Problem with regex in GUIDE

Hello everybody,

I'm trying to write a regular expression my code is the next one:

expression = 'pi|[0-9]\s[0-9]|pi';
aux = get(handles.intervalo, 'String');
index = regexp(aux, expression);
empty = [];
c = isequal(index, empty);
if c == 1
    errordlg('Tiene que ser de la forma [a b]', 'Error');
end

The problem is when a I write a expression like [pi e] don't give any error can anyone help?

Thank you.