$re = '/, \d{4}(?:-\d{2})?$/m';
$str = 'Percent of adults with a high school diploma only, 2000
Percent of adults completing some college or associate\'s degree, 2000
Percent of adults with a bachelor\'s degree or higher, 2000
Percent of adults with less than a high school diploma, 2014-18
Percent of adults with a high school diploma only, 2014-18
Percent of adults completing some college or associate\'s degree, 2014-18';
$subst = "";
$result = preg_replace($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