Java - Count the number of times a word appears in a string

• Apache Common StringUtils function

1:       public static int countMatches(String str, String sub) {  
2:         if (isEmpty(str) || isEmpty(sub)) {  
3:           return 0;  
4:         }  
5:         int count = 0;  
6:         int idx = 0;  
7:         while ((idx = str.indexOf(sub, idx)) != -1) {  
8:           count++;  
9:           idx += sub.length();  
10:         }  
11:         return count;  
12:       }  
13:       public static boolean isEmpty(String str) {  
14:     return str == null || str.length() == 0;  
15:    }  

• Use Regex

1:       public static int countMatchesWithRegex(String str, String reg) {  
2:            Matcher m = Pattern.compile(reg).matcher(str);  
3:            int matches = 0;  
4:            while(m.find())  
5:              matches++;  
6:            return matches;  
7:       }  

• Difference: Regex approach can get correct counting over "Africa" and "African"

1:            String target = "South Africa South African South Africa.";  
2:            System.out.println(StringUtils.countMatches(target, "South Africa")); // Return 3  
3:            System.out.println(StringUtils.countMatchesWithRegex(target, "\\bSouth African\\b")); // Return 1