bigdata.txt

The Fast Lexical Analyser Generator
Copyright c 1998–2015 by Gerwin Klein, Steve Rowe, and Régis Décamps.
JFlex User’s Manual
Version 1.6.1, March 16, 2015
Contents
1 Introduction
1.1 Design goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 About this manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
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2 Installing and Running JFlex
2.1 Installing JFlex . . . . . . . .
2.1.1 Windows . . . . . . .
2.1.2 Unix with tar archive
2.2 Running JFlex . . . . . . . . .
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work with JFlex
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4 Lexical Specifications
4.1 User code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Options and declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.1 Class options and user class code . . . . . . . . . . . . . . . . . . . . . 12
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3 A simple Example: How to
3.1 Code to include . . . .
3.2 Options and Macros .
3.3 Rules and Actions . .
3.4 How to get it going . .
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5 Encodings, Platforms, and Unicode
5.1 The Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Scanning text files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 Scanning binaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
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6 Conformance with Unicode Regular Expressions UTS#18 33
7 A few words on performance 35
4.3
4.2.2 Scanning method . . . . . . . . . . . . .
4.2.3 The end of file . . . . . . . . . . . . . .
4.2.4 Standalone scanners . . . . . . . . . . .
4.2.5 CUP compatibility . . . . . . . . . . . .
4.2.6 BYacc/J compatibility . . . . . . . . . .
4.2.7 Character sets . . . . . . . . . . . . . .
4.2.8 Line, character and column counting . .
4.2.9 Obsolete JLex options . . . . . . . . . .
4.2.10 State declarations . . . . . . . . . . . .
4.2.11 Macro definitions . . . . . . . . . . . . .
Lexical rules . . . . . . . . . . . . . . . . . . . .
4.3.1 Syntax . . . . . . . . . . . . . . . . . . .
4.3.2 Semantics . . . . . . . . . . . . . . . . .
4.3.3 How the input is matched . . . . . . . .
4.3.4 The generated class . . . . . . . . . . .
4.3.5 Scanner methods and fields accessible in
8 Porting Issues
8.1 Porting from JLex . . . . . . . . . . .
8.2 Porting from lex/flex . . . . . . . . . .
8.2.1 Basic structure . . . . . . . . .
8.2.2 Macros and Regular Expression
8.2.3 Lexical Rules . . . . . . . . . .
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actions (API)
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10 Bugs and Deficiencies
10.1 Deficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.2 Bugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
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11 Copying and License 47
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Syntax
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9 Working together
9.1 JFlex and CUP . . . . . . . . . . . . . . . . . .
9.1.1 CUP2 . . . . . . . . . . . . . . . . . . .
9.1.2 CUP version 0.10j and above . . . . . .
9.1.3 Custom symbol interface . . . . . . . . .
9.1.4 Using existing JFlex/CUP specifications
9.2 JFlex and BYacc/J . . . . . . . . . . . . . . . .
9.3 JFlex and Jay . . . . . . . . . . . . . . . . . . .
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CUP
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.1 Introduction
JFlex is a lexical analyser generator for Java 1 written in Java. It is also a rewrite of the very
useful tool JLex [3] which was developed by Elliot Berk at Princeton University. As Vern
Paxson states for his C/C++ tool flex [13]: they do not share any code though.
1.1 Design goals
The main design goals of JFlex are:
• Full unicode support
• Fast generated scanners
• Fast scanner generation
• Convenient specification syntax
• Platform independence
• JLex compatibility
1.2 About this manual
This manual gives a brief but complete description of the tool JFlex. It assumes that you
are familiar with the issue of lexical analysis. The references [1], [2], and [15] provide a good
introduction to this topic.
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 JFlex scanner specification you
should also read Working with JFlex - an example (section 3). All options and the complete
specification syntax are presented in Lexical specifications (section 4); Encodings, Platforms,
and Unicode (section 5) provides information about scanning text vs. binary files. If you are
interested in performance considerations and comparing JLex with JFlex speed, a few words
on performance (section 7) might be just right for you. Those who want to use their old JLex
specifications may want to check out section 8.1 Porting from JLex to avoid possible problems
with not portable or non standard JLex behaviour that has been fixed in JFlex. Section 8.2
talks about porting scanners from the Unix tools lex and flex. Interfacing JFlex scanners
with the LALR parser generators CUP, CUP2 and BYacc/J is explained in working together
(section 9). Section 10 Bugs gives a list of currently known active bugs. The manual concludes
with notes about Copying and License (section 11) and references.
1
Java is a trademark of Sun Microsystems, Inc., and refers to Sun’s Java programming language. JFlex is not
sponsored by or affiliated with Sun Microsystems, Inc.
32 Installing and Running JFlex
2.1 Installing JFlex
2.1.1 Windows
To install JFlex on Windows 95/98/NT/XP, follow these three steps:
1. Unzip the file you downloaded into the directory you want JFlex in (using something
like WinZip 2 ). If you unzipped it to say C:\, the following directory structure should be
generated:
C:\jflex-1.6.1\
+--bin\
+--doc\
+--examples\
+--byaccj\
+--cup\
+--interpreter\
+--java\
+--simple-maven\
+--standalone-maven\
+--zero-reader\
+--lib\
+--src\
+--main\
+--cup\
+--java\
+--java cup\
+--runtime\
+--jflex\
+--gui\
+--jflex\
+--resources\
+--test\
(start scripts)
(FAQ and manual)
(calculator example for BYacc/J)
(calculator example for cup)
(interpreter example for cup)
(Java lexer specification)
(example scanner built with maven)
(a simple standalone scanner,
built with maven)
(example for Readers that return 0 characters)
(precompiled classes, skeleton files)
(JFlex parser spec)
(source code of cup runtime classes)
(source code of JFlex)
(source code of JFlex UI classes)
(JFlex scanner spec)
(messages and default skeleton file)
(unit tests)
2. Edit the file bin\jflex.bat (in the example it’s C:\jflex-1.6.1\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-1.6.1)
3. Include the bin\ directory of JFlex in your path. (the one that contains the start script,
in the example: C:\jflex-1.6.1\bin).
