This article says:
A backlink, such as \ 1 or \ 2, matches the line matched by the previous expression in parentheses, and only this line: (cat | dog) \ 1 matches catcat and dogdog, but not catdog and dogcat. Regarding theoretical regular expressions with backlinks are not regular expressions.
Why? For what reason has \1become a "regular expression" no longer a "regular expression"?
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