Finding Lines in a Large Text File with a Specific Text

I am trying to read a large text file (>1GB). I only want to read lines that contain a specific text. For example, I want to read every line that contains "<field name="data". Currently I am using fgetl and reading every line, checking if the text is in the line, but it takes too long. Any suggestions?

Answer 1

[Sonoma Rich]

I found the following code that works well

filetext = fileread('fileread.m');
expr = '[^\n]*fileread[^\n]*';
matches = regexp(filetext,expr,'match');
disp(matches')

but the regexp function is slower than I expected. I ended up using the following method which is significantly faster.

fid = fopen('fileread.m','r');
ftext = textscan(fid,'%s','Delimiter','\n');
fclose(fid);

matches = ftext{1}(contains(ftext{1},'fileread'));
disp(matches)

Answer 2

[KSSV]

Read about textscan. This function gives you option of running a loop and reading required chunks (lines) of the file. In these chunks, you can pick your required line.

Answer 3

[Walter Roberson]

If you have enough memory:

S = fileread('YourFileNameHere.txt');
selected = regexp('^.*<fieldname\s*=.*$', 'match', 'dotexceptnewline', 'lineanchors');

And in the case where you do not care what is at the begining or end of line and just want to know what the "data" field content is, then

S = fileread('YourFileNameHere.txt');
datas = regexp('(?<=fieldname\s*=")(?<data>[^"]*)', 'tokens');

That should get you a struct array with field name 'data' that is the content of inside the quotes.