Looks like L is a cell array of web sites, none of which is an email address with @ in it. So why do you think it should find an email there? Try searching H instead:
K = regexpi(H,E,'match')';
That will give you email addresses.
% Retrieve a web page.
%url = 'https://www.mathworks.com/matlabcentral/answers/?term=';
url = 'https://edition.cnn.com';
webPageContents = webread(url);
% Harvest web sites listed in the page.
listOfWebSites1 = regexp(webPageContents,'https?://[^"]+','match')';
listOfWebSites2 = regexp(webPageContents,'https?://[^"]+','match')';
% Throw out duplicates:
listOfWebSites = unique([listOfWebSites1;listOfWebSites2])
% Harvest email addresses listed in the page.
reForEMailAddresses ='[a-zA-Z0-9._%''+-]+@([a-zA-Z0-9._-])+\.([a-zA-Z]{2,4})';
listOfEMails = regexpi(webPageContents,reForEMailAddresses,'match')';
% Throw out duplicates:
listOfEMails = unique(listOfEMails)
