GENERATING PARSERS WITH JAVACC PDF

The Token Manager What is a token manager? In conventional compiler terminology, a token manager is a lexical analyzer. The token manager analyzes the input stream of characters, breaking it up into chunks called tokens and assigning each token a type. The sequence is usually sent on to a parser for further processing. Can I read from a String instead of a file?

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The Token Manager What is a token manager? In conventional compiler terminology, a token manager is a lexical analyzer. The token manager analyzes the input stream of characters, breaking it up into chunks called tokens and assigning each token a type. The sequence is usually sent on to a parser for further processing.

Can I read from a String instead of a file? You can do this with a java. StringReader as follows: import java. Reader; import java. BufferedReader; import java. Definition If a sequence x can be constructed by concatenating two other sequences y and z i.

Rules There are three rules for picking which regular expression to use to identify the next token: The regular expression must describe a prefix of the remaining input stream. If more than one regular expression describes a prefix, then a regular expression that describes the longest prefix of the input stream is used this is called the maximal munch rule.

If more than one regular expression describes the longest possible prefix, then the regular expression that comes first in the. The following three regular expression productions might appear in the.

Rule 2 says that production 3 is preferred over the first. The order of the productions has no effect on this example.

But, if the remaining input stream starts with: "int i; What if the chosen regular expression matches more than one prefix? Then the longest prefix is used. What if no regular expression matches a prefix of the remaining input? If the remaining input is empty, an EOF token is generated. Otherwise, a TokenMgrError is thrown. How do I make a character sequence match more than one type of token? A common misapprehension is that the token manager will make its decisions based on what the parser expects.

This is not how JavaCC works 3. As discussed previously, the first match wins see What if more than one regular expression matches a prefix of the remaining input? So what do you do? You want the parser to be able to request a member of set B. If A is not a subset of B, there is more work to do. There are two other approaches that might also be tried - one involves lexical states and the other involves semantic actions.

How do I match any character? This will match all characters up to the end of the file provided there are more than zero, which is likely not what you want see What if the chosen regular expression matches more than one prefix? Usually, what you really want is to match all characters up to either the end of the file or some stopping point.

Note that the TEXT tokens are all exactly one character long. Therefore, you should use it with discretion - aware that it can lead to a big generated tokenizer. Regular expression productions are classified as one of four types: TOKEN means that when the production is applied, a Token object should be created and passed to the parser.

SKIP means that when the production is applied, no Token object should be constructed. Each of these special tokens can be accessed from the next Token produced whether special or not , via its specialToken field. What are lexical states? Lexical states allow you to bring different sets of regular expression productions in-to and out-of effect.

Suppose you wanted to write a JavaDoc processor. Most of Java is tokenized according to regular ordinary Java rules. To solve this problem, we could use two lexical states - one for regular Java tokenizing and one for tokenizing within JavaDoc comments. It is possible to list any number of states comma separated before a production. Lexical states are also useful for avoiding complex regular expressions. Suppose you want to skip C style comments. If we use a single, complex regular expression to find comments, then the lexical error will be missed and, in this example at least, a syntactically correct sequence of seven tokens will be found.

If we use the lexical states approach then the behaviour is different although again incorrect as the comment will be skipped - an EOF token will be produced after the token for j and no error will be reported by the token manager 5.

Can the parser force a switch to a new lexical state? Yes, but it is very easy to create bugs by doing so. During parsing there are a number of tokens waiting to be used by the parser - technically this is a queue of tokens held within the parser object.

Any change of state will take effect for the first token not yet in the queue. Usually there is one token in the queue, but because of syntactic lookahead there may be many more. If you are going to force a state change from the parser be sure that at that point in the parsing the token manager is a known and fixed number of tokens ahead of the parser, and that you know what that number is.

If you ever feel tempted to call SwitchTo from the parser, stop and try to think of an alternative method that is harder to get wrong. Is there a way to make SwitchTo safer? The following code makes sure that when a SwitchTo is done any queued tokens are removed from the queue. There are three parts to the solution: In the parser add a subroutine SetState to change states within semantic actions of the parser.

Convenience one-arg constructor provided. Otherwise, we swap buffers.

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GENERATING PARSERS WITH JAVACC PDF

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