You can replace entire words or any number of letters by executing a so-called wildcard search, i.e. by using an asterisk (*). For example, in the BYU interface typing in *ground* will return background, grounding, groundwater, and also just ground. Thus, the * replaces 0 or more characters. It can also be used to replace words, but in that case, it only stands in for one word.
Similarly, ? replaces any one character, so looking for ?ling will return cling, fling, sling, bling, etc. However, this wildcard always stands for exactly one character, so looking for ?sing will not return sing, but only using.
What exactly a symbol replaces depends on the corpus. You can find complete overviews of the search syntax in the BYU interface, the Lancaster BNC interface, AntConc and regular expressions in AntConc here.