$re = '/(?i)(?:a\/c|account)\s*no\.?\s+(\w+)/m';
$str = 'bank of india account statement name abcd account no. 123456 account type savings account
statement for a/c no 11111111 between 16-09-2019 and 16-03-2020
shyam alaspure period 01-12-2019 to 29-02-2020 cust.reln.no XXXXXXXX account no 9XXX99999';
preg_match_all($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