I might reasonably expect that regexp-group(4) would get hold of the fourth
  match string? 

I think it would be  unreasonable to expect that.
regexp-group (4) is the fourth parameterised group, ie what's known as
$4 or \4 in regexp replace languages of old.

  This isn't true if there is a mismatch in between?
  Yet is true if I get sequential matches?

Can't guess what you mean here:-)

David



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