patch 8.0.0645: no error for illegal back reference in NFA engine commit https://github.com/vim/vim/commit/1ef9bbe215e13a273e74fccaddd8fc5a42c76b6e Author: Bram Moolenaar <Bram@vim.org> Date: Sat Jun 17 20:08:20 2017 +0200 patch 8.0.0645: no error for illegal back reference in NFA engine Problem: The new regexp engine does not give an error for using a back reference where it is not allowed. (Dominique Pelle) Solution: Check the back reference like the old engine. (closes #1774)

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