echo f1($str); //返回你好
echo f2($str); //返回我
echo f3($str); //返回爱
echo f4($str); //返回北京
echo f5($str); //返回天安门

function f1($str)
{
$result = array();
preg_match_all(\"/^(.*)(?:<)/i\",$str, $result);
return $result[1][0];
}

function f2($str)
{
$result = array();
preg_match_all(\"/(?:<)(.*)(?:>)/i\",$str, $result);
return $result[1][0];
}

function f3($str)
{
$result = array();
preg_match_all(\"/(?:\\()(.*)(?:\\))/i\",$str, $result);
return $result[1][0];
}

function f4($str)
{
$result = array();
preg_match_all(\"/(?:\\[)(.*)(?:\\])/i\",$str, $result);
return $result[1][0];
}

function f5($str)
{
$result = array();
preg_match_all(\"/(?:\\{)(.*)(?:\\})/i\",$str, $result);
return $result[1][0];
}