Making Emacs see ♥ as punctuation

Question

I'm trying to get emacs to understand ♥ as punctuation.
(modify-syntax-entry ?♥ "." text-mode-syntax-table)

I'm not sure what the issue is. (That is a unicode heart.)
(modify-syntax-entry ?\u2665 "." text-mode-syntax-table)

For purposes of recognizing that a new sentence starts after "Hi!♥"

How can I troubleshoot this?

Examples:

Lorem impsum dolor.♥ Sit amet.|

(where the pipe indicates the starting cursor position.)
Hitting M-a moves the cursor to the left of "Lorem".

Lorem impsum dolor♥. Sit amet.|

Hitting M-a moves the cursor to the left of "Sit".

I want both cases for it to move to the left of "Sit". In some modes.

Answer 1

Sentence movement functions use the regexp returned by the (sentence-end) function to find the end of a sentence. The (sentence-end) function returns the value of the sentence-end variable if it is not nil. (The function and variable share the same name).

You can modify the value of the sentence-end variable to any regexp you want. In order to maintain all other functionallity provided by the defaults for sentence movement, I recommend you set the value of sentence-end to be a regexp matching either ♥ or the default return value of the (sentence-end) function.

Below is a function that sets the local value of the sentence-end variable as described.

(defun my-setup-sentence-end ()
   "Modify the local value of the `sentence-end'
variable to be a regexp matching either ♥ or the default
regexp returned by the `sentence-end' function."
   (setq-local
    sentence-end
    (rx (or "♥" (eval (let ((sentence-end nil)) (sentence-end)))))))

Once you have this function you can add it as a hook to any mode you would like.

;; For example, you can add this behavior to `text-mode' like so
(add-hook 'text-mode-hook 'my-setup-sentence-end)

Now you can use (forward-sentence), (backward-sentence) and other text movemnt functions to navigate to ♥ characters. This regexp may not fit your exact use case, but should provide the basic understanding for you to create your own.

Take a look at the paragraph.el source file for more information about the modification of sentence-end.