JFlex User’s Manual

JFlex User’s Manual

The Fast Lexical Analyser Generator

Copyright © 1998–2020 by Gerwin Klein, Steve Rowe, and Régis Décamps.

Version 1.9.1, 11 March 2023

Table of Contents


JFlex is a lexical analyser generator for Java written in Java. It is also a rewrite of the tool JLex (Berk 1996) which was developed by Elliot Berk at Princeton University. As Vern Paxson states for his C/C++ tool flex (Paxson 1995): they do not share any code though.

A lexical analyser generator takes as input a specification with a set of regular expressions and corresponding actions. It generates a program (a lexer) that reads input, matches the input against the regular expressions in the spec file, and runs the corresponding action if a regular expression matched. Lexers usually are the first front-end step in compilers, matching keywords, comments, operators, etc, and generating an input token stream for parsers. They can also be used for many other purposes.

Design goals

The main design goals of JFlex are:

About this manual

This manual gives a brief but complete description of the tool JFlex. It assumes that you are familiar with the topic of lexical analysis in parsing. The references Aho, Sethi, and Ullman (1986) and Appel (1998) provide a good introduction.

The next section of this manual describes installation procedures for JFlex. Working with JFlex - an example runs through an example specification and explains how it works. The section on Lexical specifications presents all JFlex options and the complete specification syntax; Encodings, Platforms, and Unicode provides information about Unicode and scanning text vs. binary files. A few words on performance gives tips on how to write fast scanners. The section on porting scanners shows how to port scanners from JLex, and from the lex and flex tools for C. Finally, working together discusses interfacing JFlex scanners with the LALR parser generators CUP, CUP2, BYacc/J, Jay.

Installing and Running JFlex

Installing JFlex


To install JFlex on Windows, follow these three steps:

  1. Unzip the file you downloaded into the directory you want JFlex in. If you unzipped it to say C:\, the following directory structure should be generated:
        +--bin\                      (start scripts)
        +--doc\                      (manual)
            +--byaccj\               (calculator example for BYacc/J)
            +--common\               (shared Makefile and ant build files)
            +--cup-interpreter\      (interpreter example for cup)
            +--cup-java\             (Java lexer specification for use with cup)
            +--cup-java-minijava\    (example scanner from the manual)
            +--cup-lcalc\            (calculator example for cup and maven)
            +--simple\               (example scanner with no parser)
            +--standalone\           (a simple standalone scanner)
            +--zero-reader\          (Readers that return 0 characters)
        +--lib\                      (precompiled classes)
                +--config\           (PMD source analyzer configuration)
                +--cup\              (JFlex parser spec)
                +--java\jflex\       (source code of JFlex)
                +--jflex\            (JFlex scanner spec)
                +--resources\        (messages and default skeleton file)
            +--test\                 (unit tests)
  1. Edit the file bin\jflex.bat (in the example it’s C:\jflex-1.9.1\bin\jflex.bat) such that

    • JAVA_HOME contains the directory where your Java JDK is installed (for instance C:\java) and

    • JFLEX_HOME the directory that contains JFlex (in the example: C:\jflex-1.9.1)

  2. Include the bin\ directory of JFlex in your path. (the one that contains the start script, in the example: C:\jflex-1.9.1\bin).

Mac/Unix with tar

To install JFlex on a Mac or Unix system, follow these two steps:

You can verify the integrity of the downloaded file with the SHA1 checksum available on the JFlex download page. If you put the checksum file in the same directory as the archive, and run:

shasum --check jflex-1.9.1.tar.gz.sha1

it should tell you

jflex-1.9.1.tar.gz: OK

Running JFlex

You run JFlex with:

jflex <options> <inputfiles>

It is also possible to skip the start script in bin/ and include the file lib/jflex-1.9.1.jar in your CLASSPATH environment variable instead.

Then you run JFlex with:

java jflex.Main <options> <inputfiles>

or with:

java -jar jflex-1.9.1.jar <options> <inputfiles>

The input files and options are in both cases optional. If you don’t provide a file name on the command line, JFlex will pop up a window to ask you for one.

JFlex knows about the following options:

-d <directory>
writes the generated file to the directory <directory>

--encoding <name>
uses the character encoding <name> (e.g. utf-8) to read lexer specifications and write java files.

--skel <file>
uses external skeleton <file> in UTF-8 encoding. This is mainly for JFlex maintenance and special low level customisations. Use only when you know what you are doing! JFlex comes with a skeleton file in the src directory that reflects exactly the internal, pre-compiled skeleton and can be used with the -skel option.

skip the DFA minimisation step during scanner generation.

tries even harder to comply to JLex interpretation of specs.

generate graphviz dot files for the NFA, DFA and minimised DFA. This feature is still in alpha status, and not fully implemented yet.

display transition tables of NFA, initial DFA, and minimised DFA

dot (.) meta character matches [^\n] instead of

--verbose or -v
display generation progress messages (enabled by default)

--quiet or -q
display error messages only (no chatter about what JFlex is currently doing)

switch on warning type <warning>, where <warning> is one of
unused, never-match, empty-match, cupsym-after-cup, unicode-too-long, all.

suppress warnings of type <warning>, where <warning> is one of
unused, never-match, empty-match, cupsym-after-cup, unicode-too-long, all.

display time statistics about the code generation process (not very accurate)

print version number

print system and JDK information (useful if you’d like to report a problem)

--unicodever <ver>
print all supported properties for Unicode version <ver>

--help or -h
print a help message explaining options and usage of JFlex.

JFlex Maven plugin

The plugin reads JFlex grammar specification files (.jflex) and generates a corresponding Java parser (in target/generated-source/jflex by default).


Minimal configuration

This configuration generates java code of a parser for all grammar files (*.jflex, *.jlex, *.lex, *.flex) found in src/main/jflex/ and its sub-directories.

The name and package of the generated Java source code are the ones defined in the grammar. The generated Java source code is placed in target/generated-source/jflex, in sub-directories following the Java convention on package names.

