I don't use a regular expression to verify answers because like most people I don't understand them. I simply add an answer for every possible combo someone is likely to use...
For example to answer the qestion "How many fingers do most people have?" I have the following allowed answers:
10
ten
Ten
TEN
X (for the Romans

)
You can add as many answers as you want, if the user enters any of them they will pass the test.