Always case-insensitive?
Global?
Enter your regular expression:
Enter a string to match:

Result:
Matched string:

Decent high-level intro here
The javadoc for the Pattern object, explains all of the quantifiers
A nice java tutorial on regex, ignore the parts with java code - it has nice non-code examples as well
A shameless plug :) An article I wrote for a co-worker's blog about email address validation using regular expressions
Countless more: LMGTFY