Update the pom.xml to add the plugin:

  <!-- ... -->
    <!-- ... -->
  <!-- ... -->

More complex configuration

This example generates the source for the two grammars src/main/lex/preprocessor.jflex and /pub/postprocessor.jflex, as well as all grammar files found in src/main/jflex (and its sub-directories). The generated Java code is placed into src/main/java instead of target/generated-sources/jflex.


Even more complex configuration, using several executions

This generates the source for

            <id>strict jlex</id>

More information

JFlex Ant Task

JFlex can easily be integrated with the Ant build tool. To use JFlex with Ant, simply copy the lib/jflex-1.9.1.jar file to the $ANT_HOME/lib/ directory or explicitly set the path to lib/jflex-1.9.1.jar in the task definition (see example below).

The JFlex Ant Task invokes JFlex on a grammar file.

To use the JFlex task, place the following line in the Ant build file:

<taskdef classname="jflex.anttask.JFlexTask" name="jflex" />

Or, setting the path to the JFlex jar explicitly:

<taskdef classname="jflex.anttask.JFlexTask" name="jflex"
         classpath="path-to-jflex.jar" />

The JFlex task requires the file attribute to be set to the source grammar file (*.flex). Unless the target directory is specified with the destdir option, the generated class will be saved to the same directory where the grammar file resides. Like javac, the JFlex task creates subdirectories in destdir according to the generated class package.

This task only invokes JFlex if the grammar file is newer than the generated files.


The following attributes are available for invoking the JFlex task.


After the task definition, the <jflex ..> task is available in Ant. For example:


JFlex generates the scanner for src/parser/Scanner.flex and saves the result to build/generated/parser/, providing Scanner.flex is declared to be in package parser.


The same as above plus compile generated classes to build/classes

A simple Example: How to work with JFlex

To demonstrate how a lexical specification with JFlex looks like, this section presents a part of the specification for the Java language.

The example does not describe the whole lexical structure of Java programs, but only a small and simplified part of it: - some keywords, - some operators, - comments - and only two kinds of literals.

It also shows how to interface with the LALR parser generator CUP (Hudson 1996) and therefore uses a class sym (generated by CUP), where integer constants for the terminal tokens of the CUP grammar are declared.

You can find this example in examples/cup-java-minijava.

The examples/cup-java directory also contains a complete JFlex specification of the lexical structure of Java programs together with the CUP parser specification for Java by C. Scott Ananian, obtained from the CUP (Hudson 1996) web site (modified to interface with the JFlex scanner). Both specifications adhere to the Java Language Specification (Gosling, Joy, and Steele 1996).

In examples/standalone, you can find a small standalone scanner that doesn’t need other dependencies or tools like CUP to give you working code.

    /* JFlex example: partial Java language lexer specification */
    import java_cup.runtime.*;

     * This class is a simple example lexer.

    %class Lexer

      StringBuffer string = new StringBuffer();

      private Symbol symbol(int type) {
        return new Symbol(type, yyline, yycolumn);
      private Symbol symbol(int type, Object value) {
        return new Symbol(type, yyline, yycolumn, value);

    LineTerminator = \r|\n|\r\n
    InputCharacter = [^\r\n]
    WhiteSpace     = {LineTerminator} | [ \t\f]

    /* comments */
    Comment = {TraditionalComment} | {EndOfLineComment} | {DocumentationComment}

    TraditionalComment   = "/*" [^*] ~"*/" | "/*" "*"+ "/"
    // Comment can be the last line of the file, without line terminator.
    EndOfLineComment     = "//" {InputCharacter}* {LineTerminator}?
    DocumentationComment = "/**" {CommentContent} "*"+ "/"
    CommentContent       = ( [^*] | \*+ [^/*] )*

    Identifier = [:jletter:] [:jletterdigit:]*

    DecIntegerLiteral = 0 | [1-9][0-9]*

    %state STRING


    /* keywords */
    <YYINITIAL> "abstract"           { return symbol(sym.ABSTRACT); }
    <YYINITIAL> "boolean"            { return symbol(sym.BOOLEAN); }
    <YYINITIAL> "break"              { return symbol(sym.BREAK); }

      /* identifiers */ 
      {Identifier}                   { return symbol(sym.IDENTIFIER); }
      /* literals */
      {DecIntegerLiteral}            { return symbol(sym.INTEGER_LITERAL); }
      \"                             { string.setLength(0); yybegin(STRING); }

      /* operators */
      "="                            { return symbol(sym.EQ); }
      "=="                           { return symbol(sym.EQEQ); }
      "+"                            { return symbol(sym.PLUS); }

      /* comments */
      {Comment}                      { /* ignore */ }
      /* whitespace */
      {WhiteSpace}                   { /* ignore */ }

    <STRING> {
      \"                             { yybegin(YYINITIAL); 
                                       return symbol(sym.STRING_LITERAL, 
                                       string.toString()); }
      [^\n\r\"\\]+                   { string.append( yytext() ); }
      \\t                            { string.append('\t'); }
      \\n                            { string.append('\n'); }

      \\r                            { string.append('\r'); }
      \\\"                           { string.append('\"'); }
      \\                             { string.append('\\'); }

    /* error fallback */
    [^]                              { throw new Error("Illegal character <"+
                                                        yytext()+">"); }

From this specification JFlex generates a .java file with one class that contains code for the scanner. The class will have a constructor taking a java.io.Reader from which the input is read. The class will also have a function yylex() that runs the scanner and that can be used to get the next token from the input (in this example the function actually has the name next_token() because the specification uses the %cup switch).

As with JLex, the specification consists of three parts, divided by %%:

Code to include

Let’s take a look at the first section, user code: The text up to the first line starting with %% is copied verbatim to the top of the generated lexer class (before the actual class declaration). Next to package and import statements there is usually not much to do here. If the code ends with a javadoc class comment, the generated class will get this comment, if not, JFlex will generate one automatically.

Options and Macros

The second section options and declarations is more interesting. It consists of a set of options, code that is included inside the generated scanner class, lexical states and macro declarations. Each JFlex option must begin a line of the specification and starts with a %. In our example the following options are used:

The code between %{ and %} is copied verbatim into the generated lexer class source. Here you can declare member variables and functions that are used inside scanner actions. In our example we declare a StringBuffer string in which we will store parts of string literals and two helper functions symbol that create java_cup.runtime.Symbol objects with position information of the current token (see also JFlex and CUP for how to interface with the parser generator CUP). As with all JFlex options, both %{ and %} must begin a line.

The specification continues with macro declarations. Macros are abbreviations for regular expressions, used to make lexical specifications easier to read and understand. A macro declaration consists of a macro identifier followed by =, then followed by the regular expression it represents. This regular expression may itself contain macro usages. Although this allows a grammar-like specification style, macros are still just abbreviations and not non-terminals – they cannot be recursive. Cycles in macro definitions are detected and reported at generation time by JFlex.

Here some of the example macros in more detail:

The last part of the second section in our lexical specification is a lexical state declaration: state STRING declares a lexical state STRING that can be used in the lexical rules part of the specification. A state declaration is a line starting with %state followed by a space or comma separated list of state identifiers. There can be more than one line starting with %state.

Rules and Actions

The lexical rules section of a JFlex specification contains regular expressions and actions (Java code) that are executed when the scanner matches the associated regular expression. 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. Our specification above for instance would with input breaker match the regular expression for Identifier and not the keyword break followed by the Identifier er, because rule {Identifier} matches more of this input at once than any other rule in the specification. If two regular expressions both have the longest match for a certain input, the scanner chooses the action of the expression that appears first in the specification. In that way, we get for input break the keyword break and not an Identifier break.

In addition to regular expression matches, one can use lexical states to refine a specification. A lexical state acts like a start condition. If the scanner is in lexical state STRING, only expressions that are preceded by the start condition <STRING> can be matched. A start condition of a regular expression can contain more than one lexical state. It is then matched when the lexer is in any of these lexical states. The lexical state YYINITIAL is predefined and is also the state in which the lexer begins scanning. If a regular expression has no start conditions it is matched in all lexical states.

Since there often are sets of expressions with the same start conditions, they can be grouped:

  expr1   { action1 }
  expr2   { action2 }

means that both expr1 and expr2 have start condition <STRING>.

The first three rules in our example demonstrate the syntax of a regular expression preceded by the start condition <YYINITIAL>.

<YYINITIAL> "abstract"           { return symbol(sym.ABSTRACT); }

matches the input abstract only if the scanner is in its start state YYINITIAL. When the string abstract is matched, the scanner function returns the CUP symbol sym.ABSTRACT. If an action does not return a value, the scanning process is resumed immediately after executing the action.

The rules enclosed in


demonstrate the abbreviated syntax and are also only matched in state YYINITIAL.

Of these rules, one is of special interest:

\"  { string.setLength(0); yybegin(STRING); }

If the scanner matches a double quote in state YYINITIAL we have recognised the start of a string literal. Therefore we clear our StringBuffer that will hold the content of this string literal and tell the scanner with yybegin(STRING) to switch into the lexical state STRING. Because we do not yet return a value to the parser, our scanner proceeds immediately.

In lexical state STRING another rule demonstrates how to refer to the input that has been matched:

[^\n\r\"\\]+                   { string.append( yytext() ); }

The expression [^\n\r\"\\]+ matches all characters in the input up to the next backslash (indicating an escape sequence such as \n), double quote (indicating the end of the string), or line terminator (which must not occur in a Java string literal). The matched region of the input is referred to by yytext() and appended to the content of the string literal parsed so far.

The last lexical rule in the example specification is used as an error fallback. It matches any character in any state that has not been matched by another rule. It doesn’t conflict with any other rule because it has the least priority (because it’s the last rule) and because it matches only one character (so it can’t have longest match precedence over any other rule).

How to get it building

Lexical Specifications

As shown above, a lexical specification file for JFlex consists of three parts divided by a single line starting with %%:

Options and declarations
Lexical rules

In all parts of the specification comments of the form /* comment text */ and Java-style end-of-line comments starting with // are permitted. JFlex comments do nest - so the number of /* and */ should be balanced.

User code

The first part contains user code that is copied verbatim to the beginning of the generated source file before the scanner class declaration. As shown in the example spec, this is the place to put package declarations and import statements. It is possible, but not considered good Java style to put helper classes, such as token classes, into this section; they are usually better declared in their own .java files.

Options and declarations

The second part of the lexical specification contains options and directives to customise the generated lexer, declarations of lexical states and macro definitions.

Each JFlex directive must sit at the beginning of a line and starts with the % character. Directives that have one or more parameters are described as follows.

%class "classname"

means that you start a line with %class followed by a space followed by the name of the class for the generated scanner (the double quotes are not to be entered, see also the example specification).

Class options and user class code

These options regard name, constructor, API, and related parts of the generated scanner class.

Scanning method

This section shows how the scanning method can be customised. You can redefine the name and return type of the method and it is possible to declare exceptions that may be thrown in one of the actions of the specification. If no return type is specified, the scanning method will be declared as returning values of class Yytoken.

The end of file

There is always a default value that the scanning method will return when the end of file has been reached. 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.

The default end of file value depends on the return type of the scanning method:

User values and code to be executed at the end of file can be defined using these directives:

Standalone scanners

CUP compatibility

You may also want to read the CUP section if you are interested in how to interface your generated scanner with CUP.

BYacc/J compatibility

You may also want to read JFlex and BYacc/J if you are interested in how to interface your generated scanner with Byacc/J.

Input Character sets

Line, character and column counting


Warning directives

Warning Types

Obsolete JLex options

State declarations

State declarations have the following form:

%s[tate] "state identifier" [, "state identifier", ... ] for inclusive or
%x[state] "state identifier" [, "state identifier", ... ] for exclusive states

There may be more than one line of state declarations, each starting with %state or %xstate. State identifiers are letters followed by a sequence of letters, digits or underscores. State identifiers can be separated by white-space or comma.

The sequence

%state STATE1
%xstate STATE3, XYZ, STATE_10
%state ABC STATE5

declares the set of identifiers STATE1, STATE3, XYZ, STATE_10, ABC, STATE5 as lexical states, STATE1, ABC, STATE5 as inclusive, and STATE3, XYZ, STATE_10 as exclusive. See also How the Input is Matched on the way lexical states influence how the input is matched.