2
http://www.winzip.com
42.1.2 Unix with tar archive
To install JFlex on a Unix system, follow these two steps:
• Decompress the archive into a directory of your choice with GNU tar, for instance to
/usr/share:
tar -C /usr/share -xvzf jflex-1.6.1.tar.gz
(The example is for site wide installation. You need to be root for that. User installation
works exactly the same way—just choose a directory where you have write permission)
• Make a symbolic link from somewhere in your binary path to bin/jflex, for instance:
ln -s /usr/share/jflex-1.6.1/bin/jflex /usr/bin/jflex
If the Java interpreter is not in your binary path, you need to supply its location in the
script bin/jflex.
You can verify the integrity of the downloaded file with the MD5 checksum available on the
JFlex download page. If you put the checksum file in the same directory as the archive, you
run:
md5sum --check jflex-1.6.1.tar.gz.md5
It should tell you
jflex-1.6.1.tar.gz:
OK
2.2 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-1.6.1.jar
in your CLASSPATH environment variable instead.
Then you run JFlex with:
java jflex.Main <options> <inputfiles>
or with:
java -jar jflex-1.6.1.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>
--skel <file>
uses external skeleton <file>. 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.
5--nomin
skip the DFA minimisation step during scanner generation.
--jlex
tries even harder to comply to JLex interpretation of specs.
--dot
generate graphviz dot files for the NFA, DFA and minimised DFA. This feature is still
in alpha status, and not fully implemented yet.
--dump
display transition tables of NFA, initial DFA, and minimised DFA
--legacydot
dot (.) meta character matches [^\n] instead of
[^\n\r\u000B\u000C\u0085\u2028\u2029]
--noinputstreamctor
don’t include an InputStream constructor in the generated scanner
--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)
--warn-unused
warn about unused macros (by default true in verbose mode and false in quiet mode)
--no-warn-unused
do not warn about unused macros (by default true in verbose mode and false in quiet
mode)
--time
display time statistics about the code generation process (not very accurate)
--version
print version number
--info
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.
3 A simple Example: How to work with JFlex
To demonstrate what 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 [8] and therefore uses a class sym (generated by CUP), where
6integer constants for the terminal tokens of the CUP grammar are declared. JFlex comes
with a directory examples, where you can find a small standalone scanner that doesn’t need
other tools like CUP to give you a running example. The ”examples” 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 [8] web site
(it was modified to interface with the JFlex scanner). Both specifications adhere to the Java
Language Specification [7].
/* JFlex example: part of Java language lexer specification */
import java_cup.runtime.*;
/**
* This class is a simple example lexer.
*/
%%
%class Lexer
%unicode
%cup
%line
%column
%{
StringBuffer string = new StringBuffer();
private Symbol symbol(int
return new Symbol(type,
}
private Symbol symbol(int
return new Symbol(type,
}
type) {
yyline, yycolumn);
type, Object value) {
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]*
7%state STRING
%%
/* keywords
<YYINITIAL>
<YYINITIAL>
<YYINITIAL>
*/
"abstract"
"boolean"
"break"
<YYINITIAL> {
/* identifiers */
{Identifier}
{ return symbol(sym.ABSTRACT); }
{ return symbol(sym.BOOLEAN); }
{ return symbol(sym.BREAK); }
{ 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> {
\"
[^\n\r\"\\]+
\\t
\\n { yybegin(YYINITIAL);
return symbol(sym.STRING_LITERAL,
string.toString()); }
{ string.append( yytext() ); }
{ string.append(’\t’); }
{ 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
8is 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 %%:
• usercode,
• options and declarations and
• lexical rules.
3.1 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).
Beside 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.
3.2 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:
• %class Lexer tells JFlex to give the generated class the name “Lexer” and to write the
code to a file “Lexer.java”.
• %unicode defines the set of characters the scanner will work on. For scanning text files,
%unicode should always be used. The Unicode version may be specified, e.g. %unicode
4.1. If no version is specified, the most recent supported Unicode version will be used -
in JFlex 1.6.1, this is Unicode 7.0. See also section 5 for more information on character
sets, encodings, and scanning text vs. binary files.
• %cup switches to CUP compatibility mode to interface with a CUP generated parser.
• %line switches line counting on (the current line number can be accessed via the variable
yyline)
• %column switches column counting on (current column is accessed via yycolumn)
The code included in %{...%} 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 section 9.1 JFlex and CUP for how to interface with the
parser generator CUP). As 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
9it 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 or mutually recursive. Cycles in macro definitions are detected and
reported at generation time by JFlex.
Here some of the example macros in more detail:
• LineTerminator stands for the regular expression that matches an ASCII CR, an ASCII
LF or an CR followed by LF.
• InputCharacter stands for all characters that are not a CR or LF.
• TraditionalComment is the expression that matches the string "/*" followed by a
character that is not a *, followed by anything that does not contain, but ends in "*/".
As this would not match comments like /****/, we add "/*" followed by an arbitrary
number (at least one) of "*" followed by the closing "/". This is not the only, but one
of the simpler expressions matching non-nesting Java comments. It is tempting to just
write something like the expression "/*" .* "*/", but this would match more than we
want. It would for instance match the whole of /* */ x = 0; /* */, instead of two
comments and four real tokens. See DocumentationComment and CommentContent for
an alternative.
