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copied!<p>The following ANTLR grammar:</p> <pre><code>parse : exp EOF ; exp : add_exp ; add_exp : mul_exp ('+' mul_exp)* ; mul_exp : atom ('*' atom)* ; atom : Number | '(' exp ')' ; Number : 'a'..'z' ; </code></pre> <p>parses the input <code>a + b * c + d + e</code> as:</p> <p><a href="http://img266.imageshack.us/img266/7099/17212574.png" rel="nofollow noreferrer">alt text http://img266.imageshack.us/img266/7099/17212574.png</a></p> <p>As you can see, the <code>mul_exp</code> is the furthest away in the tree and (using an appropriate "walk" through your tree) will be evaluated first.</p> <p>and the input <code>a + b * (c + d) + e</code> is parsed as:</p> <p><a href="http://img688.imageshack.us/img688/2207/89332200.png" rel="nofollow noreferrer">alt text http://img688.imageshack.us/img688/2207/89332200.png</a></p> <p>The images were generated with <a href="http://www.antlr.org/works/index.html" rel="nofollow noreferrer">ANTLRWorks</a>. </p> <p><strong>EDIT:</strong></p> <p>A tool like <a href="http://www.antlr.org/works/index.html" rel="nofollow noreferrer">ANTLRWorks</a> makes debugging a grammar a breeze! For example, if I click on the <code>atom</code> rule in the grammar above, the following is automatically generated and displayed at the bottom of the screen:</p> <p><a href="http://img340.imageshack.us/img340/6793/53395907.png" rel="nofollow noreferrer">alt text http://img340.imageshack.us/img340/6793/53395907.png</a></p> <p>Of course, that rule isn't complex at all, but when you do get to work with more complex rules, it's pretty darn easy to visualize them like that.</p> <p>HTH.</p>

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