Filter the content read from file

Question

(This follows the answer to the question: Completion of sentences stored in a file.)

Suppose I have a file, the content of which being organized thanks to org-mode:

* Dreams ≠ reality 
Please, don't take your dreams for reality: (a + b)² ≠ a² + b².
Please, don't take your dreams for reality: cos(a + b) ≠ cos a + cos b.
No, √(a²) ≠ a.

* Derivatives
You should revise the derivatives.

* General remarks
This only works for a positive x.

Edit after @NickD comments:

AFAICS, the function used for reading this file, insert-file-contents, filters (I mean ignores) empty lines. Is it possible to ask it to filter lines starting with a * as well?

Is it possible to ask the function used for reading this file, insert-file-contents, or a consequent one, to filter some lines, e.g. those starting with a *

Answer 1

The answer to any "Is it possible" question is almost always yes (there is a theorem by Turing which tells us the limit of what is computable, but in practice people rarely come up against that limit). It is better to rephrase this type of question to ask how to do something, rather than merely if it is possible, since that's usually what most people mean by it. It's also a good idea to list what you've already tried, and possibly why that didn't work.

You want to use the seq-remove function. You can read the documentation for it by typing C-h f seq-remove <RET>. You'll see that it takes a predicate function and a sequence, and returns a new sequence comprised of those elements for which the predicate function returns nil.

For the predicate, I recommend writing a lambda that calls string-match-p. This returns true when a string matches a regular expression. Again, you can see the documentation for functions with C-h f.

Putting those together, something like this ought to work:

(seq-remove (lambda (line)
              (string-match-p "^\\*+ " line))
            your-list-of-strings)

The elisp manual includes additional documentation about these topics that you might want to read, along with documentation about similar functions that you may soon need. You can open the info browser with C-h i. You may have many info manuals installed on your computer, but you should specifically take a look at the Emacs manual and the Elisp manual. Chapters 6.1 Sequences and 34.4 Regular Expression Searching would be good places to start.