Java Regular Expression Extract String Between Two Words

String test = 
        "<br/><description>Using a combination of remote probes, " +
        "(TCP/IP, SMB, HTTP, NTP, SNMP, etc...) it is possible to guess " +
        "the name of the remote operating system in use, and sometimes " +
        "its version.</description><br/><fname>os_fingerprint.nasl</fname>" +
        "<br/><plugin_modification_date>2012/12/01</plugin_modification_date>" +
        "<br/><plugin_name>OS Identification</plugin_name><br/>" +
        "<plugin_publication_date>2003/12/09</plugin_publication_date><br/>" +
        "<plugin_type>combined</plugin_type><br/><risk_factor>None</risk_factor>" +
        "<br/><solution>n/a</solution><br/><synopsis>It is possible to guess the " +
        "remote operating system.</synopsis><br/><plugin_output><br/>Remote operating " +
        "system : Microsoft Windows Server 2008 R2 Enterprise Service Pack 1<br/>" +
        "Confidence Level : 99<br/>Method : MSRPC<br/><br/> <br/>The remote host is " +
        "running Microsoft Windows Server 2008 R2 Enterprise Service Pack 1" +
        "</plugin_output><br/>";
        Pattern pattern = Pattern.compile("Remote\\soperating\\ssystem\\s:\\s(.+?)\\<br/>");
        Matcher matcher = pattern.matcher(test);
        if (matcher.find()) {
            System.out.println(matcher.group(1));
        }

Output:

Microsoft Windows Server 2008 R2 Enterprise Service Pack 1

Note that in general, using regex against markup language is not advised. However here you are using regex against a specific string of text, that only happens to be inside markup, so I guess it's ok.

There is no space after : and before \\b in your regex.

Try this way:

Pattern.compile("(?<=\\bRemote operating system : \\b).*?(?=\\b<br/>\\b)");
//                                               ^additional space

Without that space \\b wont match start of new word (Microsoft) (it also will never match end of word since : cant be end of correct word).

Try the next:

if (str.matches("^.*Remote operating system : ([^<]*).*$")) {
    System.out.println(
        str.replaceAll("^.*Remote operating system : ([^<]*).*$", "$1")
    );
}