// regexp: optional '-' followed by between 1 and 3 digits
QRegularExpression rx("-?\\d{1,3}");
QValidator *validator = new QRegularExpressionValidator(rx, this);

QLineEdit *edit = new QLineEdit(this);
edit->setValidator(validator);

// integers 1 to 9999
QRegularExpression re("[1-9]\\d{0,3}");
// the validator treats the regexp as "^[1-9]\\d{0,3}$"
QRegularExpressionValidator v(re, 0);
QString s;
int pos = 0;

s = "0";     v.validate(s, pos);    // returns Invalid
s = "12345"; v.validate(s, pos);    // returns Invalid
s = "1";     v.validate(s, pos);    // returns Acceptable

re.setPattern("\\S+");            // one or more non-whitespace characters
v.setRegularExpression(re);
s = "myfile.txt";  v.validate(s, pos); // Returns Acceptable
s = "my file.txt"; v.validate(s, pos); // Returns Invalid

// A, B or C followed by exactly five digits followed by W, X, Y or Z
re.setPattern("[A-C]\\d{5}[W-Z]");
v.setRegularExpression(re);
s = "a12345Z"; v.validate(s, pos);        // Returns Invalid
s = "A12345Z"; v.validate(s, pos);        // Returns Acceptable
s = "B12";     v.validate(s, pos);        // Returns Intermediate

// match most 'readme' files
re.setPattern("read\\S?me(\.(txt|asc|1st))?");
re.setPatternOptions(QRegularExpression::CaseInsensitiveOption);
v.setRegularExpression(re);
s = "readme";      v.validate(s, pos); // Returns Acceptable
s = "README.1ST";  v.validate(s, pos); // Returns Acceptable
s = "read me.txt"; v.validate(s, pos); // Returns Invalid
s = "readm";       v.validate(s, pos); // Returns Intermediate