A way using negative lookbehind as been shown. Others are: 
/(?:^.{0,4}|[^e].{4}|[^c].{3}|[^h].{2}|[^o].|[^ ])done/is
/(.....)done(??{$1 eq "echo " ? "(*FAIL)" : ""})|^.{0,4}done/is # Repl
+ace (*FAIL) with (?!) pre-5.10

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Of course, if you know you only need to look at DONE at the end, you can use: 
!/echo done$/i && /done$/i

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In reply to Re: regex help needed by JavaFan
in thread regex help needed by Anonymous Monk

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