function [i1,i2,j1,j2] = get_file_index(ref_i,ref_j,PairString)
%get the first file name set by the GUI series

if iscell(PairString)
    PairString=PairString{1};
end
i1=ref_i;
j1=ref_j;
i2=ref_i;
j2=ref_j;
% case of pairs
if ~isempty(PairString)
    r=regexp(PairString,'(?<mode>(Di=)|(Dj=)) -*(?<num1>\d+)\|(?<num2>\d+)','names');
    if isempty(r)
        r=regexp(PairString,'(?<num1>\d+)(?<mode>-)(?<num2>\d+)','names');
    end
    switch r.mode
        case 'Di='  %  case 'series(Di)')
            i1=ref_i-str2num(r.num1);
            i2=ref_i+str2num(r.num2);
        case 'Dj='  %  case 'series(Dj)'
            j1=ref_j-str2num(r.num1);
            j2=ref_j+str2num(r.num2);
        case '-'  % case 'bursts'
            j1=str2num(r.num1)*ones(size(ref_i));
            j2=str2num(r.num2)*ones(size(ref_i));
    end
end