How to replace a string with another string in specific table columns

Hello,

Im trying to replace all [] with '-99' only in the columns that begin with Q_ in the table t below. How would I do that with an index ? Its important that I dont use the variables names.

% data
Q_x = {'1'; '[]'; '3'};
a = {'A'; 'B'; 'C'};
o = {'A'; '[]'; 'C'};
Q_y = {'1'; '[]'; '3'};

t = table(Q_x, a, o, Q_y)

all_vars = t.Properties.VariableNames;
index = find(strncmp(all_vars, 'Q_', 2));

Thank you,

Answer 1

[Daniel M]

t{:,index} = regexprep(t{:,index},'\[\]','-99')

Answer 2

[Guillaume]

One easy way:

missinglocs = ismissing(t, '[]'); 
newt = fillmissing(t, 'constant', '-99', 'DataVariables', startsWith(t.Properties.VariableNames, 'Q_'), 'MissingLocations', missinglocs)