$re = '/^(.{10}).*$/xm';
$str = 'Mary had a little lamb,
little lamb, little lamb.
Mary had a little lamb, its fleece was white as snow.
And everywhere that Mary went,
Mary went, Mary went,
and everywhere that Mary went, the lamb was sure to go.';
$subst = "$1";
$result = preg_replace($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