How to delete all lines in a text file after a certain word matches

What I need to do is to delete all lines (all the way to the end of the file) after some "keyword" and then add whatever lines I need, essentially "replacing" those last lines of a file with new lines. Attached is an example of the file I am dealing with.

Keyword in example file attached : 'KEY' <--- this is a variable!

Text to be replace after deleting all lines after KEY: 'LINES DELETION COMPLETE!' <--- this is a variable!

note: all lines with "blah" are what I want to delete, but they could be anything, I just want to delete all lines after KEY and replace them with specific text, using fprintf for example.

Any help would be much appreciated!

Answer 1

[Walter Roberson]

keyword = 'KEY';
newtext = "LINES DELETED" + newline + "Yes indeed, deleted" + newline;

%this part has to do with fetching your file over the network for the
%purposes of running the code with MATLAB Answers
filename = 'https://www.mathworks.com/matlabcentral/answers/uploaded_files/1119660/example.txt';
newfilename = 'example_out.txt';

tempfilename = tempname();
tempoutname = websave(tempfilename, filename);

%this is the important part for your purposes -- you would be using
%fileread() directly from your local file system instead of fetching over
%the network

S = fileread(tempoutname);

%do the substitution work
newS = regexprep(S, "^(?<=" + keyword + "\s*).*$", newtext, 'lineanchors');

%write result to a new file
fid = fopen(newfilename, 'w');
fwrite(fid, newS);
fclose(fid);

%cross-checking the results
dbtype(newfilename)

1     .
2     .
3     .
4     .
5     .
6     1 2 3 4
7     5 6 7 8 9
8     .
9     .
10    .
11    KEY
12    LINES DELETED
13    Yes indeed, deleted

[Omar Khammash]

Amazing, thank you!