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)
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All of these are equivalent:

• / (?(?{ $d{$1}  })         | (*FAIL) )/x
• / (?(?{ !$d{$1} }) (*FAIL) |         )/x
• / (?(?{ !$d{$1} }) (*FAIL)           )/x
• / (?(?{ !$d{$1} }) (?!)              )/x

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