That is totally a non-trivial problem, which requires a lot more conception before it can be solved. Even for English, this is not well defined: Does "don't" consist of one or two words? Should "www.google.com" be split into three parts? Etc.
And don't let me start with other languages:
In French, "qu'est-ce que" is one word (what).
In the German sentence "Ruf mich an.", the "Ruf an" is one word (call) while mich is another word (me).
In Chinese, you usually don't even have spaces between words.
If I got that feature request in a ticket, I'd send it back to conception. If you asked me this question in an interview, I'd ask if you wanted a programmer, a requirements analysis, or a linguist and why you invite people for a job interview if you don't even know what role you are hiring for.