Macro definitions

A macro definition has the form

macroidentifier = regular expression

That means, a macro definition is a macro identifier (letter followed by a sequence of letters, digits or underscores), that can later be used to reference the macro, followed by optional white-space, followed by an =, followed by optional white-space, followed by a regular expression (see Lexical Rules for more information about the regular expression syntax).

The regular expression on the right hand side must be well formed and must not contain the ^, / or $ operators. Differently to JLex, macros are not just pieces of text that are expanded by copying - they are parsed and must be well formed.

This is a feature. It eliminates some very hard to find bugs in lexical specifications (such like not having parentheses around more complicated macros - which is not necessary with JFlex). See Porting from JLex for more details on the problems of JLex style macros.

Since it is allowed to have macro usages in macro definitions, it is possible to use a grammar-like notation to specify the desired lexical structure. However, macros remain just abbreviations of the regular expressions they represent. They are not non-terminals of a grammar and cannot be used recursively. JFlex detects cycles in macro definitions and reports them at generation time. JFlex also warns you about macros that have been defined but never used in the lexical rules section of the specification.

Lexical rules

The lexical rules section of a JFlex specification contains a set of regular expressions and actions (Java code) that are executed when the scanner matches the associated regular expression.

The %include directive may be used in this section to include lexical rules from a separate file. The directive will be replaced verbatim by the contents of the specified file.


The syntax of the lexical rules section is described by the following EBNF grammar (terminal symbols are enclosed in ’quotes’):

LexicalRules ::= (Include|Rule)+
Include      ::= '%include' (' '|'\t'|'\b')+ File
Rule         ::= [StateList] ['^'] RegExp [LookAhead] Action 
               | [StateList] '<<EOF>>' Action
               | StateGroup 
StateGroup   ::= StateList '{' Rule+ '}' 
StateList    ::= '<' Identifier (',' Identifier)* '>' 
LookAhead    ::= '$' | '/' RegExp
Action       ::= '{' JavaCode '}' | '|'

RegExp       ::= RegExp '|' RegExp 
               | RegExp RegExp 
               | '(' RegExp ')'
               | ('!'|'~') RegExp
               | RegExp ('*'|'+'|'?')
               | RegExp "{" Number ["," Number] "}" 
               | CharClass
               | PredefinedClass 
               | MacroUsage 
               | '"' StringCharacter+ '"' 
               | Character 

CharClass    ::= '[' ['^'] CharClassContent* ']'
               | '[' ['^'] CharClassContent+ 
                     CharClassOperator CharClassContent+ ']'
CharClassContent    ::= CharClass | Character |
                        Character'-'Character | 
                        MacroUsage | PredefinedClass

CharClassOperator   ::= '||' | '&&' | '--' | '~~'

MacroUsage          ::= '{' Identifier '}'

PredefinedClass     ::= '[:jletter:]' 
                      | '[:jletterdigit:]' 
                      | '[:letter:]' 
                      | '[:digit:]'
                      | '[:uppercase:]' 
                      | '[:lowercase:]'
                      | '\d' | '\D'
                      | '\s' | '\S'
                      | '\w' | '\W'
                      | '\p{' UnicodePropertySpec '}'
                      | '\P{' UnicodePropertySpec '}'
                      | '\R'
                      | '.'          
UnicodePropertySpec ::= BinaryProperty | 
                        EnumeratedProperty (':' | '=') PropertyValue

BinaryProperty      ::= Identifier

EnumeratedProperty  ::= Identifier

PropertyValue       ::= Identifier

The grammar uses the following terminal symbols:

Please note that the \n escape sequence stands for the ASCII LF character - not for the end of line. If you would like to match the line terminator, you should use the expression \r|\n|\r\n if you want the Java conventions, or \r\n|[\r\n\u2028\u2029\u000B\u000C\u0085] (provided as predefined class \R) if you want to be fully Unicode compliant (see also (Davis and Heninger 2013)).

The white-space characters " " (space) and \t (tab) can be used to improve the readability of regular expressions. They will be ignored by JFlex. In character classes and strings, however, white-space characters keep standing for themselves (so the string " " still matches exactly one space character and [ \n] still matches an ASCII LF or a space character).

JFlex applies the following standard operator precedences in regular expression (from highest to lowest):

So the expression a | abc | !cd* for instance is parsed as (a|(abc)) | ((!c)(d*)).


This section gives an informal description of which text is matched by a regular expression, i.e. an expression described by the RegExp production of the grammar above.

A regular expression that consists solely of

If a and b are regular expressions, then

In a lexical rule, a regular expression r may be preceded by a ^ (the beginning of line operator). r is then only matched at the beginning of a line in the input. A line begins after each occurrence of \r|\n|\r\n|\u2028|\u2029|\u000B|\u000C|\u0085 (see also (Davis and Heninger 2013)) and at the beginning of input. The preceding line terminator in the input is not consumed and can be matched by another rule.

In a lexical rule, a regular expression r may be followed by a look-ahead expression. A look-ahead expression is either $ (the end of line operator) or / followed by an arbitrary regular expression. In both cases the look-ahead is not consumed and not included in the matched text region, but it is considered while determining which rule has the longest match (see also How the input is matched).

In the $ case, r is only matched at the end of a line in the input. The end of a line is denoted by the regular expression \r|\n|\r\n|\u2028|\u2029|\u000B|\u000C|\u0085. So a$ is equivalent to a / \r|\n|\r\n|\u2028|\u2029|\u000B|\u000C|\u0085. This is different to the situation described in (Davis and Heninger 2013): since in JFlex $ is a true trailing context, the end of file does not count as end of line.

For arbitrary look-ahead (also called trailing context) the expression is matched only when followed by input that matches the trailing context.

JFlex allows lex/flex style <<EOF>> rules in lexical specifications. A rule

[StateList]  <<EOF>>    { action code }

is very similar to the %eofval directive. The difference lies in the optional StateList that may precede the <<EOF>> rule. The action code will only be executed when the end of file is read and the scanner is currently in one of the lexical states listed in StateList. The same StateGroup (see section How the input is matched) and precedence rules as in the “normal” rule case apply (i.e. if there is more than one <<EOF>> rule for a certain lexical state, the action of the one appearing earlier in the specification will be executed). <<EOF>> rules override settings of the %cup and %byaccj options and should not be mixed with the %eofval directive.

