Unix: What is the difference between a wild-card and a regular expression? Write the wildcard ls *.c as a regular expression in ls | grep 'put your regular expression here'.

I'd grin and point out that trick questions aren't very nice, then tell you that there is no difference. A wildcard is a regular expression, it just happens to have different syntax from the regular expressions you use in perl or grep. I'd also ask you which version of grep you meant, and enquire as to whether or not it supported PCRE or not.

And IMO this isn't a "unix" question, grep has been ported to just about every OS, and likewise just about every OS has ways of using regular expressions to query the file system.

PS: apologies, I just hate it when people put trick questions into tests. Like many people I'm not particularly good in test situations at all, and I dont need the stress of second guessing the question on top of trying to get the right answer while dealing with the stress of the test enviornment.