Assuming your string is named, str,
str = '@(S)sigmoid(S,-1.588276e+00,-2.598747e-01)';
numstr = regexp(str,'(-)?\d+(\.\d+)?(e(-|+)\d+)?','match')
% numstr =
% 1×2 cell array
% {'-1.588276e+00'} {'-2.598747e-01'}
to convert to double,
num = str2double(numstr)
% num =
% -1.5883 -0.25987
