how to find most common words in text by matlab

Finding the most common words is easy with Text Analytics Toolbox:

>> sonnets = extractFileText("sonnets.txt");
>> sonnets = erasePunctuation(sonnets);
>> tokenizedSonnets = tokenizedDocument(lower(sonnets));
>> bag = bagOfWords(tokenizedSonnets);
>> topkwords(bag, 10)

ans =

    10×2 table

       Word     Count
      ______    _____

      "and"      490 
      "the"      436 
      "to"       409 
      "my"       371 
      "of"       370 
      "i"        344 
      "in"       321 
      "that"     320 
      "thy"      281 
      "thou"     234

You probably want to remove some words (check out removeWords and stopWords). POS tagging is supported in release R2018b and later.