$re = '/(\/.+?)(२)(.+?\/)/m';
$str = '{{https://www.test.com/events/test-event-२०१८/२०१८-.१-entry-list|caption=test event of २०१८}}{{https://www.test.com/events/test-event-३१/३१-.१-entry-list|caption=test event of ३१}}{{https://www.test.com/events/test-event-१८/१८-.१-entry-list|caption=test event of १८}}';
$subst = "$12$3";
$result = preg_replace($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