How to use wild cards in Search

Hello All,
I am using DASL to search contents as well as Property search. I need some
guideness how to use Wild Cards.

For example:-
<D:contains>???.txt</D:contains> // What it will search
<D:propcontains><D:prop><D:displayname/></D:prop><D:literal>*jj*</D:literal></D:propcontains>

Confuse with * , % and ? , _
Plz let me clear.

Also there is a big problem when i use 

<D:contains>*****************line*</D:contains>
The server gets hang and not working. How to restrict this?

Thanks...
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