JLex directives: This includes macro definitions (described below). See the JLex Reference Manual for more information about this part of the specification. ~appel/modern/java/CUP/ □. A ready-to-use JLex spec. (*.lex). CUP spec. (*.cup). Lexical analyzer. (*.java). Nodes of the. The next section of this manual describes installation procedures for JFlex. If you never worked with JLex or just want to compare a JLex and a.

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This manual gives a brief but complete description of the tool JFlex. The decimal integer n must be positive. JFlex attempts to report these cases as errors at generation time, but the warnings are overeager. The values returned by the scanner are ignored, but any unmatched text is printed to the Java console instead.

One rule is matched in the innermost loop of the scanner. The int member variable yychar contains the number of characters starting with 0 from the beginning of input to the beginning of the current token. JFlex can easily be integrated with the Ant build tool. This will be faster, but can in some cases become quite messy.

There are however some more subtle kinds of errors that can be introduced by JLex macros. The generated class contains among other things the DFA tables, an input buffer, the lexical states of the specification, a constructor, and the scanning method with the user supplied actions.

Mnaual works, but error reporting can be strange if a syntax error occurs on the last token in the included file. If it was your specification, you may use the generated code without restriction. It is then matched when the lexer is in any of these lexical states. The references Aho, Sethi, and Ullman and Appel provide a good introduction.


The main interface to the outside world is the generated scanning method default name yylexdefault return type Yytoken. Another point are predefined character classes. Identifier matches each string that starts with a character of class jletter followed by zero or more characters of class jletterdigit.

If you don’t provide a file name on the command line, JFlex will pop up a window to ask you for one. Causes the scanning method to be declared as of the Java wrapper type Integer. Unicode aims to be a superset of all known character sets and is therefore a perfect base for encoding things that might get used all over the world. If something goes wrong, it returns -1 to indicate an error.

See section 6 performance of this manual to compare these methods. The first part contains user code that is copied verbatim to the beginning of the mznual source file before the scanner class declaration. How jldx work with JFlex To demonstrate what a lexical specification with JFlex looks like, this section presents a part of the manuxl for the Java language.

JLex: A Lexical Analyzer Generator for Java(TM)

The example is for site wide installation. This is were the mess starts. In most scanners it is possible to do the line counting in the specification by incrementing yyline each time a line terminator has been matched.

They do not share jleex code though.

Reader reader closes the current input stream, and resets the scanner to read from a new input stream. The generated class JFlex generates exactly one file containing one class from the specification unless you have declared another class in the first specification section.

The last part of the second section in our lexical specification is a lexical state declaration: It costs multiple additional comparisons per input character and the matched text has to be re-scanned for counting.


Since user code is part of the same class, JFlex has no language means like the private modifier to indicate which members and methods are internal and which ones belong to the API. The definitions section of a flex specification is quite similar to the options and declarations part of JFlex specs.

It was however possible to further improve the performance of generated scanners using JFlex.

See also section 5 for information about character encodings. If a rule is contained in one or more StateGroupsthen the states of these are also associated with the rule, i.

Moreover, a file written on one platform, say Windows, may later be read by a mmanual on another platform, for instance Linux. Lexer lexer; public parser java.

JFlex User’s Manual

As the scanner reads its input, it keeps track of all regular expressions and activates the action of the expression that has the longest match. Basic structure A lexical specification for flex has the following basic structure: The preceding line terminator in the input is not consumed and can be matched by another rule.

The pushed back characters will after the call of yypushback not be included in yylength and yytext. You may however define a specific value to return and a specific piece of code that should be executed when the end of file is reached. Of course the encoding to use can also come from the data itself: