How to add a regex to a gtkmm3 combo-box/Best way to grep a regex into a gtkmm3/c++ combo box?

Question

I am working on a project to list items of a file in a gtkmm combo box.

//I declared the signal handler as void on_combo_changed();

//And the widget Gtk::ComboBoxText m_Combo;

//All void ABC::on_combo_changed()

I want to add a code that will only show the content of a file. Example, a linux command would be grep -o \b[A-Z0-9._%+-]+@[A-Z0-9.-]+.[A-Z]{2,}\b /Documents/email/. How do I implement this on a combox box so when clicked it will show a list of emails?