I'm not sure I entirely understand your requirements, but /t(?!est\b)\w*t/ would match any 't' not followed by 'est' and a word break then match 0 or more word characters and then match one last 't'. For example (anchors added): 

#!/usr/bin/perl -w
use strict;

/^t(?!est\b)\w*t$/ and print while <DATA>;

__DATA__
test
testset
tot
tesset
tt

[download]
prints everything but 'test'.

Adapting it slightly for your original problem like this 

#!/usr/bin/perl -lw
use strict;

$_ = "thought test tot 1 2 3 tesset";

s/t(?!est\b)\w*t\s*//g;

print;

[download]
prints "test 1 2 3" just as you want. 

-sauoq
"My two cents aren't worth a dime.";

In reply to Re: A regex that does this, but not that? by sauoq
in thread A regex that does this, but not that? by bradcathey

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