unescape.php

<?php
$d = file_get_contents('index.html');
//$d = htmlspecialchars($d);
$d = strtr($d, array('&'=>'&amp;', '<'=>'&lt;', '>'=>'&gt;'));
for ($i =0; $i < 10000; ++$i) {
    strtr($d, array('&amp;'=>'&', '&lt;'=>'<', '&gt;'=>'>'));
}

unescape.py

import re

def unescape_replace(s):
    return s.replace('&lt;', '<').replace('&gt;', '>').replace('&amp;', '&')

def unescape_sub(s,
        _sub=re.compile('&lt;|&gt;|&amp;').sub,
        _d = {"&amp;": "&", "&gt;": ">", "&lt;": "<"}
        ):
    return _sub(lambda m: _d[m.group()], s)


import cgi
d = cgi.escape(open('index.html').read())
assert unescape_replace(d) == unescape_sub(d)

N=10000
import timeit
print "replace", timeit.timeit(lambda: unescape_replace(d), number=N)
print "re.sub", timeit.timeit(lambda: unescape_sub(d), number=N)

"""
$ wget http://www.python.org/
$ python unescape.py
replace 2.49956488609
re.sub 7.33878493309
"""