Counting Words In a String

Hi, I want to check a string and return the amount of words present in it. These strings could be quite large, some higher than 100,000 characters. I realize I could explode the string on the whitespace and count the number of values. However, I'm not sure this would be the most optimized way to do this, especially considering there will be some quite large strings passed. Any ideas on the most efficient to count the number of words in a large string? Thanks