• CommentContent matches zero or more occurrences of any character except a * or any
number of * followed by a character that is not a /
• Identifier matches each string that starts with a character of class jletter followed
by zero or more characters of class jletterdigit. jletter and jletterdigit are
predefined character classes. jletter includes all characters for which the Java function
Character.isJavaIdentifierStart returns true and jletterdigit all characters for
that Character.isJavaIdentifierPart returns true.
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.
3.3 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 (i.e. it matches all of it) 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”.
Additional to regular expression matches, one can use lexical states to refine a specification.
10A 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 you often have a bunch of expressions with the same start conditions, JFlex allows the
same abbreviation as the Unix tool flex:
<STRING> {
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
<YYINITIAL> {
...
}
demonstrate the abbreviated syntax and are also only matched in state YYINITIAL.
Of these rules, one may be 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 string literal). The matched region of the input is
referred to with 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
11other 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).
3.4 How to get it going
• Install JFlex (see section 2 Installing JFlex )
• If you have written your specification file (or chosen one from the examples directory),
save it (say under the name java-lang.flex).
• Run JFlex with
jflex java-lang.flex
• JFlex should then report some progress messages about generating the scanner and
write the generated code to the directory of your specification file.
• Compile the generated .java file and your own classes. (If you use CUP, generate your
parser classes first)
• That’s it.
4 Lexical Specifications
As shown above, a lexical specification file for JFlex consists of three parts divided by a single
line starting with %%:
UserCode
%%
Options and declarations
%%
Lexical rules
In all parts of the specification comments of the form /* comment text */ and the Java style
end of line comments starting with // are permitted. JFlex comments do nest - so the number
of /* and */ should be balanced.
4.1 User code
The first part contains user code that is copied verbatim into the beginning of the source file
of the generated lexer before the scanner class is declared. As shown in the example above,
this is the place to put package declarations and import statements. It is possible, but not
considered good Java programming style to put own helper classes (such as token classes) in
this section. They should get their own .java file instead.
4.2 Options and declarations
The second part of the lexical specification contains options to customise your generated lexer
(JFlex directives and Java code to include in different parts of the lexer), declarations of
12lexical states and macro definitions for use in the third section “Lexical rules” of the lexical
specification file.
Each JFlex directive must be situated 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 the example
specification in section 3).
4.2.1 Class options and user class code
These options regard name, constructor, API, and related parts of the generated scanner class.
• %class "classname"
Tells JFlex to give the generated class the name ”classname” and to write the generated
code to a file ”classname.java”. If the -d <directory> command line option is not
used, the code will be written to the directory where the specification file resides. If no
%class directive is present in the specification, the generated class will get the name
”Yylex” and will be written to a file ”Yylex.java”. There should be only one %class
directive in a specification.
• %implements "interface 1"[, "interface 2", ..]
Makes the generated class implement the specified interfaces. If more than one %imple-
ments directive is present, all the specified interfaces will be implemented.
• %extends "classname"
Makes the generated class a subclass of the class “classname”. There should be only
one %extends directive in a specification.
• %public
Makes the generated class public (the class is only accessible in its own package by
default).
• %final
Makes the generated class final.
• %abstract
Makes the generated class abstract.
• %apiprivate
Makes all generated methods and fields of the class private. Exceptions are the construc-
tor, user code in the specification, and, if %cup is present, the method next token. All
occurrences of " public " (one space character before and after public) in the skeleton
file are replaced by " private " (even if a user-specified skeleton is used). Access to
the generated class is expected to be mediated by user class code (see next switch).
• %{
...
13%}
The code enclosed in %{ and %} is copied verbatim into the generated class. Here you
can define your own member variables and functions in the generated scanner. Like all
options, both %{ and %} must start a line in the specification. If more than one class
code directive %{...%} is present, the code is concatenated in order of appearance in
the specification.
• %init{
...
%init}
The code enclosed in %init{ and %init} is copied verbatim into the constructor of
the generated class. Here, member variables declared in the %{...%} directive can be
initialised. If more than one initialiser option is present, the code is concatenated in
order of appearance in the specification.
• %initthrow{
"exception1"[, "exception2", ...]
%initthrow}
or (on a single line) just
%initthrow "exception1" [, "exception2", ...]
Causes the specified exceptions to be declared in the throws clause of the constructor. If
more than one %initthrow{ ... %initthrow} directive is present in the specification,
all specified exceptions will be declared.
• %ctorarg "type" "ident"
Adds the specified argument to the constructors of the generated scanner. If more
than one such directive is present, the arguments are added in order of occurrence in
the specification. Note that this option conflicts with the %standalone and %debug
directives, because there is no sensible default that can be created automatically for such
parameters in the generated main methods. JFlex will warn in this case and generate
an additional default constructor without these parameters and without user init code
(which might potentially refer to the parameters).
• %scanerror "exception"
Causes the generated scanner to throw an instance of the specified exception in case
of an internal error (default is java.lang.Error). Note that this exception is only for
internal scanner errors. With usual specifications it should never occur (i.e. if there is an
error fallback rule in the specification and only the documented scanner API is used).
• %buffer "size"
Set the initial size of the scan buffer to the specified value (decimal, in bytes). The
default value is 16384.
• %include "filename"
Replaces the %include verbatim by the specified file. This feature is still experimental.
It works, but error reporting can be strange if a syntax error occurs on the last token in
the included file.
144.2.2 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.
• %function "name"
Causes the scanning method to get the specified name. If no %function directive is
present in the specification, the scanning method gets the name “yylex”. This directive
overrides settings of the %cup switch. Please note that the default name of the scanning
method with the %cup switch is next token. Overriding this name might lead to the
generated scanner being implicitly declared as abstract, because it does not provide
the method next token of the interface java cup.runtime.Scanner. It is of course
possible to provide a dummy implementation of that method in the class code section if
you still want to override the function name.
