(?<!foo) Negative Lookbehind Asserts that what immediately precedes the current position in the string is not foo

update

maybe try s/(?<!\bisn)(?<!\baren)'t/ 't/g

\b ensures a word boundary.

(actually in order to play save you should also check for a trailing boundary 't\b

Negative look-behind only works for fixed length patterns, "aren" and "isn" are of different lengths.

(?<!n)'t
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might work, but if you need to exclude both, you can't use

(?<!aren|isn)'t

Variable length lookbehind not implemented in regex m/(?<!aren|isn)'t/
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You can specify them one by one, though:

(?<!aren)(?<!isn)'t
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($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
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Tsk, you replied to the king of updates without checking ?

;-p

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}

Looks good :-)

(Note: Link requires a fairly modern browser)

WOW. It works perfectly.

Perhaps a bit more fancy than necessary, but a lot is factored out. Needs Perl version 5.10+ for  (*SKIP) (*FAIL) (see Special Backtracking Control Verbs in perlre):

c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 my $text = q{x aren't y ain't z isn't w won't u 't v daren't t};
 print qq{'$text'};
 ;;
 my $sq_t = qr{ 't }xms;
 my $skip_sq_t = qr{ aren | isn }xms;
 ;;
 my $exceptions_sq_t = qr{ \b $skip_sq_t $sq_t (*SKIP) (*FAIL) }xms;
 ;;
 $text =~ s{ $exceptions_sq_t? (?<= [[:alpha:]]) (?= $sq_t) }{ }xmsg;
 print qq{'$text'};
"
'x aren't y ain't z isn't w won't u 't v daren't t'
'x aren't y ain 't z isn't w won 't u 't v daren 't t'
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Update: Another, more highly factored approach. See haukex's article Building Regex Alternations Dynamically for more info on how the  $exception_n_sq_t regex is built. Something like this could be used to manufacture countless target/exception regex pairs.

c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 my $sq_t = qr{ 't }xms;
 ;;
 my $target_n_sq_t = qr{ (?<= [[:alpha:]]) (?= $sq_t) }xms;
 ;;
 my ($exception_n_sq_t) =
   map  qr{ \b (?: $_) n $sq_t (*SKIP) (*FAIL) }xms,
   join q{ | },
   qw(are is)
   ;
 print $exception_n_sq_t;
 ;;
 my $text = q{x aren't y ain't z isn't w won't u 't v daren't t};
 print qq{'$text'};
 ;;
 $text =~ s{ $exception_n_sq_t? $target_n_sq_t }{ }xmsg;
 print qq{'$text'};
"
(?msx-i: \b (?: are | is) n (?msx-i: 't ) (*SKIP) (*FAIL) )
'x aren't y ain't z isn't w won't u 't v daren't t'
'x aren't y ain 't z isn't w won 't u 't v daren 't t'
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