reversing a regular expression in php - php

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:
Strip everything between [ and ].
Replace entities like <, { and encoded entities like &lt; to a hyphen -.
Escape special HTML characters (< → <, & → & etc.)
Remove accents of accented characters (é (=é) → e, etc.)
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');
}