$re = '/(err\s*?\d{1})|\s+(\d{3})\b/mi';
$str = 'The below items should match:
-----------------------------
123
456 123
123 324 324
Match the number in this sentence 123 as well
999
The below items should NOT match:
---------------------------------
1234
12345
123456
1234567
£123
$456
Err404
ERR404
err404
Err 404
ERR 404
err 404
there is err 404 on page
this err 1232222222222 as well
a string 12323 like this
asd
4444333322221111
4444 3333 2222 1111
Err123
02012341234
920 1234 1234';
preg_match_all($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