One day I thought, "I'm having a hard time reading these C
expressions, because I haven't internalized C's relatively complex
rules of precedence.  I'll just write a program to parse them for me.
I'll write it in C, so that by using the language, I can learn it
better."

Once I started writing it, I realized that I would have to test it, so
I would have to learn the precedence rules well enough to write
correct tests.  So my new hope was to write a Wittgensteinian program:

"My propositions serve as elucidations in the following way: anyone
who understands me eventually recognizes them as nonsensical, when he
has used them - as steps - to climb beyond them. He must, so to speak,
throw away the ladder after he has climbed up it."

I'm still not 100% confident on some of the rules, but at least now
I have a tool.  And you can try it out too at:
http://novalis.org/parse_c_expressions.html

This package includes the following tools:

expr.cgi: a CGI script that accepts a single variable, expr, via HTTP
GET, and returns a SVG of the parse tree.

The command-line program expr_parse, which takes an expression as an
argument, and prints a fully-parenthesized version of the expression.