OK so that's \bt[^t]+t\b -- word-boundary, then a t, then one or more other characters not a t, then a t, then a word boundary.

Apart from the abbreviation "tt" this should be fine.

So "tent", "tesseract", "tot", "tort" and "test" itself will match this pattern.

However, "testament" will fail it because of the "t" in the middle.

Then you need a special case for "test" itself, which you can do with the /e modifier and the ternary operator, as in pg's example above.

So something like this:

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

my $words='test Buffy testament Anya tot Willow tesseract
 Faith tent';
$words =~ s/\b(t[^t]+t)\b/$1 eq "test" ? $1  : ''/ge; 
print $words;
# prints 'test Buffy testament Anya  Willow  Faith';
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Where the regex means "Find words matching t, something-not-t, then t at the end. Replace them with nothing, unless they're the word test, in which case, replace them with themselves".

You could replace the ternary thing with this more longwinded version if you liked:

$words =~ s/\b(t[^t]+t)\b/
           my $temp = $1;
           if($temp eq 'test'){
             $temp
           }else{
             ''
           }/xge;
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