Flyspell and Ispell find different incorrect words

Question

There are words that are considered to be wrong by Flyspell that are instead considered correct by Ispell. I thought that Flyspell was merely an async version of Ispell, I must be wrong...

To add to the confusion, there cases in which ispell-buffer finds nothing, but ispell-word instead considers some words in the buffer to be wrong, coherently to what Flyspell has to say.

Here's an example:

$ echo http://example.com >/tmp/test
$ emacs -Q /tmp/test

Then execute M-x flyspell-buffer. The word http is now marked as wrong, in fact, ispell-word shows the usual interactive menu. Yet, M-x ispell-buffer completes without finding any wrong words.

Can you help me understanding what's going on here?

Answer 1

Flyspell tries to respect Ispell as much as possible (e.g., ispell-program-name and ispell-dictionary), but does not use the functionalities exposed by Ispell to do the spell checking.

ispell-buffer uses ispell-begin-skip-region-regexp to skip over some parts of the buffer; this is what causes it to skip over http.

If you create a buffer called *Ispell Debug* and then enable debugging by evaluating (defvar ispell-debug-buffer "*Ispell Debug*"), you will see ispell-buffer print something along the lines of

ispell-region: Search for first region to skip after (ispell-begin-skip-region-regexp)
ispell-region: First skip: http://example.com at (pos,line,column): (1,1,0).

in *Ispell Debug*. ispell-word does not do any of this skipping.