All other slash commands start with a forward slash. Why not the replace command? The syntax could be:

/s/old_text/new_text

or even something easier to remember:

/replace/old_text/new_text

I have seen people struggling with this, typing /s/old_text/new_text without success.

I don't know why, but many times I also try to type: i/old_text/new_txt and the text is just written to the channel. I really don't know why I have this association. The replace command is the only one that does not have a name that refers to an understandable english word.