• %integer
%int
Both cause the scanning method to be declared as of Java type int. Actions in the
specification can then return int values as tokens. The default end of file value under
this setting is YYEOF, which is a public static final int member of the generated
class.
• %intwrap
Causes the scanning method to be declared as of the Java wrapper type Integer. Actions
in the specification can then return Integer values as tokens. The default end of file
value under this setting is null.
• %type "typename"
Causes the scanning method to be declared as returning values of the specified type.
Actions in the specification can then return values of typename as tokens. The de-
fault end of file value under this setting is null. If typename is not a subclass of
java.lang.Object, you should specify another end of file value using the %eofval{ ...
%eofval} directive or the <<EOF>> rule. The %type directive overrides settings of the
%cup switch.
• %yylexthrow{
"exception1"[, "exception2", ...
%yylexthrow}
]
or (on a single line) just
%yylexthrow "exception1" [, "exception2", ...]
The exceptions listed inside %yylexthrow{ ... %yylexthrow} will be declared in the
throws clause of the scanning method. If there is more than one %yylexthrow{ ...
%yylexthrow} clause in the specification, all specified exceptions will be declared.
154.2.3 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:
• For %integer, the scanning method will return the value YYEOF, which is a public
static final int member of the generated class.
• For %intwrap,
• no specified type at all, or a
• user defined type, declared using %type, the value is null.
• In CUP compatibility mode, using %cup, the value is
new java cup.runtime.Symbol(sym.EOF)
User values and code to be executed at the end of file can be defined using these directives:
• %eofval{
...
%eofval}
The code included in %eofval{ ... %eofval} will be copied verbatim into the scanning
method and will be executed each time when the end of file is reached (this is possible
when the scanning method is called again after the end of file has been reached). The code
should return the value that indicates the end of file to the parser. There should be only
one %eofval{ ... %eofval} clause in the specification. The %eofval{ ... %eofval}
directive overrides settings of the %cup switch and %byaccj switch. As of version 1.2
JFlex provides a more readable way to specify the end of file value using the <<EOF>>
rule (see also section 4.3.2).
• %eof{
...
%eof}
The code included in %{eof ... %eof} will be executed exactly once, when the end
of file is reached. The code is included inside a method void yy do eof() and should
not return any value (use %eofval{...%eofval} or <<EOF>> for this purpose). If more
than one end of file code directive is present, the code will be concatenated in order of
appearance in the specification.
• %eofthrow{
"exception1"[,"exception2", ...
%eofthrow}
]
or (on a single line) just
%eofthrow "exception1" [, "exception2", ...]
The exceptions listed inside %eofthrow{...%eofthrow} will be declared in the throws
clause of the method yy do eof() (see %eof for more on that method). If there is more
than one %eofthrow{...%eofthrow} clause in the specification, all specified exceptions
16will be declared.
• %eofclose
Causes JFlex to close the input stream at the end of file. The code yyclose() is appended
to the method yy do eof() (together with the code specified in %eof{...%eof}) and
the exception java.io.IOException is declared in the throws clause of this method
(together with those of %eofthrow{...%eofthrow})
• %eofclose false
Turns the effect of %eofclose off again (e.g. in case closing of input stream is not
wanted after %cup).
4.2.4 Standalone scanners
• %debug
Creates a main function in the generated class that expects the name of an input file on
the command line and then runs the scanner on this input file by printing information
about each returned token to the Java console until the end of file is reached. The
information includes: line number (if line counting is enabled), column (if column
counting is enabled), the matched text, and the executed action (with line number in
the specification).
• %standalone
Creates a main function in the generated class that expects the name of an input file on
the command line and then runs the scanner on this input file. The values returned by
the scanner are ignored, but any unmatched text is printed to the Java console instead
(as the C/C++ tool flex does, if run as standalone program). To avoid having to use
an extra token class, the scanning method will be declared as having default type int,
not YYtoken (if there isn’t any other type explicitly specified). This is in most cases
irrelevant, but could be useful to know when making another scanner standalone for
some purpose. You should also consider using the %debug directive, if you just want to
be able to run the scanner without a parser attached for testing etc.
4.2.5 CUP compatibility
You may also want to read section 9.1 JFlex and CUP if you are interested in how to interface
your generated scanner with CUP.
• %cup
The %cup directive enables the CUP compatibility mode and is equivalent to the following
set of directives:
%implements java_cup.runtime.Scanner
%function next_token
%type java_cup.runtime.Symbol
%eofval{
return new java_cup.runtime.Symbol(<CUPSYM>.EOF);
17%eofval}
%eofclose
The value of <CUPSYM> defaults to sym and can be changed with the %cupsym directive.
In JLex compatibility mode (--jlex switch on the command line), %eofclose will not
be turned on.
• %cup2
The %cup2 directive is similar to CUP mode, just for the CUP2 generator from TU
Munich at http://www2.in.tum.de/cup2. It does the following:
– adds CUP2 package import declarations
– implements the CUP2 scanner interface
– switches on line and column count
– sets the scanner function to readNextTerminal
– sets the token type to ScannerToken<? extends Object>
– returns the special CUP2 EOF token at end of file
– switches on unicode
• %cupsym "classname"
Customises the name of the CUP generated class/interface containing the names of
terminal tokens. Default is sym. The directive should not be used after %cup, but before.
• %cupdebug
Creates a main function in the generated class that expects the name of an input file
on the command line and then runs the scanner on this input file. Prints line, column,
matched text, and CUP symbol name for each returned token to standard out.
4.2.6 BYacc/J compatibility
You may also want to read section 9.2 JFlex and BYacc/J if you are interested in how to
interface your generated scanner with Byacc/J.
