Abstract: Antlr4, a popular parsing tool, encounters an issue when parsing certain input strings, such as '3+2*5'. This article explains how to handle extraneous input and skip it during parsing.
2024-08-30 by DevCodeF1 Editors
Introduction
ANTLR4 (Another Tool for Language Recognition) is a powerful parser generator that can generate a lexer and parser in multiple programming languages. It can be used to build languages, tools, and frameworks. However, sometimes you may encounter issues while parsing certain inputs, such as the expression ""3+2*5"" which fails to parse and reports an error message: ""1:5 extraneous input '*5' expecting {',', WS}"". This article will focus on the concept of skipping extraneous input during parsing in ANTLR4 and how to handle such cases.
Skipping Extraneous Input Parsing in ANTLR4
Extraneous input is the input that is not part of the grammar's language. In other words, it is the input that the parser does not expect. When the parser encounters extraneous input, it reports an error message indicating the location of the extraneous input and the input that it expected instead.
Skipping extraneous input is a technique used to handle such cases where the input is not part of the grammar's language. ANTLR4 provides a built-in mechanism to skip extraneous input using the skip() method in the lexer or parser rules. By using the skip() method, the parser ignores the extraneous input and continues parsing the rest of the input.
Skipping Extraneous Input in Lexer Rules
To skip extraneous input in lexer rules, you can use the skip() method in the lexer rule. For example, to skip whitespace characters, you can define a lexer rule as follows:
WS : [ \t\r]+ -> skip;
This rule matches any whitespace character and skips it. By using this rule, the parser ignores any whitespace characters in the input and continues parsing the rest of the input.
Skipping Extraneous Input in Parser Rules
To skip extraneous input in parser rules, you can use the skip() method in the parser rule. For example, to skip comments in a grammar, you can define a parser rule as follows:
comment : '/*' .*? '*/' -> skip ;
This rule matches any text between the delimiters '"/*"' and '"*/"' and skips it. By using this rule, the parser ignores any comments in the input and continues parsing the rest of the input.
Skipping Extraneous Input in Grammar
To skip extraneous input in the entire grammar, you can use the skip() method in the grammar's entry point rule. For example, to skip any input that is not part of the grammar's language, you can define the entry point rule as follows:
parse : (expr NEWLINE)* EOF ;expr : expr op=('*' | '/') expr -> ^(MUL $op expr expr) | expr op=('+' | '-') expr -> ^(ADD $op expr expr) | INT | '(' expr ')' -> expr ;MUL : '*' ; // assign token name to '*' operatorADD : '+' ; // assign token name to '+' operatorINT : [0-9]+ ; // match integersNEWLINE : [\r]+ ; // skip newlinesWS : [ \t]+ -> skip ; // skip whitespace
This grammar defines a simple expression language that supports multiplication and addition. The entry point rule is 'parse', which matches any number of expressions separated by newlines. The 'expr' rule matches an expression, which can be an integer, a parenthesized expression, or an expression with an operator. The 'MUL' and 'ADD' rules define the token names for the multiplication and addition operators. The 'INT' rule matches integers. The 'NEWLINE' rule skips newlines, and the 'WS' rule skips whitespace.
By using the 'skip()' method in the 'WS' rule, the parser ignores any whitespace characters in the input and continues parsing the rest of the input. This way, the parser can handle inputs that contain extraneous whitespace characters, such as ""3 + 2 \* 5"", without reporting an error message.
Skipping extraneous input is an essential technique in ANTLR4 to handle inputs that contain characters or tokens that are not part of the grammar's language. By using the skip() method in lexer or parser rules, you can ignore the extraneous input and continue parsing the rest of the input. By using the skip() method in the grammar's entry point rule, you can skip any input that is not part of the grammar's language. By using these techniques, you can build robust and flexible parsers that can handle a wide range of inputs.
References
- ANTLR4: The Definitive Guide by Terence Parr
- ANTLR4 Documentation: https://www.antlr.org/doc/index.html
- ANTLR4 Grammar for ANTLR4: https://github.com/antlr/grammars-v4/tree/master/antlr4
Learn how to handle extraneous input during Antlr4 parsing and improve your software development skills.
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