It's invalid because there's no space before -->
That's bogus. There's no need for space to be there. Nor does there have to be space as the first character following a COM sequence (COM being --).

OTOH, your pattern falsely considers <!-- -- --> to be a valid comment, while it doesn't consider <!-- <!-- --> --> to be valid.

This matches HTML comments: 

  <!(?:--(?:[^-]*(?:-[^-]+)*)--\s*)*>

[download]
although if you are truely pedantic, you'd replace the \s with the set of characters the HTML DTD defines as white space characters.

In reply to Re^2: HTML stripper... by JavaFan
in thread HTML stripper... by Anonymous Monk

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