An Action consists either of a piece of Java code enclosed in curly braces or is the special | action. The | action is an abbreviation for the action of the following expression.


expression1   |
expression2   |
expression3   { some action }

is equivalent to the expanded form

expression1   { some action }
expression2   { some action }
expression3   { some action }

They are useful when working with trailing context expressions. The expression a | (c / d) | b is not a syntactically legal regular expression, but can be expressed using the | action:

a       |
c / d   |
b       { some action }

How the input is matched

When consuming its input, the scanner determines the regular expression that matches the longest portion of the input (longest match rule). If there is more than one regular expression that matches the longest portion of input (i.e. they all match the same input), the generated scanner chooses the expression that appears first in the specification. After determining the active regular expression, the associated action is executed. If there is no matching regular expression, the scanner terminates the program with an error message (if the %standalone directive has been used, the scanner prints the unmatched input to java.lang.System.out instead and resumes scanning).

Lexical states can be used to further restrict the set of regular expressions that match the current input.

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 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.

Name of the generated class

The name of the class is by default Yylex. The name is customisable with the %class directive. The input buffer of the lexer is connected with external input through the java.io.Reader object which is passed to the lexer in the generated constructor. If you provide your own constructor for the lexer, you should always chain-call the generated one to initialise the input buffer. The input buffer should not be accessed directly, but only through the advertised API (see also Scanner Methods). Its internal implementation may change between releases or skeleton files without notice.

Scanning method

The main interface to the outside world is the generated scanning method (default name yylex, default return type Yytoken). Most of its aspects are customisable (name, return type, declared exceptions etc.). If it is called, it will consume input until one of the expressions in the specification is matched or an error occurs. If an expression is matched, the corresponding action is executed. It may return a value of the specified return type (in which case the scanning method returns with this value), or, if it does not return a value, the scanner resumes consuming input until the next expression is matched. If the end of file is reached, the scanner executes the EOF action, and (also upon each further call to the scanning method) returns the specified EOF value.

Scanner methods and fields accessible in actions (API)

Generated methods and member fields in JFlex scanners are prefixed with yy to indicate that they are generated and to avoid name conflicts with user code copied into the class. 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. Instead, JFlex follows a naming convention: everything starting with zz, such as zzStartRead, is internal and subject to change without notice between JFlex releases. Methods and members of the generated class that do not have a zz prefix, such as yycharat, belong to the API that the scanner class provides to users in action code of the specification. They will remain stable and supported between JFlex releases as long as possible.

Currently, the API consists of the following methods and member fields:

Encodings, Platforms, and Unicode

This section discusses Unicode and encodings, cross platform scanning, and how to deal with binary data.

The Problem

Java aims to be implementation platform independent, yet different platforms use different ways to encode characters. Moreover, a file written on one platform, say Windows, may later be read by a scanner on another platform, for instance Linux.

If a program reads a file from disk, what it really reads is a stream of bytes. These bytes can be mapped to characters in different ways. For instance, in standard ASCII, the byte value 65 stands for the character A, and in the encoding iso-latin-1, the byte value 213 stands for the umlaut character ä, but in the encoding iso-latin-2 the value 213 is é instead. As long as one encoding is used consistently, this is no problem. Some characters may not be available in the encoding you are using, but at least the interpretation of the mapping between bytes and characters agrees between different programs.

When your program runs on more than one platform, however, as is often the case with Java, things become more complex. Java’s solution to this is to use Unicode internally. Unicode aims to be able to represent all known character sets and is therefore a perfect base for encoding things that might get used all over the world and on different platforms. To make things work correctly, you still have to know where you are and how to map byte values to Unicode characters and vice versa, but the important thing is, that this mapping is at least possible (you can map Kanji characters to Unicode, but you cannot map them to ASCII or iso-latin-1).

Scanning text files

Scanning text files is the standard application for scanners like JFlex. Therefore it should also be the most convenient one. Most times it is.

The following scenario works fine: You work on a platform X, write your lexer specification there, can use any obscure Unicode character in it as you like, and compile the program. Your users work on any platform Y (possibly but not necessarily something different from X), they write their input files on Y and they run your program on Y. No problems.

Java does this as follows: If you want to read anything in Java that is supposed to contain text, you use a FileReader, which converts the bytes of the file into Unicode characters with the platform’s default encoding. If a text file is produced on the same platform, the platform’s default encoding should do the mapping correctly. Since JFlex also uses readers and Unicode internally, this mechanism also works for the scanner specifications. If you write an A in your text editor and the editor uses the platform’s encoding (say A is 65), then Java translates this into the logical Unicode A internally. If a user writes an A on a completely different platform (say A is 237 there), then Java also translates this into the logical Unicode A internally. Scanning is performed after that translation and both match.

Note that because of this mapping from bytes to characters, you should always use the %unicode switch in you lexer specification if you want to scan text files. %8bit may not be enough, even if you know that your platform only uses one byte per character. The encoding Cp1252 used on many Windows machines for instance knows 256 characters, but the character ' with Cp1252 code \x92 has the Unicode value \u2019, which is larger than 255 and which would make your scanner throw an ArrayIndexOutOfBoundsException if it is encountered.

So for the usual case you don’t have to do anything but use the %unicode switch in your lexer specification.

Things may break when you produce a text file on platform X and consume it on a different platform Y. Let’s say you have a file written on a Windows PC using the encoding Cp1252. Then you move this file to a Linux PC with encoding ISO 8859-1 and there you run your scanner on it. Java now thinks the file is encoded in ISO 8859-1 (the platform’s default encoding) while it really is encoded in Cp1252. For most characters Cp1252 and ISO 8859-1 are the same, but for the byte values \x80 to \x9f they disagree: ISO 8859-1 is undefined there. You can fix the problem by telling Java explicitly which encoding to use. When constructing the InputStreamReader, you can give the encoding as argument. The line

Reader r = new InputStreamReader(input, Cp1252);

will do the trick.

Of course the encoding to use can also come from the data itself: for instance, when you scan an HTML page, it may have embedded information about its character encoding in the headers.