• %byacc
The %byacc directive enables the BYacc/J compatibility mode and is equivalent to the
following set of directives:
%integer
%eofval{
return 0;
%eofval}
%eofclose
4.2.7 Character sets
• %7bit
18Causes the generated scanner to use an 7 bit input character set (character codes 0-127).
If an input character with a code greater than 127 is encountered in an input at runtime,
the scanner will throw an ArrayIndexOutofBoundsException. Not only because of this,
you should consider using the %unicode directive. See also section 5 for information
about character encodings. This is the default in JLex compatibility mode.
• %full
%8bit
Both options cause the generated scanner to use an 8 bit input character set (character
codes 0-255). If an input character with a code greater than 255 is encountered in an
input at runtime, the scanner will throw an ArrayIndexOutofBoundsException. Note
that even if your platform uses only one byte per character, the Unicode value of a
character may still be greater than 255. If you are scanning text files, you should consider
using the %unicode directive. See also section 5 for more information about character
encodings.
• %unicode
%16bit
Both options cause the generated scanner to use the full Unicode input character set,
including supplementary code points: 0-0x10FFFF. %unicode does not mean that the
scanner will read two bytes at a time. What is read and what constitutes a character
depends on the runtime platform. See also section 5 for more information about character
encodings. This is the default unless the JLex compatibility mode is used (command
line option --jlex).
• %caseless
%ignorecase
This option causes JFlex to handle all characters and strings in the specification as if
they were specified in both uppercase and lowercase form. This enables an easy way
to specify a scanner for a language with case insensitive keywords. The string ”break”
in a specification is for instance handled like the expression ([bB][rR][eE][aA][kK]).
The %caseless option does not change the matched text and does not affect character
classes. So [a] still only matches the character a and not A, too. Which letters are
uppercase and which lowercase letters, is defined by the Unicode standard. In JLex
compatibility mode (--jlex switch on the command line), %caseless and %ignorecase
also affect character classes.
4.2.8 Line, character and column counting
• %char
Turns character counting on. 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.
• %line
Turns line counting on. The int member variable yyline contains the number of lines
(starting with 0) from the beginning of input to the beginning of the current token.
19• %column
Turns column counting on. The int member variable yycolumn contains the number of
characters (starting with 0) from the beginning of the current line to the beginning of
the current token.
4.2.9 Obsolete JLex options
• %notunix
This JLex option is obsolete in JFlex but still recognised as valid directive. It used
to switch between Windows and Unix kind of line terminators (\r\n and \n) for the
$ operator in regular expressions. JFlex always recognises both styles of platform
dependent line terminators.
• %yyeof
This JLex option is obsolete in JFlex but still recognised as valid directive. In JLex it
declares a public member constant YYEOF. JFlex declares it in any case.
4.2.10 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
(the first character is sufficient, %s and %x works, too). 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
section 4.3.3 on the way lexical states influence how the input is matched.
4.2.11 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 section 4.3 lexical rules for more information about regular expressions).
20The 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 section 8.1 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. Macros however remain just abbreviations
of the regular expressions they represent. They are not non terminals of a grammar and
cannot be used recursively in any way. 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.
4.3 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.
4.3.1 Syntax
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
21| 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:
• File
a file name, either absolute or relative to the directory containing the lexical specification.
• JavaCode
a sequence of BlockStatements as described in the Java Language Specification [7],
section 14.2.
• Number
22a non negative decimal integer.
• Identifier
a letter [a-zA-Z] followed by a sequence of zero or more letters, digits or underscores
[a-zA-Z0-9_]
• Character
an escape sequence or any unicode character that is not one of these meta characters:
| ( ) { } [ ] < > \ . * + ? ^ $ / . " ~ !
• StringCharacter
an escape sequence or any unicode character that is not one of these meta characters:
\ "
• An escape sequence
– \n \r \t \f \b
– a \x followed by two hexadecimal digits [a-fA-F0-9] (denoting a standard ASCII
escape sequence);
– a \u followed by four hexadecimal digits [a-fA-F0-9] (denoting a unicode escape
sequence);
– a \U (note that the ’U’ is uppercase) followed by six hexadecimal digits [a-fA-F0-9]
(denoting a unicode code point escape sequence);
– \u{H+( H+)*}, where H+ is one or more hexadecimal digits [a-fA-F0-9], each H+
denotes a code point - note that in character classes, only one code point is allowed;
– a backslash followed by a three digit octal number from 000 to 377 (denoting a
standard ASCII escape sequence); or
– a backslash followed by any other unicode character that stands for this character.
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 [5]).
As of version 1.1 of JFlex 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):
• unary postfix operators (’*’, ’+’, ’?’, {n}, {n,m})
• unary prefix operators (’!’, ’~’)
• concatenation (RegExp::= RegExp Regexp)
• union (RegExp::= RegExp ’|’ RegExp)
So the expression a | abc | !cd* for instance is parsed as (a|(abc)) | ((!c)(d*)).
234.3.2 Semantics
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 presented above).
A regular expression that consists solely of
• a Character matches this character.
• a character class ’[...]’ matches any character in that class. A Character is to be
considered an element of a class, if it is listed in the class or if its code lies within a
listed character range Character’-’Character or Macro or predefined character class.
So [a0-3\n] for instance matches the characters
a 0 1 2 3 \n
If the list of characters is empty (i.e. just []), the expression matches nothing at all
(the empty set), not even the empty string. This may be useful in combination with the
negation operator ’!’.
Character sets may be nested, e.g. [[[abc]d[e]]fg] is equivalent to [abcdefg].
Supported character set operations:
– Union (||), e.g. [[a-c]||[d-f]], equivalent to [a-cd-f]: this is the default
character set operation when no operator is specified.
– Intersection (&&), e.g. [[a-f]&&[f-m]], equivalent to [f].
