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    <p>You first create a grammar. Below is a small grammar that you can use to evaluate expressions that are built using the 4 basic math operators: +, -, * and /. You can also group expressions using parenthesis.</p> <p>Note that this grammar is just a very basic one: it does not handle unary operators (the minus in: -1+9) or decimals like .99 (without a leading number), to name just two shortcomings. This is just an example you can work on yourself.</p> <p>Here's the contents of the grammar file <strong>Exp.g</strong>:</p> <pre><code>grammar Exp; /* This will be the entry point of our parser. */ eval : additionExp ; /* Addition and subtraction have the lowest precedence. */ additionExp : multiplyExp ( '+' multiplyExp | '-' multiplyExp )* ; /* Multiplication and division have a higher precedence. */ multiplyExp : atomExp ( '*' atomExp | '/' atomExp )* ; /* An expression atom is the smallest part of an expression: a number. Or when we encounter parenthesis, we're making a recursive call back to the rule 'additionExp'. As you can see, an 'atomExp' has the highest precedence. */ atomExp : Number | '(' additionExp ')' ; /* A number: can be an integer value, or a decimal value */ Number : ('0'..'9')+ ('.' ('0'..'9')+)? ; /* We're going to ignore all white space characters */ WS : (' ' | '\t' | '\r'| '\n') {$channel=HIDDEN;} ; </code></pre> <p><em>(Parser rules start with a lower case letter, and lexer rules start with a capital letter)</em></p> <p>After creating the grammar, you'll want to generate a parser and lexer from it. Download the <a href="http://www.antlr.org/download/antlr-3.2.jar" rel="noreferrer">ANTLR jar</a> and store it in the same directory as your grammar file.</p> <p>Execute the following command on your shell/command prompt:</p> <pre><code>java -cp antlr-3.2.jar org.antlr.Tool Exp.g </code></pre> <p>It should not produce any error message, and the files <strong>ExpLexer.java</strong>, <strong>ExpParser.java</strong> and <strong>Exp.tokens</strong> should now be generated.</p> <p>To see if it all works properly, create this test class:</p> <pre class="lang-java prettyprint-override"><code>import org.antlr.runtime.*; public class ANTLRDemo { public static void main(String[] args) throws Exception { ANTLRStringStream in = new ANTLRStringStream("12*(5-6)"); ExpLexer lexer = new ExpLexer(in); CommonTokenStream tokens = new CommonTokenStream(lexer); ExpParser parser = new ExpParser(tokens); parser.eval(); } } </code></pre> <p>and compile it:</p> <pre><code>// *nix/MacOS javac -cp .:antlr-3.2.jar ANTLRDemo.java // Windows javac -cp .;antlr-3.2.jar ANTLRDemo.java </code></pre> <p>and then run it:</p> <pre><code>// *nix/MacOS java -cp .:antlr-3.2.jar ANTLRDemo // Windows java -cp .;antlr-3.2.jar ANTLRDemo </code></pre> <p>If all goes well, nothing is being printed to the console. This means the parser did not find any error. When you change <code>"12*(5-6)"</code> into <code>"12*(5-6"</code> and then recompile and run it, there should be printed the following:</p> <pre><code>line 0:-1 mismatched input '&lt;EOF&gt;' expecting ')' </code></pre> <p>Okay, now we want to add a bit of Java code to the grammar so that the parser actually does something useful. Adding code can be done by placing <code>{</code> and <code>}</code> inside your grammar with some plain Java code inside it.</p> <p>But first: all parser rules in the grammar file should return a primitive double value. You can do that by adding <code>returns [double value]</code> after each rule:</p> <pre><code>grammar Exp; eval returns [double value] : additionExp ; additionExp returns [double value] : multiplyExp ( '+' multiplyExp | '-' multiplyExp )* ; // ... </code></pre> <p>which needs little explanation: every rule is expected to return a double value. Now to "interact" with the return value <code>double value</code> (which is NOT inside a plain Java code block <code>{...