More information about encodings, which ones are supported, how they are called, and how to set them may be found in the official Java documentation in the chapter about internationalisation. The link http://docs.oracle.com/javase/7/docs/technotes/guides/intl/ leads to an online version of this for Oracle’s JDK 1.7.

Scanning binaries

Scanning binaries is both easier and more difficult than scanning text files. It’s easier because you want the raw bytes and not their meaning, i.e. you don’t want any translation. It’s more difficult because it’s not so easy to get “no translation” when you use Java readers.

The problem (for binaries) is that JFlex scanners are designed to work on text. Therefore the interface is the Reader class. You can still get a binary scanner when you write your own custom InputStreamReader class that explicitly does no translation, but just copies byte values to character codes instead. It sounds quite easy, and actually it is no big deal, but there are a few pitfalls on the way. In the scanner specification you can only enter positive character codes (for bytes that is \x00 to \xFF). Java’s byte type on the other hand is a signed 8 bit integer (-128 to 127), so you have to convert them accordingly in your custom Reader. Also, you should take care when you write your lexer spec: if you use text in there, it gets interpreted by an encoding first, and what scanner you get as result might depend on which platform you run JFlex on when you generate the scanner (this is what you want for text, but for binaries it gets in the way). If you are not sure, or if the development platform might change, it’s probably best to use character code escapes in all places, since they don’t change their meaning.

Conformance with Unicode Regular Expressions UTS#18

This section gives details about JFlex 1.9.1’s conformance with the requirements for Basic Unicode Support Level 1 given in UTS#18 (Davis and Heninger 2013).

RL1.1 Hex Notation

To meet this requirement, an implementation shall supply a mechanism for specifying any Unicode code point (from U+0000 to U+10FFFF), using the hexadecimal code point representation.

JFlex conforms. Syntax is provided to express values across the whole range, via \uXXXX, where XXXX is a 4-digit hex value; \Uyyyyyy, where yyyyyy is a 6-digit hex value; and \u{X+( X+)*}, where X+ is a 1-6 digit hex value.

RL1.2 Properties

To meet this requirement, an implementation shall provide at least a minimal list of properties, consisting of the following: General_Category, Script and Script_Extensions, Alphabetic, Uppercase, Lowercase, White_Space, Noncharacter_Code_Point, Default_Ignorable_Code_Point, ANY, ASCII, ASSIGNED.

The values for these properties must follow the Unicode definitions, and include the property and property value aliases from the UCD. Matching of Binary, Enumerated, Catalog, and Name values, must follow the Matching Rules from [UAX44].

JFlex conforms. The minimal set of properties is supported, as well as a few others. To see the full list of supported properties, use the JFlex command line option --uniprops <ver>, where <ver> is the Unicode version. Loose matching is performed: case distinctions, whitespace, underscores and hyphens in property names and values are ignored.

RL1.2a Compatibility Properties

To meet this requirement, an implementation shall provide the properties listed in Annex C: Compatibility Properties, with the property values as listed there. Such an implementation shall document whether it is using the Standard Recommendation or POSIX-compatible properties.

JFlex does not fully conform. The Standard Recommendation version of the Annex C Compatibility Properties are provided, with two exceptions: \X Extended Grapheme Clusters; and \b Default Word Boundaries.

RL1.3 Subtraction and Intersection

To meet this requirement, an implementation shall supply mechanisms for union, intersection and set-difference of Unicode sets.

JFlex conforms by providing these mechanisms, as well as symmetric difference.

RL1.4 Simple Word Boundaries

To meet this requirement, an implementation shall extend the word boundary mechanism so that:

  1. The class of <word_character> includes all the Alphabetic values from the Unicode character database, from UnicodeData.txt [UData], plus the decimals (General_Category = Decimal_Number, or equivalently Numeric_Type = Decimal), and the U+200C ZERO WIDTH NON-JOINER and U+200D ZERO WIDTH JOINER (Join_Control=True). See also Annex C: Compatibility Properties.

  2. Nonspacing marks are never divided from their base characters, and otherwise ignored in locating boundaries.

JFlex does not conform: \b does not match simple word boundaries.

RL1.5 Simple Loose Matches

To meet this requirement, if an implementation provides for case-insensitive matching, then it shall provide at least the simple, default Unicode case-insensitive matching, and specify which properties are closed and which are not.

To meet this requirement, if an implementation provides for case conversions, then it shall provide at least the simple, default Unicode case folding.

JFlex conforms. All supported Unicode Properties are closed.

RL1.6 Line Boundaries

To meet this requirement, if an implementation provides for line-boundary testing, it shall recognize not only CRLF, LF, CR, but also NEL (U+0085), PARAGRAPH SEPARATOR (U+2029) and LINE SEPARATOR (U+2028).

JFlex conforms.

RL1.7 Supplementary Code Points

To meet this requirement, an implementation shall handle the full range of Unicode code points, including values from U+FFFF to U+10FFFF. In particular, where UTF-16 is used, a sequence consisting of a leading surrogate followed by a trailing surrogate shall be handled as a single code point in matching.

JFlex conforms.

A few words on performance

This section gives tips on how to make your specification produce a faster scanner.

In general, the regular expression matching generated by JFlex has very good performance. It is DFA-based (deterministic finite automata) and does not require backtracking over alternative as for instance perl-style regular expression matching does. In the optimal case, each character is only examined once, in some situations explained below, a small amount of backtracking is necessary to determine the longest match.

Even within the class of DFA-based scanners, JFlex generated scanners usually show very good performance without special optimisations. The following lists a few heuristics that can make a lexical specification produce an even faster scanner. Those are (roughly in order of performance gain):

Note that writing more rules in a specification does not make the generated scanner slower.

The two main rules of optimisation apply also for lexical specifications:

  1. don’t do it
  2. (for experts only) don’t do it yet

Some of the performance tips above contradict a readable and compact specification style. When in doubt or when requirements are not or not yet fixed: don’t use them — the specification can always be optimised in a later state of the development process.

Porting Issues

Porting from JLex

JFlex was designed to read old JLex specifications unchanged and to generate a scanner which behaves exactly the same as the one generated by JLex with the only difference of being faster.

