How to find only negative root of any polynomial

I am looking for a filter design. and from all possible roots of transfer function, i have to filter only left half s-plane (negative) roots. please suggest any code as "roots()" gives all +ve and _ve roots

Answer 1

[Azzi Abdelmalek]

r=roots(your_array)
r=r(r<0))

Answer 2

[Mikhail]

If you have symbolic math toolbox, function vpasolve will find all roots. After that you can filter it with something like:

    a=vpasolve(eqn);
    b=a(find(real(a)<0));