By default regular expression matching return true if there is at least one match for the regular expression in given string (answer). Exact matching means match must be an entire string.
Set this to Yes, if you write regular expressions for full student's answers. Setting this to No gives you additional flexibility: you can specify some answer with low (or zero) grade to catch common errors and give comments on them. You still can specify exact matches for some of you regular expressions if you start them with ^ and end with $.