the colon depicts an (unrecognized) regexp extension

No, it doesn't. The colon is a literally matching atom, and the question mark is a quantifier. With some hopefully clearer /x spacing: 

$foo =~ /( ( :? .          # 0 or 1 colon + 1 non-newline char
             |             # or
             \n (?! \n\n)  # 1 newline not followed by two more newlin
+es
            )+
          ) /x

[download]

print "Just another Perl ${\(trickster and hacker)},"
The Sidhekin proves Sidhe did it!

In reply to Re^2: Non-capturing zero-width negative lookahead by Sidhekin
in thread Non-capturing zero-width negative lookahead by throop

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