/(?<!$pre)$searchTerm?!$post/

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should be 
/(?<!$pre)$searchTerm(?!$post)/

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Actually, if $pre, $searchTerm and $post are text rather than regexp patterns, it should be 
/(?<!\Q$pre\E)\Q$searchTerm\E(?!\Q$post\E)/

[download]

In reply to Re: Using lookaround with variables by ikegami
in thread Using lookaround with variables by mwunderlich

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