Jo Ann perhaps you can confirm, but I think what's being asked for here is for a "Search & Replace" function while editing a post or reply. i.e. Not any kind of "global" or "replace this word in all posts from the beginning of time." Just a "normal" search & replace option like you would have when editing any document, but in this context is while you're composing a message or reply on phpBB.
"As a BBCode" was really just in reference to "being available as a button at the top of the full editor" like the other BBCodes are, and not literally that BBCode can accomplish this type of action. Because no, BBCode cannot accomplish this action. What we're really asking for here is a new feature of the rich editor used for "Full Editor & Preview" editing in phpBB.
What's available from the "Edit" menu of the web browser itself is going to vary from browser to browser. But no, I agree, it's not typical that they have "editing" in mind to the extent of providing a "Search & Replace" option while you're in an editable field. In the near term, the best workaround when you have a long response that needs substantial editing like that would be to cut-n-paste it out into a normal document editor such as Word.