Thanks a lot everyone for all the hints. Just to clarify, the variables would contain plain text, not a pattern. I have now tried the following two expressions, but neither of the works: 
   if( $line =~ m/(?<!\Q$pre\E)\Q$searchTerm\E(?!\Q$post\E)/ )
       {print $line};   

   if( $line =~ m/(?<!$pre)$searchTerm(?!$post)/ )
       {print $line};

[download]
Strangely enough, if I put in the contents of the variables $pre and $post directly into the RegEx, then it does work. Any ideas why that might be?

Cheers,

Martin

In reply to Re^2: Using lookaround with variables by mwunderlich
in thread Using lookaround with variables by mwunderlich

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