$re = '/(lang=)[^&#]*/';
$str = 'http://winner.com/?page=home&lang=es&lang=en&lang=es&bas=2
http://winner.com/?page=home&lang=es&lang=en#hash
http://winner.com/?page=home&lang=es&lang=en&lang=es&base=2#hash
http://winner.com/test.html#HashValue?lang=QueryValue';
$subst = "$1en";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
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