valac --pkg gtk+-3.0 EntryNumber.vala

EntryNumber.vala

using Gtk;
using Gdk;

public static int main (string[] args) {
    Gtk.init (ref args);

    Application app = new Application ();
    app.show_all ();
    Gtk.main ();
    return 0;
}

public class Application : Gtk.Window {
    
    public Application () {
        this.title = "My Gtk.Window";
        this.window_position = Gtk.WindowPosition.CENTER;
        this.set_default_size (350, 300);
        this.hide_titlebar_when_maximized = false;

        EntryNumber entry_number = new EntryNumber (Gtk.InputPurpose.DIGITS);
        entry_number.include_icon_clean ();
        entry_number.changed.connect (() => {
            entry_number.input_digits(); 
        });

        this.add (entry_number);
    }
}

public class EntryNumber : Gtk.Entry {

    private string old = "";

    public EntryNumber (Gtk.InputPurpose purpose) {
        this.input_purpose = purpose;
    }   

    public  void input_digits () {
        try {
            this.text = this.text.replace (" ", "");
            
            var regex = new Regex ("""^[0-9]{1,}$""");
            var regex_match = regex.match (this.text);
            
            if (regex_match) {
                this.old = this.text;
            } else if (this.text != "") {
                this.text = this.old;
            }    
        } catch (Error e) {
            message (e.message);
        }    
    }

    public void include_icon_clean () {
        this.set_icon_from_icon_name (Gtk.EntryIconPosition.SECONDARY, "edit-clear");
        this.icon_press.connect ((pos, event) => {
            if (pos == Gtk.EntryIconPosition.SECONDARY) {
                this.set_text ("");
            }
        });
    }
}