$re = '/\?ab=(\d+)/m';
$str = '<li><a href="xyzw.do?ab=1111111111110&method=abcde&maintab=D&_UID_=111Y-T00O-U0‌​0U-PCDE-ZA7C-QQ99-0I00-0550" target="_top"> Letter</a>
<li><a href="xyzw.do?ab=2763276324872&method=abcde&maintab=D&_UID_=111Y-ABCD-EF‌​GH-IHJK-ZA7C-QQ99-0I00-0550" target="_top"> Letter</a> ';
preg_match_all($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