The zero length pattern always matches, so (?!) never matches. 5.10 introduced alternative (*FAIL). (?!) is optimized into the same op as (*FAIL) since the latter was introduced. 

(?(?{ cond-expr })then-pat|else-pat)

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is the conditional operator for regex. It can also appear in the form 

(?(?{ cond-expr })then-pat)

[download]

All of these are equivalent:

Update: Fixed to replace non-existent (*PASS) with empty pattern.

In reply to Re: Regular expression operators (?{code}), (?!pattern), and (?...) by ikegami
in thread Regular expression operators (?{code}), (?!pattern), and (?...) by jffry

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