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).

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