This works as expected on all well formed JLex specifications.

Since the statement above is somewhat absolute, let’s take a look at what well formed means for this purpose. A JLex specification is well formed, when it

Porting from lex/flex

This section gives an incomplete overview of potential pitfalls and steps for porting a lexical specification from the C/C++ tools lex and flex (Paxson 1995) available on most Unix systems to JFlex.

Most of the C/C++ specific features are naturally not present in JFlex, but most “clean” lex/flex lexical specifications can be ported to JFlex without too much work.

Basic structure

A lexical specification for flex has the following basic structure:

user code

The user code section usually contains C code that is used in actions of the rules part of the specification. For JFlex, this code will have to be translated to Java, and most of it will then go into the class code %{..%} directive in the options and declarations section.

Macros and Regular Expression Syntax

The definitions section of a flex specification is quite similar to the options and declarations part of JFlex specs.

Macro definitions in flex have the form:

<identifier>  <expression>

To port them to JFlex macros, just insert a = between <identifier> and <expression>.

The syntax and semantics of regular expressions in flex are pretty much the same as in JFlex. Some attention is needed for escape sequences present in flex (such as \a) that are not supported in JFlex. These escape sequences should be transformed into their unicode equivalent.

Character Classes

Flex offers the character classes directly supported by C, JFlex offers the ones supported by Java. These classes will sometimes have to be listed manually.

In flex more special characters lose their meaning in character classes. In particular1:

Lexical Rules

Since flex is mostly Unix based, the ’^’ (beginning of line) and ’$’ (end of line) operators, consider the \n character as only line terminator. This should usually not cause much problems, but you should be prepared for occurrences of \r or \r\n or one of the characters \u2028, \u2029, \u000B, \u000C, or \u0085. They are considered to be line terminators in Unicode and therefore may not be consumed when ^ or $ is present in a rule.

Working together

JFlex and CUP

One of the design goals of JFlex was to make interfacing with the parser generators CUP (Hudson 1996) and CUP2 (Petter 2008) as easy as possible. This has been done by providing the %cup and %cup2 directives in JFlex. However, each interface has two sides. This section concentrates on the CUP side of the story.


Please refer to the CUP2 (Petter 2008) documentation, which provides instructions on how to interface with JFlex. The CUP2 JFlex patch provided there is not necessary any more for JFlex versions greater than 1.5.0.

CUP version 0.10j and above

Since CUP version 0.10j, interfacing with JFlex has been simplified greatly by the new CUP scanner interface java_cup.runtime.Scanner. JFlex lexers now implement this interface automatically when the %cup switch is used. There are no special parser code, init code or scan with options any more that you have to provide in your CUP parser specification. You can just concentrate on your grammar.

If your generated lexer has the class name Scanner, the parser is started from the main program like this:

  try {
    parser p = new parser(new Scanner(new FileReader(fileName)));
    Object result = p.parse().value;
  catch (Exception e) {

Custom symbol interface

If you have used the -symbol command line switch of CUP to change the name of the generated symbol interface, you have to tell JFlex about this change of interface so that correct end-of-file code is generated. You can do so either by using an %eofval{ directive or by using an <<EOF>> rule.

If your new symbol interface is called mysym for example, the corresponding code in the jflex specification would be either

  return mysym.EOF;

in the macro/directives section of the spec, or it would be

  <<EOF>>  { return mysym.EOF; }

in the rules section of your spec.

Using existing JFlex/CUP specifications with CUP 0.10j and above

If you already have an existing specification and you would like to upgrade both JFlex and CUP to their newest version, you will probably have to adjust your specification.

The main difference between the %cup switch in JFlex 1.2.1 and lower, and more recent versions is that JFlex scanners now automatically implement the java_cup.runtime.Scanner interface. This means the scanning function changes its name from yylex() to next_token().

The main difference from older CUP versions to 0.10j is, that CUP now has a default constructor that accepts a java_cup.runtime.Scanner as argument and that uses this scanner as default (so no scan with code is necessary any more).

If you have an existing CUP specification, it will probably look somewhat like this:

parser code {:
  Lexer lexer;

  public parser (java.io.Reader input) {
    lexer = new Lexer(input);

scan with {: return lexer.yylex(); :};

To upgrade to CUP 0.10j, you could change it to look like this:

parser code {:
  public parser (java.io.Reader input) {
    super(new Lexer(input));

If you don’t mind changing the method that is calling the parser, you could remove the constructor entirely (and if there is nothing else in it, the whole parser code section). The main method calling the parser would then construct the parser as shown in the section above.

The JFlex specification does not need to be changed.

JFlex and BYacc/J

JFlex has built-in support for the Java extension BYacc/J (Jamison, n.d.) by Bob Jamison to the classical Berkeley Yacc parser generator. This section describes how to interface BYacc/J with JFlex. It builds on many helpful suggestions and comments from Larry Bell.

Since Yacc’s architecture is a bit different from CUP’s, the interface setup also works in a slightly different manner. BYacc/J expects a function int yylex() in the parser class that returns each next token. Semantic values are expected in a field yylval of type parserval where parser is the name of the generated parser class.

For a small calculator example, one could use a setup like the following on the JFlex side:



  /* store a reference to the parser object */
  private parser yyparser;

  /* constructor taking an additional parser object */
  public Yylex(java.io.Reader r, parser yyparser) {
    this.yyparser = yyparser;

NUM = [0-9]+ ("." [0-9]+)?
NL  = \n | \r | \r\n


/* operators */
"+" | 
"(" | 
")"    { return (int) yycharat(0); }

/* newline */
{NL}   { return parser.NL; }

/* float */
{NUM}  { yyparser.yylval = new parserval(Double.parseDouble(yytext()));
         return parser.NUM; }

The lexer expects a reference to the parser in its constructor. Since Yacc allows direct use of terminal characters like ’+’ in its specifications, we just return the character code for single char matches (e.g. the operators in the example). Symbolic token names are stored as public static int constants in the generated parser class. They are used as in the NL token above. Finally, for some tokens, a semantic value may have to be communicated to the parser. The NUM rule demonstrates how.

