s/

This comes from the command s (substitute), originally in ed and also found in Perl, to replace one string with another. Although the command does not require slashes (other punctuation can be used as delimiters) in this informal use (i.e., outside of scripting) slashes are virtually universally used. In the original command, a trailing g means that the change in strings should be effected every time the first string appears (not just the first time it appears); this g is often used in this informal verb also, as described in the usage note below.