How odd. It works fine exactly as posted for me in Perl 5.20.3. Which Perl version are you using which requires the escape?

Update: also tested successfully on v5.10.1, v5.16.3, v5.26.1 and v5.30.0.

In reply to Re^3: Regex AND NOT with zero-width negative lookahead assertion by hippo
in thread Regex AND NOT with zero-width negative lookahead assertion by mldvx4

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