}</code>) from inside a code block, you'll need to add a dollar sign in front of <code>value</code>:</p> <pre><code>grammar Exp; /* This will be the entry point of our parser. */ eval returns [double value] : additionExp { /* plain code block! */ System.out.println("value equals: "+$value); } ; // ... </code></pre> <p>Here's the grammar but now with the Java code added:</p> <pre><code>grammar Exp; eval returns [double value] : exp=additionExp {$value = $exp.value;} ; additionExp returns [double value] : m1=multiplyExp {$value = $m1.value;} ( '+' m2=multiplyExp {$value += $m2.value;} | '-' m2=multiplyExp {$value -= $m2.value;} )* ; multiplyExp returns [double value] : a1=atomExp {$value = $a1.value;} ( '*' a2=atomExp {$value *= $a2.value;} | '/' a2=atomExp {$value /= $a2.value;} )* ; atomExp returns [double value] : n=Number {$value = Double.parseDouble($n.text);} | '(' exp=additionExp ')' {$value = $exp.value;} ; Number : ('0'..'9')+ ('.' ('0'..'9')+)? ; WS : (' ' | '\t' | '\r'| '\n') {$channel=HIDDEN;} ; </code></pre> <p>and since our <code>eval</code> rule now returns a double, change your ANTLRDemo.java into this:</p> <pre class="lang-java prettyprint-override"><code>import org.antlr.runtime.*; public class ANTLRDemo { public static void main(String[] args) throws Exception { ANTLRStringStream in = new ANTLRStringStream("12*(5-6)"); ExpLexer lexer = new ExpLexer(in); CommonTokenStream tokens = new CommonTokenStream(lexer); ExpParser parser = new ExpParser(tokens); System.out.println(parser.eval()); // print the value } } </code></pre> <p>Again (re) generate a fresh lexer and parser from your grammar (1), compile all classes (2) and run ANTLRDemo (3):</p> <pre><code>// *nix/MacOS java -cp antlr-3.2.jar org.antlr.Tool Exp.g // 1 javac -cp .:antlr-3.2.jar ANTLRDemo.java // 2 java -cp .:antlr-3.2.jar ANTLRDemo // 3 // Windows java -cp antlr-3.2.jar org.antlr.Tool Exp.g // 1 javac -cp .;antlr-3.2.jar ANTLRDemo.java // 2 java -cp .;antlr-3.2.jar ANTLRDemo // 3 </code></pre> <p>and you'll now see the outcome of the expression <code>12*(5-6)</code> printed to your console!</p> <p>Again: this is a very brief explanation. I encourage you to browse the <a href="http://www.antlr.org/wiki/display/ANTLR3/ANTLR+3+Wiki+Home" rel="noreferrer">ANTLR wiki</a> and read some tutorials and/or play a bit with what I just posted.</p> <p>Good luck!</p> <p>EDIT:</p> <p><a href="https://stackoverflow.com/questions/2042353/extending-simple-antlr-grammer-to-support-input-variables">This post</a> shows how to extend the example above so that a <code>Map&lt;String, Double&gt;</code> can be provided that holds variables in the provided expression.</p> <p>And this <a href="https://stackoverflow.com/questions/15610183/if-else-statements-in-antlr-using-listeners">Q&amp;A</a> demonstrates how to create a simple expression parser, and evaluator using <strong>ANTLR4</strong>.</p> <p>To get this code working with a current version of Antlr (June 2014) I needed to make a few changes. <code>ANTLRStringStream</code> needed to become <code>ANTLRInputStream</code>, the returned value needed to change from <code>parser.eval()</code> to <code>parser.eval().value</code>, and I needed to remove the <code>WS</code> clause at the end, because attribute values such as <code>$channel</code> are no longer allowed to appear in lexer actions.</p>
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    1. COWhere does the implemenations of `parser.eval()` happen? That isn't clear HERE or on the ANTLR3 Wiki!
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    2. CO@Jarrod, err, sorry, I don't really understand you. `eval` is a parser rule which returns a `double`. So there is an `eval()` method you can call on an instance of an `ExpParser`, just as I demonstrated in the `ANTLRDemo.main(...)`. After generating a lexer/parser, simply open the file `ExpParser.java` and you'll see that there's an `eval()` method returning a `double`.
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