It is probably easier to write all lines that have the specific string into a new file so you don't have to worry about the file pointer location.
You use fopen to open the file, use fgetl to get the line. Then you can use strfind to check if it has the specific string. If so, write it to the new file using fprintf.
doc fopen
doc fgetl
doc strfind
doc fprintf
Here is an example
fid1 = fopen('old.txt','r');
fid2 = fopen('new.txt','w');
while 1
tline = fgetl(fid1);
if strfind(tline,SPECIFI_STRING)
fprintf(fid2,'%s\n',tline);
end
end
fclose(fid1);
fclose(fid2);
