Finding a character in a string?

so I have the following line of code:

 hashtag=lower(input('Which hashtag would you like information on?','s'));

This prompts the user to input a hashtag, like say '#helloworld'. However, the program runs even without the user putting the hashtag. So it runs even if they only input "helloworld", which presents problems for me later on. How can I get the code to stop reading and present an error message if there is no hashtag symbol in the user's input? Apparently the # character means something in matlab as well which is causing me problems.

I have this so far but it seems not to work:

 if k=(strfind(hashtag,'#')))
    error('Please include the hashtag')
 end

Answer 1

[The Matlab Spot]

using start anchor with regexp will ensure that you '#' as the first character of you input string.

hashtag=lower(input('Which hashtag would you like information on?','s'));
if(isempty(regexp(hashtag,'^#','once')))
    error('Please include the hashtag')
end

[Johny]

This answer is more robust than the one above, so best answer goes to you. I didn't know the ^ symbol did anything at all actually. I learned something!Thanks to both of you guys though.

[Jos (10584)]

using regexp for this problem is like killing a mosquito with a nuclear missile ...

Answer 2

[sixwwwwww]

Dear Johny, try this:

hashtag=lower(input('Which hashtag would you like information on?','s'));
if isempty(strfind(hashtag, '#'))
    error('Please include the hashtag')
end

Answer 3

[Jos (10584)]

Per definition a hashtag starts with a # sign followed by one or more letters or numbers

So, quite simply, check all these three requirements:

str = lower(input('Which hashtag would you like information on?','s'));

if numel(str)<2 || str(1) ~= '#' || ~all(ismember(str(2:end),['a':z' '_' '0':'9']))
   error('Input is not a valid hashtag') ;
end