– Set difference (--), e.g. [[a-z]--m], equivalent to [a-ln-z].
– Symmetric difference (~~): the union of two classes minus their intersection. For in-
stance [\p{Letter}~~\p{ASCII}] is equivalent to [[\p{Letter}||\p{ASCII}]--
[\p{Letter}&&\p{ASCII}]]: the set of characters that are present in either
\p{Letter} or in \p{ASCII}, but not in both.
• a negated character class ’[^...]’ matches all characters not listed in the class. If the
list of characters is empty (i.e. [^]), the expression matches any character of the input
character set.
• a string ’"’ StringCharacter+ ’"’ matches the exact text enclosed in double quotes.
All meta characters but \ and " lose their special meaning inside a string. See also the
%ignorecase switch.
• a macro usage ’{’ Identifier ’}’ matches the input that is matched by the right
hand side of the macro with name ”Identifier”.
• a predefined character class matches any of the characters in that class. There are the
following predefined character classes:
– These two predefined character classes are determined by Java functions of class
java.lang.Character:
[:jletter:]
[:jletterdigit:]
isJavaIdentifierStart()
isJavaIdentifierPart()
– These four predefined character classes are equivalent to the following Unicode
24properties (described below):
[:letter:]
[:digit:]
[:uppercase:]
[:lowercase:]
\p{Letter}
\p{Digit}
\p{Uppercase}
\p{Lowercase}
– These meta characters are equivalent to the following (sets of) Unicode Properties
(described below):
\d
\D
\s
\S
\w
\W
\p{Digit}
\P{Digit}
\p{Whitespace}
\P{Whitespace}
[\p{Alpha}\p{Digit}\p{Mark}
\p{Connector Punctuation}\p{Join Control}]
[^\p{Alpha}\p{Digit}\p{Mark}
\p{Connector Punctuation}\p{Join Control}]
– Unicode Properties are character classes specified by each version of the Unicode
Standard. JFlex supports a subset of all defined Properties for each supported
Unicode version. To see the full list of supported Properties, give the --uniprops
<ver> option on the JFlex command line, where <ver> is the Unicode version. Some
Properties have aliases; JFlex recognizes all aliases for all supported properties.
JFlex supports loose matching of Properties: case distinctions, whitespace, hyphens,
and underscores are ignored.
To refer to a Unicode Property, use the \p{...} syntax, e.g. the Greek Block can
be referred to as \p{Block:Greek}. To match the all characters not included in a
property, use the \P{...} syntax (note that the ’P’ is uppercase), e.g. to match all
characters that are not letters: \P{Letter}.
See UTS#18 [5] for a description of and links to definitions of some supported
Properties. UnicodeSet [16] is an online utility to show the character sets corre-
sponding to Unicode Properties and set operations on them, but only for the most
recent Unicode version.
– Dot (.) matches [^\r\n\u2028\u2029\u000B\u000C\u0085].
Use the --legacydot option to instead match [^\n].
– \R matches any newline: \r\n|[\r\n\u2028\u2029\u000B\u000C\u0085].
If a and b are regular expressions, then
a | b (union)
is the regular expression that matches all input matched by a or by b.
a b (concatenation)
is the regular expression that matches the input matched by a followed by the input
matched by b.
25a* (Kleene closure)
matches zero or more repetitions of the input matched by a
a+ (iteration)
is equivalent to aa*
a? (option)
matches the empty input or the input matched by a
!a (negation)
matches everything but the strings matched by a. Use with care: the construction of !a
involves an additional, possibly exponential NFA to DFA transformation on the NFA for
a. Note that with negation and union you also have (by applying DeMorgan) intersection
and set difference: the intersection of a and b is !(!a|!b), the expression that matches
everything of a not matched by b is !(!a|b)
~a (upto)
matches everything up to (and including) the first occurrence of a text matched by a.
The expression ~a is equivalent to !([^]* a [^]*) a. A traditional C-style comment
is matched by "/*" ~"*/"
a{n} (repeat)
is equivalent to n times the concatenation of a. So a{4} for instance is equivalent to the
expression a a a a. The decimal integer n must be positive.
a{n,m} is equivalent to at least n times and at most m times the concatenation of a. So a{2,4}
for instance is equivalent to the expression a a a? a?. Both n and m are non negative
decimal integers and m must not be smaller than n.
( a ) matches the same input as a.
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 [5]) 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 a ’$’ (the end of line operator) or a ’/’ followed by an arbitrary
regular expression. In both cases the look-ahead is not consumed and not included in the
matched text region, but it is considered while determining which rule has the longest match
(see also 4.3.3 How the input is matched ).
In the ’$’ 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 [5]: 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.
26As of version 1.2, JFlex allows lex/flex style <<EOF>> rules in lexical specifications. A rule
[StateList]
<<EOF>>
{ some action code }
is very similar to the %eofval directive (section 4.2.3). 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 4.3.3 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.
Example:
expression1
expression2
expression3
|
|
{ some action }
is equivalent to the expanded form
expression1
expression2
expression3
{ some action }
{ some action }
{ some action }
They are useful when you work with trailing context expressions. The expression a | (c /
d) | b is not syntactically legal, but can easily be expressed using the | action:
a
c / d
b
|
|
{ some action }
4.3.3 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.
27• A regular expression can only be matched when its associated set of lexical states includes
the currently active lexical state of the scanner or if the set of associated lexical states is
empty and the currently active lexical state is inclusive. Exclusive and inclusive states
only differ at this point: rules with an empty set of associated states.
• The currently active lexical state of the scanner can be changed from within an action
of a regular expression using the method yybegin().
• The scanner starts in the inclusive lexical state YYINITIAL, which is always declared by
default.
