Since they have the same meaning, it is natural for you to assume you are dealing with a single word; yet technically these are three different forms.

Most corpus query processors have an option for ignoring case, but case can at times also be meaningful.

Consider the two forms "polish" and "Polish". The first can be a verb or a noun and can never refer to Poland; the second is an adjective (or a noun meaning "the Polish language") and nearly always refers to Poland.

Summing up:

• remember to instruct your programme to ignore case if you want to maximise the scope of your query;
• remember to specify the appropriate case if you want to restrict the scope of your query.