Question

suppose I have this function:

function f($string){
    $string = preg_replace("`\[.*\]`U","",$string);
    $string = preg_replace('`&(amp;)?#?[a-z0-9]+;`i','-',$string);
    $string = htmlentities($string, ENT_COMPAT, 'utf-8');
    $string = preg_replace( "`&([a-z])(acute|uml|circ|grave|ring|cedil|slash|tilde|caron|lig|quot|rsquo);`i","\\1", $string );
    $string = preg_replace( array("`[^a-z0-9]`i","`[-]+`") , "-", $string);
    return $string;
}

how can I reverse this function…ie. how should I write the function fReverse() such that we have the following:

$s = f("some string223---");
$reversed = fReverse($s);
echo $s;

and output: some string223—

Answer 1

f is lossy. It is impossible to find an exact reverse. For example, both "some string223---" and "some string223--------" gives the same output (see http://ideone.com/DtGQZ).

---

Nevertheless, we could find a pre-image of f. The 5 replacements of f are:

1. Strip everything between [ and ].
2. Replace entities like <, { and encoded entities like < to a hyphen -.
3. Escape special HTML characters (< → <, & → & etc.)
4. Remove accents of accented characters (é (=é) → e, etc.)
5. Turn non-alphanumerics and consecutive hyphens into a single hyphen -.

Out of these, it is possible that 1, 2, 4 and 5 be identity transforms. Therefore, one possible preimage is just reverse step 3:

function fReverse($string) {
   return html_entity_decode($string, ENT_COMPAT, 'utf-8');
}