A matching BYacc/J parser specification would look like this:

  import java.io.*;
%token NL          /* newline  */
%token <dval> NUM  /* a number */

%type <dval> exp

%left '-' '+'
%right '^'         /* exponentiation */

exp:     NUM          { $$ = $1; }
       | exp '+' exp  { $$ = $1 + $3; }
       | exp '^' exp  { $$ = Math.pow($1, $3); }
       | '(' exp ')'  { $$ = $2; }

  /* a reference to the lexer object */
  private Yylex lexer;

  /* interface to the lexer */
  private int yylex () {
    int yyl_return = -1;
    try {
      yyl_return = lexer.yylex();
    catch (IOException e) {
      System.err.println("IO error :"+e);
    return yyl_return;

  /* error reporting */
  public void yyerror (String error) {
    System.err.println ("Error: " + error);

  /* lexer is created in the constructor */
  public parser(Reader r) {
    lexer = new Yylex(r, this);

  /* that's how you use the parser */
  public static void main(String args[]) throws IOException {
    parser yyparser = new parser(new FileReader(args[0]));

Here, the customised part is mostly in the user code section. We create the lexer in the constructor of the parser and store a reference to it for later use in the parser’s int yylex() method. This yylex in the parser only calls int yylex() of the generated lexer and passes the result on. If something goes wrong, it returns -1 to indicate an error.

Runnable versions of the specifications above are located in the examples/byaccj directory of the JFlex distribution.

JFlex and Jay

Combining JFlex with the Jay Parser Generator (Schreiner 2006) is quite simple. The Jay Parser Generator defines an interface called <parsername>.yyInput. In the JFlex source the directive

%implements <parsername>.yyInput

tells JFlex to generate the corresponding class declaration.

The three interface methods to implement are

The following shows a small example with Jay parser specification and corresponding JFlex code. First of all the Jay code (in a file MiniParser.jay):

// Prefix Code like Package declaration, 
// imports, variables and the parser class declaration

import java.io.*;
import java.util.*;

public class MiniParser 


// Token declarations, and types of non-terminals

%token <Integer> NUMBER

%token <String> NAME

%type <Gameresult> game
%type <Vector<Gameresult>> gamelist

// start symbol
%start gamelist


gamelist: game        { $$ = new Vector<Gameresult>();
  |  gamelist game    { $1.add($2); }

      $$ = new Gameresult($1, $3, $4, $6); }


  // supporting methods part of the parser class
  public static void main(String[] argv)
    MiniScanner scanner = new MiniScanner(new InputStreamReader(System.in));
    MiniParser parser = new MiniParser();
    try {
      parser.yyparse (scanner);
    } catch (final IOException ioe) {
      System.out.println("I/O Exception : " + ioe.toString());
    } catch (final MiniParser.yyException ye) {
      System.out.println ("Oops : " + ye.toString());

} // closing brace for the parser class

class Gameresult {
  String homeTeam;
  String outTeam;
  Integer homeScore;
  Integer outScore;

  public Gameresult(String ht, String ot, Integer hs, Integer os)
    homeTeam = ht;
    outTeam = ot;
    homeScore = hs;
    outScore = os;

The corresponding JFlex code (MiniScanner.jflex) could be


%class MiniScanner
%implements MiniParser.yyInput


private int token;
private Object value;

// the next 3 methods are required to implement the yyInput interface

public boolean advance() throws java.io.IOException {
  value = new String("");
  token = yylex();
  return (token != YYEOF);

public int token() {
  return token;

public Object value() {
  return value;


nl =     [\n\r]+
ws =     [ \t\b\015]+
number = [0-9]+
name =   [a-zA-Z]+
dash =   "-"
colon =  ":"


{nl}      { /* do nothing */ }
{ws}      { /* happy meal */ }
{name}    { value = yytext(); return MiniParser.NAME; }
{dash}    { return MiniParser.DASH; }
{colon}   { return MiniParser.COLON; }
{number}  { try  {
              value = Integer.valueOf(Integer.parseInt(yytext()));
            } catch (NumberFormatException nfe) {
              // shouldn't happen
              throw new Error();
            return MiniParser.NUMBER;

This small example reads an input like

Borussia - Schalke 3:2
ACMilano - Juventus 1:4

Bugs and Deficiencies


JFlex 1.9.1 conforms with Unicode Regular Expressions UTS#18 (Davis and Heninger 2013) Basic Unicode Support Level 1, with a few exceptions - for details see UTS # 18 Conformance.


As of 11 March 2023, no major open problems are known for JFlex version 1.9.1.

Please use the JFlex github issue tracker for any problems that have been reported since then.

Copying and License

JFlex is free software, published under a BSD-style license.

There is NO WARRANTY for JFlex, its code and its documentation.

See the file COPYRIGHT for more information.


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Appel, Andrew W. 1998. Modern Compiler Implementation in Java: Basic Techniques. Cambridge University Press.
Berk, Elliot. 1996. JLex: A Lexical Analyzer Generator for Java.” http://www.cs.princeton.edu/~appel/modern/java/JLex/.
Davis, Mark, and Andy Heninger. 2013. “Unicode Regular Expressions.” http://www.unicode.org/reports/tr18/tr18-17.html.
Gosling, James, Bill Joy, and Guy Steele. 1996. The Java Language Specification. Addison-Wesley. https://docs.oracle.com/javase/specs/.
Hudson, Scott E. 1996. CUP LALR Parser Generator for Java.” http://www2.cs.tum.edu/projects/cup/.
Jamison, Bob. n.d. BYacc/J.” http://byaccj.sourceforge.net.
Paxson, Vern. 1995. “Flex - the Fast Lexical Analyzer Generator.” http://flex.sourceforge.net.
Petter, Michael. 2008. CUP2 User Manual.” http://www2.in.tum.de/cup2.
Schreiner, Axel T. 2006. “Jay Parser Generator.” http://www.cs.rit.edu/~ats/projects/lp/doc/jay/package-summary.html.
“Unicode Utilities: UnicodeSet.” 2015. http://unicode.org/cldr/utility/list-unicodeset.jsp.

  1. Thanks to Dimitri Maziuk for pointing these out.↩︎