Alternative version of my answer in http://stackoverflow.com/questions/24674318/efficient-regular-expression-for-big-data-if-a-string-contains-a-word

IndexOfWord.java

public class IndexOfWord {
    public static void main(String[] args) {
        String input = "There are longer strings than this not very long one.";
        String search = "long";
        
        // using a StringBuilder enables passing a string as 
        // a function argument without creating a copy of it
        int index = indexOfWord(new StringBuilder(input), search);

        if (index > -1) {
            System.out.println("Hit for \"" + search + "\" at position " + index + ".");
        } else {
            System.out.println("No hit for \"" + search + "\".");
        }
    }

    public static int indexOfWord(StringBuilder input, String word) {
        int index, before_i, after_i = 0;

        while (true) {
            index = input.indexOf(word, after_i);
            if (index == -1 || word.isEmpty()) break;

            before_i = index - 1;
            after_i = index + word.length();

            if (isNonWord(input, before_i) && isNonWord(input, after_i)) {
                return index;
            }
        }
        return -1;
    }
    
    private static boolean isNonWord(StringBuilder input, int index) {
        if (index < 0 || index > input.length() - 1) {
            return true;
        } else {
            char c = input.charAt(index);
            return !Character.isAlphabetic(c); // add more checks, as needed
        }
    }
}