<85>login[2775]: ROOT LOGIN  on '/dev/tty1'

QRegExp re1_1(".*: (?<username>.*) LOGIN.*");
foreach(const QString& str, strings)
{
    if(re1_1.exactMatch(str))
        qDebug() << "Found: " << ":" << str;
}

.*:s+(.*)s+LOGIN.*

QString source{"<85>login[2775]: ROOT LOGIN  on '/dev/tty1'"};
QRegExp regexp{R"-(.*:s+(.*)s+LOGIN.*)-"};
if(regexp.exactMatch(source))
    qDebug() << "The found login is: " << regexp.cap(1);

QString source{"<85>login[2775]: ROOT LOGIN  on '/dev/tty1'"};
QRegularExpression regexp{R"-(.*?:s+(.*?)s+LOGIN.*)-"};
QRegularExpressionMatch match = regexp.match(source);
if(match.hasMatch())
    qDebug() << "The found login is: " << match.captured(1);

std::string s ("<85>login[2775]: ROOT LOGIN  on '/dev/tty1'");
std::regex e (".*:\s+(.*?)\s+LOGIN.*");
std::smatch m;
if(std::regex_search (s,m,e)) std::cout << m[1];