Trouble importing data from folder

ls returns a 2d character array in general (on Windows), but regexp expects a character row vector. You can covert the output of ls to a cell array in order to use it in regexp. Also, you can use things like "Incidents*" in the ls call directly in order to narrow down the file names returned.

files = ls(fullfile(data_folder,"Incidents*.csv"));
files = strtrim(cellstr(files));
index_wanted = regexp(files, "\d+", 'match', 'once');

However, dir may be easier to use than ls

files = dir(fullfile(data_folder,"Incidents*.csv"));
index_wanted = regexp({files.name}, "\d+", 'match', 'once');

By the way, the 'undefined function' errors you got when testing your anonymous functions are due to the use of @ when using the function. You only need @ when defining the function, not when using it, e.g.:

temp= table(files) ;
file_check= @(x ) regexp(  x , "Incident*" );
rowfun( file_check , temp)