"I have a recipe that calls for sherry vinegar: can I use regular red wine vinegar instead?"
Sherry vinegar has a distinctive flavor that sets it apart from all other vinegars. What you substitute for sherry vinegar depends in large part on the role the vinegar plays in your dish. Fruity raspberry vinegar lends a mildly sweet full flavor, while red wine vinegar is all about the bite of acid. If you are looking for sherry flavor, try substituting 3/4 parts red wine vinegar and 1/4 part sherry.
For more information about using vinegars in recipes, see 7 Ways To Cook With Vinegar.