$re = '/{if +([a-z0-9:]{1,20})+ +(==|>|<|\!=) +([0-9a-z]{1,20})}(.*?){else}(.*?){endif}/s';
$str = '...
{if var > 0}
тут хтмл код
{else}тут другой хтмл код
{endif}
.....
{if id < 6}тут хтмл код{else}тут другой ко д{endif}
...';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php