off-by-one string comparison

[argv]

I want to see if a given string is off by 1 character from the test string. Thus, If my test string is "foo bar baz", I want to have a function that matches any one of:

fooo bar baz
foo bar baaz
foo bbar baz
[etc]

Here, I'm looking for off-by-one errors. Extra bonus for a function that matches if it's off by an arbitrary N. That is, a function that returned the number of characters that were different between the strings rather than just a binary true/false.