Detecting Capital Letter Strings in a Larger String

Question

Is there a nice and clean way to find strings of capital letters of size 2-4 in length within a larger string in matlab. For example, lets say I have a string...

 stringy = 'I imagine I could FLY';

Is there a nice way to just extract the FLY portion of the string? Currently I'm using the upper() function to identify all the characters in the string that are upper case like this...

 for count = 1:length(stringy)
     if upper(stringy(count))==stringy(count)
          isupper(count)=1;
     else
          isupper(count)=0;
     end
 end

And then, I'm just going through the binary vector and identifying when there there are 2-4 1's in the row.

This method is working... but I'm wondering if there is a cleaner way to be doing this... thanks!!!

Answer 1

You can use regular expressions for this. The regular expression [A-Z]{2,4} will search for 2-4 capital letters in a string.

The corresponding matlab function is called regexp.

regexp(string,pattern) returns subindexes into string of all the places it matches pattern.

For your pattern I have two suggestions:

1. \<[A-Z]{2,4}\>. This searches for whole words that consist of 2-4 capital letters (so it doesn't grab TOUCH below):

   stringy = 'I imagine I could FLY and TOUCH THE SKY';
   regexp(stringy,'\<[A-Z]{2,4}\>') % returns 19, 33, 37 ('FLY','THE','SKY')
   

   (Edit: Matlab uses \< and \> for word boundaries not the standard \b).

2. If you have strings where case can be mixed within a word and you want to extract those, try (?<![A-Z])[A-Z]{2,4}(?![A-Z]) (which means "2-4 capital letters that aren't surrounded by capital letters):

   stringy = 'I image I could FLYandTouchTHEsky';
   % returns 17 and 28 ('FLY', 'THE')
   regexp(stringy,'(?<![A-Z])[A-Z]{2,4}(?![A-Z])') 
   
   % note '\<[A-Z]{2,4}\>' wouldn't match anything here since it looks for
   % *whole words* that consist of 2-4 capital letters only.
   % 'FLYandTouchTHEsky' doesn't satisfy this.
   

Pick the regex based on what behaviour you want to occur.