I have some text stored in a string $text that’s filtered through the_content and therefore, wptexturize().

wptexturize() is a function that replaces various characters into more prettier ones.

Is there any way that I can unwptexturize() the string? Revert it back to what it was.

What I’ve danced with so far

I can replace smart quotes with regular ones like this:

$text = iconv('UTF-8', 'ASCII//TRANSLIT', $text);

But that will only cover the smart quotes, there are a lot more characters.

Another alternative is to write a custom regex to replace all these characters back to their original ones.

Don’t think so after skimming through the wptexturize() code, but what about using the run_wptexturize filter (untested):

// Turn off wptexturize
add_filter( 'run_wptexturize', '__return_false' );

// Your text handling here ...
$text = apply_filters( 'the_content', $text );

// Remove filter
remove_filter( 'run_wptexturize', '__return_false' );

i.e. just turn off the wptexturize only for your text handling?