italic

    _foo_b must be "foo" highlighted as italic followed by "b" highlighted as normal

    _foo_bar_baz_ must be "foo" highlighted as italic followed by "bar{UNDERSCORE}baz{UNDERSCORE}" highlighted as normal

    _foo_bar _baz_ must be "foo" and "baz" highlighted as italic and "bar" highlighted as normal

    Two underscores __ or two asterisks ** must not be highlighted without something between them

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

    FOO_BAR must be normal

    FOO_BAR and BAR_FOO must be normal

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