Searching for "anne" finds:
Anne
raggedy anne
ANNeNiPoT
SiMPLy_ANNe
Annilein
but not anne216.
I thought this was weird until I noticed that ANNeNiPoT, SiMPLy_ANNe, Annilein all have "Anne" listed at their first name in the real name blank.
So, it looks like partial searches only find players which have the entered text as an entire "word" in either their username or their real name, but not when it is just a string in a bigger "word". I.e. searching for "g33k" doesn't find g33kguy or g33kgurrl.