$re = '/^(?:(?:56|78)-[0-9]+|[0-9]{2,})$/m';
$str = 'Valid matches:
minimun two characters and only numbers OR
only numbers with dash in third position and first two numbers should be either 56 or 78. So if there is dash then there should be 56 or 78 next to it
12
123456789
423432423423423423423432
56-1
56-23456789
78-12
78-34234234234234234234234
Invalid matches:
1
1-
11-
1-112
44-2342424
64-4345334
55-sdrfewrwe
56-
5678-234324123423154';
preg_match_all($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