• The set of lexical states associated with a regular expression is the StateList that
precedes the expression. If a rule is contained in one or more StateGroups, then the
states of these are also associated with the rule, i.e. they accumulate over StateGroups.
Example:
%states A, B
%xstates C
%%
expr1
<YYINITIAL, A> expr2
<A> {
expr3
<B,C> expr4
}
{ yybegin(A); action }
{ action }
{ action }
{ action }
The first line declares two (inclusive) lexical states A and B, the second line an exclusive
lexical state C. The default (inclusive) state YYINITIAL is always implicitly there and
doesn’t need to be declared. The rule with expr1 has no states listed, and is thus
matched in all states but the exclusive ones, i.e. A, B, and YYINITIAL. In its action, the
scanner is switched to state A. The second rule expr2 can only match when the scanner
is in state YYINITIAL or A. The rule expr3 can only be matched in state A and expr4 in
states A, B, and C.
• Lexical states are declared and used as Java int constants in the generated class under
the same name as they are used in the specification. There is no guarantee that the
values of these integer constants are distinct. They are pointers into the generated DFA
table, and if JFlex recognises two states as lexically equivalent (if they are used with the
exact same set of regular expressions), then the two constants will get the same value.
4.3.4 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.
The name of the class is by default Yylex, it is customisable with the %class directive (see
also section 4.2.1). The input buffer of the lexer is connected with an input stream over the
28java.io.Reader object which is passed to the lexer in the generated constructor. If you want
to provide your own constructor for the lexer, you should always call the generated one in it
to initialise the input buffer. The input buffer should not be accessed directly, but only over
the advertised API (see also section 4.3.5). Its internal implementation may change between
releases or skeleton files without notice.
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., see also section 4.2.2). 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 doesn’t 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 (see also section 4.2.3).
4.3.5 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 a zz prefix like zzStartRead is to
be considered internal and subject to change without notice between JFlex releases. Methods
and members of the generated class that do not have a zz prefix like 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:
• String yytext()
returns the matched input text region
• int yylength()
returns the length of the matched input text region (does not require a String object
to be created)
• char yycharat(int pos)
returns the character at position pos from the matched text. It is equivalent to
yytext().charAt(pos), but faster. pos must be a value from 0 to yylength()-1.
• void yyclose()
closes the input stream. All subsequent calls to the scanning method will return the end
of file value
• void yyreset(java.io.Reader reader)
closes the current input stream, and resets the scanner to read from a new Reader. All
internal variables are reset, the old Reader cannot be reused (content of the internal
buffer is discarded and lost). The lexical state is set to YY INITIAL. The %{init code is
not included in yyreset, because it is assumed to run in the context of a constructor,
not a normal method. If %{init does need to be repeated, consider constructing a
29new lexer object instead or additionally calling a custom function that performs any
additional user-level state reset.
• void yypushStream(java.io.Reader reader)
Stores the current input stream on a stack, and reads from a new stream. Lexical state,
line, char, and column counting remain untouched. The current input stream can be
restored with yypopStream (usually in an <<EOF>> action).
A typical example for this are include files in style of the C pre-processor. The corre-
sponding JFlex specification could look somewhat like this:
"#include" {FILE} { yypushStream(new FileReader(getFile(yytext()))); }
..
<<EOF>>
{ if (yymoreStreams()) yypopStream(); else return EOF; }
This method is only available in the skeleton file skeleton.nested. You can find it in
the src directory of the JFlex distribution.
• void yypopStream()
Closes the current input stream and continues to read from the one on top of the stream
stack.
This method is only available in the skeleton file skeleton.nested. You can find it in
the src directory of the JFlex distribution.
• boolean yymoreStreams()
Returns true iff there are still streams for yypopStream left to read from on the stream
stack.
This method is only available in the skeleton file skeleton.nested. You can find it in
the src directory of the JFlex distribution.
• int yystate()
returns the current lexical state of the scanner.
• void yybegin(int lexicalState)
enters the lexical state lexicalState
• void yypushback(int number)
pushes number characters of the matched text back into the input stream. They will
be read again in the next call of the scanning method. The number of characters to be
read again must not be greater than the length of the matched text. The pushed back
characters will not be included in yylength() and yytext(). Note that in Java strings
are unchangeable, i.e. an action code like
String matched = yytext();
yypushback(1);
return matched;
will return the whole matched text, while
yypushback(1);
return yytext();
30will return the matched text minus the last character.
• int yyline
contains the current line of input (starting with 0, only active with the %line directive)
• int yychar
contains the current character count in the input (starting with 0, only active with the
%char directive)
• int yycolumn
contains the current column of the current line (starting with 0, only active with the
%column directive)
5 Encodings, Platforms, and Unicode
This section tries to shed some light on the issues of Unicode and encodings, cross platform
scanning, and how to deal with binary data. My thanks go to Stephen Ostermiller for his
input on this topic.
5.1 The Problem
Before we dive straight into details, let’s take a look at what the problem is. The problem
is Java’s platform independence when you want to use it. For scanners the interesting part
about platform independence is character encodings and how they are handled.
If a program reads a file from disk, it gets a stream of bytes. In earlier times, when the grass
was green, and the world was much simpler, everybody knew that the byte value 65 is, of
course, an A. It was no problem to see which bytes meant which characters (actually these
times never existed, but anyway). The normal Latin alphabet only has 26 characters, so 7
bits or 128 distinct values should surely be enough to map them, even if you allow yourself
the luxury of upper and lower case. Nowadays, things are different. The world suddenly
grew much larger, and all kinds of people wanted all kinds of special characters, just because
they use them in their language and writing. This is were the mess starts. Since the 128
distinct values were already filled up with other stuff, people began to use all 8 bits of the
byte, and extended the byte/character mappings to fit their need, and of course everybody did
it differently. Some people for instance may have said “let’s use the value 213 for the German
character ä”. Others may have found that 213 should much rather mean é, because they
didn’t need German and wrote French instead. As long as you use your program and data
files only on one platform, this is no problem, as all know what means what, and everything
gets used consistently.
