Close; it was one of the Bell Labs versions of QED — https://www.bell-labs.com/usr/dmr/www/qed.html https://www.bell-labs.com/usr/dmr/www/qed.html — I suspect b

Close; it was one of the Bell Labs versions of QED — https://www.bell-labs.com/usr/dmr/www/qed.html https://www.bell-labs.com/usr/dmr/www/qed.html — I suspect but am not sure that Ken Thompson's CTSS version, that introduced regular expression text editing, used slash. The original Deutsch/Lampson QED used colons or square brackets for searches (fixed strings, not regular expressions) but did use slashes as the substitution delimiter, which may have inspired Thompson to use the same character for both search and replace.