You can put the words in a HashSet and thus quickly test if the typed word is in the set.

Thank you so much!
HashSet is something new for me, I try to use it in my code, but I'm wrong with something.
Running the sketch it doesn't print vocabulary as an array and when I type to write it gives me a NullPointerException error.

This is what I write at the global declarations level:

1. HashSet<String> vocabulary;

This is what I write in setup:

1.   String [] lines = loadStrings("list.txt");
2.   HashSet<String> vocabulary = new HashSet<String>(Arrays.asList(lines));
3.   println(vocabulary);
   

This is what I write in void keyPressed:

1.       if (vocabulary.contains(lastWord)) {
2.         background(255, 0, 0);
3.       }

This is the whole code:

1. int leftmargin = 10;
2. int rightmargin = 20;
3. String buff = "";
4. String lastWord;
5. String lines;
6. HashSet<String> vocabulary;
7. boolean didntTypeYet = true;
8. 
   
9. void setup() {
10.   size(640, 360, P3D);
11. 
    
12.   textFont(loadFont("Univers45.vlw"), 25);  //text input
13.   String [] lines = loadStrings("list.txt");
14.   HashSet<String> vocabulary = new HashSet<String>(Arrays.asList(lines));
15.   println(vocabulary);
16. }
17. 
    
18. void draw(){
19.   fill(0);
20.   background(255);
21.   text(buff, 10, height/2);
22. }
23. 
    
24. void keyPressed() {
25.   char k;
26.   k = (char)key;
27.   switch(k) {
28.   case 8:
29.     if (buff.length()>0) {
30.       buff = buff.substring(0, buff.length()-1);
31.     }
32.     break;
33.   case 27:
34.     break;
35.   default:
36.     didntTypeYet = false;
37.     buff+=k;
38.     println(buff);
39.     String[] parole =splitTokens(buff);
40.     if (parole!=null) {
41.       String lastWord=parole[parole.length-1];
42.       lastWord = lastWord.toLowerCase();            //Tutte le lettere diventano minuscole;
43.       println(lastWord);
44.       if (vocabulary.contains(lastWord)) {
45.         background(255, 0, 0);
46.       }
47.     }
48.     break;
49.   }
50. }

" it gives me a NullPointerException error."
At what line?

At a glance, the code looks OK.
At least, if your file contains one word per line.

I don't know in which line. No line is highlighted. This is what Processing says:

Exception in thread "Animation Thread" java.lang.NullPointerException
    at test_13_searchintext.keyPressed(test_13_searchintext.java:79)
    at processing.core.PApplet.handleKeyEvent(PApplet.java:1996)
    at processing.core.PApplet.dequeueKeyEvents(PApplet.java:1979)
    at processing.core.PApplet.handleDraw(PApplet.java:1643)
    at processing.core.PApplet.run(PApplet.java:1530)
    at java.lang.Thread.run(Thread.java:680)

OK, my quick glance missed something, a classical error:
you re-declare vocabulary in setup(), hiding the global one, and loosing this variable at the end of setup(), so the global one remains un-initialized.
Just change to:
vocabulary = new HashSet<String>(Arrays.asList(lines));

See the Technical FAQ.

Yeah! Now it's all OK!
Thank you very much!!!