I found out why captuuring them does not activate the optimisation. It has to do with the regex tree. A regex like /.*?foo/ becomes:

  1 MINMOD(2)
  2 STAR(4)
    3 REG_ANY(0)
  4 EXACT <foo>(6)
  6 END
[download]

whereas the tree for /(.*?)foo/ is:

  1 OPEN1(3)
  3   MINMOD(4)
  4   STAR(6)
  5     REG_ANY(0)
  6 CLOSE1(8)
  8 EXACT <foo>(10)
 10 END
[download]

In the non-capturing version, the node directly after STAR is EXACT. In the capturing version, the node is CLOSE. I patched regexec.c to allow for a CLOSE in between the two.

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Jeff[japhy]Pinyan: Perl, regex, and perl hacker.
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;