Looks like you want a Negative Lookbehind Assertion:

(?<!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

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

In reply to Re: substitution with exceptions by LanX
in thread substitution with exceptions by Anonymous Monk

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