The canonical search and replace operator, as used in sed, Perl, and any other tool that makes use of regular expressions.
Any valid regular expression may be sought as pattern. If the s/// operator is followed by g, all occurrences of the pattern are replaced, instead of just the first found; if followed by i, the matching is case-insensitive.

A full discussion of the syntax of s/// is beyond the scope of this writeup, and may be better served by the Perl manual, or the excellent O'Reilly book Mastering Regular Expressions.

The s/// form is often seen in text-based chat environments, where it is used to correct typos in one's previous statements, or humorously in the same manner ^H often is.