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    <p>If you only want a lexer, start your grammar with:</p> <pre><code>lexer grammar FooLexer; // creates: FooLexer.java </code></pre> <p><code>LT(int): Token</code> can only be used inside parser rules (on a <a href="http://www.antlr.org/api/Java/interfaceorg_1_1antlr_1_1runtime_1_1_token_stream.html" rel="nofollow"><code>TokenStream</code></a>). Inside lexer rules, you can only use <code>LA(int): int</code> that gets the next <code>int</code> (character) from the <a href="http://www.antlr.org/api/Java/interfaceorg_1_1antlr_1_1runtime_1_1_int_stream.html" rel="nofollow"><code>IntStream</code></a>. But there is no need for all the manual look ahead. Just do something like this:</p> <pre class="lang-java prettyprint-override"><code>lexer grammar FooLexer; BEGIN : 'BEGIN' ; END : 'END' ; FLOAT : DIGIT+ '.' DIGIT+ ; INT : DIGIT+ ; IDENTIFIER : LETTER (LETTER | DIGIT)* ; WS : (' ' | '\t' | '\n' | '\r' | '\f')+ {$channel = HIDDEN;} ; fragment LETTER : ('a'..'z' | 'A'..'Z'); fragment DIGIT : ('0'..'9'); </code></pre> <p>I don't see the need to create a token called <code>KEYWORD</code> that matches all keywords: you'll want to make a distinction between a <code>BEGIN</code> and <code>END</code> token, right? But if you really want this, simply do:</p> <pre class="lang-java prettyprint-override"><code>KEYWORD : 'BEGIN' | 'END' ; </code></pre> <p>and remove the <code>BEGIN</code> and <code>END</code> rules. Just make sure <code>KEYWORD</code> is defined before <code>IDENTIFIER</code>.</p> <h2>EDIT</h2> <p>Test the lexer with the following class:</p> <pre class="lang-java prettyprint-override"><code>import org.antlr.runtime.*; public class Main { public static void main(String[] args) throws Exception { String src = "BEGIN END 3.14159 42 FOO"; FooLexer lexer = new FooLexer(new ANTLRStringStream(src)); while(true) { Token token = lexer.nextToken(); if(token.getType() == FooLexer.EOF) { break; } System.out.println(token.getType() + " :: " + token.getText()); } } } </code></pre> <p>If you generate a lexer, compile the .java source files and run the Main class like this:</p> <pre class="lang-none prettyprint-override"><code>java -cp antlr-3.3.jar org.antlr.Tool FooLexer.g javac -cp antlr-3.3.jar *.java java -cp .:antlr-3.3.jar Main </code></pre> <p>the following output will be printed to the console:</p> <pre class="lang-none prettyprint-override"><code>4 :: BEGIN 11 :: 5 :: END 11 :: 7 :: 3.14159 11 :: 8 :: 42 11 :: 10 :: FOO </code></pre>
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    1. COIn your example FLOAT and INT can't both be recognized because it is an ambiguous case. I would get the following warning: Multiple token rules can match input such as "'0'..'9''0'..'9'": FLOATLITERAL, INTLITERAL As a result, token(s) INTLITERAL were disabled for that input
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    2. CO@macneil, no, that is not true. My guess is that you've not copy-pasted my suggestion. I'll add a little demo shortly.
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    3. COAs you can see, the tokens `3.14159` and `42` are of a different type (FLOAT and INT respectively).
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