Can I use my old JLex specifications with JFlex?

Yes. You usually can use them unchanged. See section porting from JLex of the manual for more information on that topic.

Where can I get the latest version of JFlex?

Check out the JFlex website. There is an announcement mailing list for announcements for new JFlex releases.

What platforms does JFlex support?

JFlex generates the scanner on platforms that supports JDK 6 or above.

The generated scanner runs with JRE 5 or above.

Can I use CUP and JFlex together?

You can. See section Working together: JFlex and CUP of the manual on how to do it.

Can I use the generated code of my JFlex specification commercially?

You can use your generated code without restriction. See the file copyright for more information.

My Scanner throws an “IOException: Reader returned 0 characters. See JFlex examples for workaround.”
What now?

Short answer: wrap the reader that returns 0 characters in one that blocks instead. Long answer: See the README and example solution in the example zero-reader.

I want my scanner to read from a network byte stream or from interactive stdin.
Can I do this with JFlex?

This actually depends on the syntax of the input you want to read. The problem is, that for some expressions the scanner needs one character of lookahead to decide which action to execute. For interactive use and network streams this is very inconvenient, because the stream doesn’t send an EOF (or any other data) when the user stops typing while the scanner just waits for the next character and doesn’t return a symbol. Since version 1.1.1 of JFlex this problem can be avoided because of a little more analysis at generation time. Take a look at these three rules:


When the scanner has read one a, an additional input character is needed to decide, if this matches rule 1 (just one a) or rule two (when the next character is another a). With input aaa the scanner also has to read one additional character, because it is supposed to return the longest match (so if there comes another a, the match is aaaa and not only aaa). But: When the scanner reads a “;”, it does not need an additional character and can immediately execute the action for rule number 3. This is the case for all rules that are not prefixes of any other rules in the specification and that have a fixed length end (so (a* b) is ok but (a b*) is not).

For your application this means: if all commands (or whatever units of input you have) are terminated by some delimiter (for instance “;” or LF or “end”) reading from a network bytestream or an interactive stream works fine with JFlex.

How can I let my scanner read its input from a string?

String myString = "some input";
Scanner myScanner = new Scanner( new );

Why do standalone scanners have a different standard return type (int instead of Yytoken)?

That’s because int is a predefined type in Java and Yytoken is not. If a scanner generated with %standalone option would have return type Yytoken, you would have to provide this class for every standalone scanner you write. In most cases you don’t want to do that, because the scanner wouldn’t be really standalone then. The standard Yytoken for non standalone cases stems from JLex and is only kept for compatibility (it’s rarely used anyway). If you still really want Yytoken as return type in a standalone scanner, you can always explicitly specify it with %type Yytoken. If you just want to test your scanner scanner and see what it does without a parser attached, use %debug instead of %standalone.

The expression ![a] seems to match “aa”. Is negation broken?

The semantics of the negation of an expression r is a literal everything not matched by r. The expression [a] matches strings that contain exactly one character (namely "a"). The string "aa" is not matched by this expression, hence it should be matched by ![a].

I use %8bit and get an Exception, but I know my platform only uses 8 bit. Is %8bit broken?

Short answer: not broken, use %unicode. Long answer: Most probably this is an encoding problem. Java uses Unicode internally and converts the bytes it reads from files (or somewhere else) to Unicode first. The 8 bit value of your platform may not be 8 bit anymore when converted to Unicode. On many Windows locales for instance Cp1252 (Windows-Latin-1) is used as standard encoding, and there the character “single right quotation mark” has code \x92 but after conversion to Unicode it’s \u2019 which is not 8 bit any more. See also the section on Encodings, platforms, and Unicode of the JFlex manual for more information.

My scanner needs to read a file that is not in my platforms standard encoding, but in encoding XYZ. How?

Since the scanner reads Java Unicode characters, it is independent of the actual character encoding a file or a string uses. The transformation byte-stream to Java characters for files usually happens in the object connected with the input stream. Class uses the platforms default encoding automatically. If you would like to explicitly specify another encoding, for instance UTF-8, you could do something like

Reader r = new InputStreamReader(new FileInputStream(file), "UTF8");

Now you have a Reader r that can be passed to the scanner’s constructor in the usual way. For more information on encodings see also Sun’s JDK documentation, especially in Guide to Features - Java Platform item Internationalization and there the FAQ and Supported Encodings.