paren matching

When an editor blinks the first parenthesis for you automatically when you type the last one. This is especially helpful when hacking Lisp.

Emacs, The One True Editor does a very nice job of this, but vi will at least do something useful when you hit the % key.

When I'm bored, I backspace over right parentheses and then retype them, just to see the cursor blink.

To do correct paren matching, you need a machine that can count. In particular, a finite automaton cannot do this (see this proof or that proof). So a real regular expression cannot do this. However, modern Perl5 regexen can do it (though the syntax is yucky).