The most popular command of sed is the <code>s</code> command for substitution. To use the command, type an s , followed by a delimiting character , followed by a regular expression to find look for, followed by the delimiting character , followed by an expression with which to replace what was found , followed by a final delimiting character. (<code>/</code> is the conventional delimiting character and potentially required by some versions of sed. GNU sed can take any character however, and when URLs or paths are involved, <code>%</code> is often a handy choice to lessen the amount of escaping required.)