Build

Example from the getting started with an Hello.g4 grammar file that was generated in:

• a lexer file called HelloLexer.java
• a parser file called HelloParser.java

The grammar below will parse every hello [a-z]+ text.

grammar Hello;
r  : 'hello' ID ;   // Top parser rule of the tree
ID : [a-z]+ ;      
WS : [ \t\r\n]+ -> skip ;

To build a tree, you need to:

• Create a ANTLRInputStream stream of characters that will be used by the lexer

ANTLRInputStream input = new ANTLRInputStream("hello nico");

• Build the lexer from the input

HelloLexer lexer = new HelloLexer(input);

• Wrap the lexer into a stream of tokens CommonTokenStream (The parser can then read this stream back and forth)

CommonTokenStream tokens = new CommonTokenStream(lexer);

• Create a parser that will build the tree

HelloParser parser = new HelloParser(tokens);

• Finally, you can build the tree (a ParseTree) from the parser rule called r (present in the grammar file and generated in the parser as a function)

ParseTree tree = parser.r();    
System.out.println(tree.toStringTree(parser)); // print LISP-style tree