erc-send-pre-hook

erc-send-pre-hook is a variable defined in `erc.el'.
Its value is

(erc-send-distinguish-noncommands erc-add-to-input-ring)

Original value was

• This variable may be risky if used as a file-local variable.

Documentation:
Hook called first when some text is sent through `erc-send-current-line'.
It gets called with one argument, STRING.

To change the text that will be sent, set the variable `str' which is
used in `erc-send-current-line'.

To change the text inserted into the buffer without changing the text
that will be sent, use `erc-send-modify-hook' instead.

Filtering functions can set `erc-send-this' to nil to avoid sending of
that particular string at all and `erc-insert-this' to prevent
inserting that particular string into the buffer.

Note that it's useless to set `erc-send-this' to nil and
`erc-insert-this' to t. ERC is sane enough to not insert the text
anyway.

You can customize this variable.