<chat> s/foo/bar/ is an idiom which means "I didn't mean to type 'foo', I meant to type 'bar'".
Its use in talk systems, especially irc, comes from the use of s/// as a substitution operator in Perl, sed and ed. In these languages and tools, s/foo/bar/ would replace any substring matching the regular expression "foo" with the string "bar".
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Nearby terms: Rx « RYFM « S « s/// » S3 » SA » sa