$re = '/\bCisco\s+((?:(?!\bSoftware\b)[A-Za-z ])*)\s.*?Software/m';
$str = 'Cisco IOS XR Software, Version 5.3.4[Default]
Cisco IOS Software, C3900 Software (C3900-UNIVERSALK9-M), Version 15.4(3)M3, RELEASE SOFTWARE (fc2)
Cisco Nexus Operating System (NX-OS) Software
Cisco IOS XE Software, Version 16.05.01b
Cisco IOS Software [Everest], ISR Software (X86_64_LINUX_IOSD-UNIVERSALK9-M), Version 16.5.1b, RELEASE SOFTWARE (fc1)
';
preg_match_all($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