A fragment is somewhat akin to an inline function: It makes the grammar more readable and easier to maintain. A fragment will never be counted as a token, it only serves to simplify a grammar. Consider: NUMBER: DIGITS | OCTAL_DIGITS | HEX_DIGITS; fragment DIGITS: ‘1’..’9′ ‘0’..’9’*; fragment OCTAL_DIGITS: ‘0’ ‘0’..’7’+; fragment HEX_DIGITS: ‘0x’ (‘0’..’9′ | … Read more
What parsers and lexers have in common: They read symbols of some alphabet from their input. Hint: The alphabet doesn’t necessarily have to be of letters. But it has to be of symbols which are atomic for the language understood by parser/lexer. Symbols for the lexer: ASCII characters. Symbols for the parser: the particular tokens, … Read more
… what are the practical differences between these two statements in ANTLR … MY_RULE will be used to tokenize your input source. It represents a fundamental building block of your language. my_rule is called from the parser, it consists of zero or more other parser rules or tokens produced by the lexer. That’s the difference. … Read more
By default, ANTLR 4 generates listeners. But if you give org.antlr.v4.Tool the command line parameter -visitor, ANTLR generates visitor classes for you. These work much like listeners, but give you more control over which (sub) trees are walked/visited. This is particularly useful if you want to exclude certain (sub) trees (like else/if blocks, as in … Read more
Would the dynamic linq library help here? In particular, I’m thinking as a Where clause. If necessary, put it inside a list/array just to call .Where(string) on it! i.e. var people = new List<Person> { person }; int match = people.Where(filter).Any(); If not, writing a parser (using Expression under the hood) isn’t hugely taxing – … Read more
Raphael wrote: Does antlr build the AST for us automatically by option{output=AST;}? Or do I have to make the tree myself? If it does, then how to spit out the nodes on that AST? No, the parser does not know what you want as root and as leaves for each parser rule, so you’ll have … Read more
Ok, let’s build a simple math example. Building an AST is totally overkill for such a task but it’s a nice way to show the principle. I’ll do it in C# but the Java version would be very similar. The grammar First, let’s write a very basic math grammar to work with: grammar Math; compileUnit … Read more
Note: this answer is for ANTLR3! If you’re looking for an ANTLR4 example, then this Q&A demonstrates how to create a simple expression parser, and evaluator using ANTLR4. 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: +, … Read more
ANTLR 4 For predicates in ANTLR 4, checkout these stackoverflow Q&A’s: Syntax of semantic predicates in Antlr4 Semantic predicates in ANTLR4? ANTLR 3 A semantic predicate is a way to enforce extra (semantic) rules upon grammar actions using plain code. There are 3 types of semantic predicates: validating semantic predicates; gated semantic predicates; disambiguating semantic … Read more