Now Java comes into play, and wants to run everywhere (once written, that is) and now there
suddenly is a problem: how do I get the same program to say ä to a certain byte when it runs
in Germany and maybe é when it runs in France? And also the other way around: when I
want to say é on the screen, which byte value should I send to the operating system?
Java’s solution to this is to use Unicode internally. 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. To make things work correctly, you still have to know where you are and how to
31map 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).
5.2 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 like a breeze: 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 or some InputStream together with an InputStreamReader.
InputStreams return the raw bytes, the InputStreamReader converts the bytes 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 want to
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.
32More 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/1.5.0/docs/guide/intl/
leads to an online version of this for Oracle’s JDK 1.5.
5.3 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 (there is a constructor for InputStream instances, but it’s just
there for convenience and wraps an InputStreamReader around it to get characters, not bytes).
You can still get a binary scanner when you write your own custom InputStreamReader class
that does explicitly 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 little 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 properly 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.
6 Conformance with Unicode Regular Expressions UTS#18
This section gives details about JFlex 1.6.1’s conformance with the requirements for Basic
Unicode Support Level 1 given in UTS#18 [5].
RL1.1 Hex Notation
To meet this requirement, an implementation shall supply a mechanism for specify-
ing 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 Exten-
33sions, 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=Deci-
mal), 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.
34RL1.5 Simple Loose Matches
To meet this requirement, if an implementation provides for case-insensitive match-
ing, 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.
7 A few words on performance
This section gives some tips on how to make your specification produce a faster scanner.
Although JFlex generated scanners show good performance without special optimisations,
there are some heuristics that can make a lexical specification produce an even faster scanner.
Those are (roughly in order of performance gain):
• Avoid rules that require backtracking
From the C/C++ flex [13] man page: “Getting rid of backtracking is messy and often
may be an enormous amount of work for a complicated scanner.” Backtracking is
introduced by the longest match rule and occurs for instance on this set of expressions:
"averylongkeyword"
.
With input "averylongjoke" the scanner has to read all characters up to ’j’ to decide
that rule . should be matched. All characters of "verylong" have to be read again for
the next matching process. Backtracking can be avoided in general by adding error rules
that match those error conditions
"av"|"ave"|"avery"|"averyl"|..
35While this is impractical in most scanners, there is still the possibility to add a “catch
all” rule for a lengthy list of keywords
"keyword1" { return symbol(KEYWORD1); }
..
"keywordn" { return symbol(KEYWORDn); }
[a-z]+
{ error("not a keyword"); }
Most programming language scanners already have a rule like this for some kind of
variable length identifiers.
• Avoid line and column counting
It costs multiple additional comparisons per input character and the matched text has
to be re-scanned for counting. 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.
Column counting could also be included in actions. This will be faster, but can in some
cases become quite messy.
• Avoid look-ahead expressions and the end of line operator ’$’
In the best case, the trailing context will first have to be read and then (because it is not
to be consumed) re-read again. The cases of fixed-length look-ahead and fixed-length
base expressions are handled efficiently by matching the concatenation and then pushing
back the required amount of characters. This extends to the case of a disjunction of
fixed-length look-ahead expressions such as r1 / \r|\n|\r\n. All other cases r1 / r2
are handled by first scanning the concatenation of r1 and r2, and then finding the
correct end of r1. The end of r1 is found by scanning forwards in the match again,
marking all possible r1 terminations, and then scanning the reverse of r2 backwards
from the end until a start of r2 intersects with an end of r1. This algorithm is linear in
the size of the input (not quadratic or worse as backtracking is), but about a factor of 2
slower than normal scanning. It also consumes memory proportional to the size of the
matched input for r1 r2.
• Avoid the beginning of line operator ’^’
It costs multiple additional comparisons per match. In some cases one extra look-ahead
character is needed (when the last character read is \r, the scanner has to read one
character ahead to check if the next one is an \n or not).
• Match as much text as possible in a rule.
One rule is matched in the innermost loop of the scanner. After each action some
overhead for setting up the internal state of the scanner is necessary.
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.
368 Porting Issues
8.1 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
here. A JLex specification is well formed, when it
• generates a working scanner with JLex
• doesn’t contain the unescaped characters ! and ~
They are operators in JFlex while JLex treats them as normal input characters. You can
easily port such a JLex specification to JFlex by replacing every ! with \! and every ~
with \~ in all regular expressions.
• has only complete regular expressions surrounded by parentheses in macro definitions
This may sound a bit harsh, but could otherwise be a major problem – it can also help
you find some disgusting bugs in your specification that didn’t show up in the first place.
In JLex, a right hand side of a macro is just a piece of text, that is copied to the point
where the macro is used. With this, some weird kind of stuff like
macro1 = ("hello"
macro2 = {macro1})*
was possible (with macro2 expanding to ("hello")*). This is not allowed in JFlex
and you will have to transform such definitions. There are however some more subtle
kinds of errors that can be introduced by JLex macros. Let’s consider a definition like
macro = a|b and a usage like {macro}*. This expands in JLex to a|b* and not to the
probably intended (a|b)*.
JFlex uses always the second form of expansion, since this is the natural form of thinking
about abbreviations for regular expressions.
Most specifications shouldn’t suffer from this problem, because macros often only contain
(harmless) character classes like alpha = [a-zA-Z] and more dangerous definitions like
ident = {alpha}({alpha}|{digit})*
are only used to write rules like
{ident}
{ .. action .. }
and not more complex expressions like
{ident}*
{ .. action .. }
where the kind of error presented above would show up.
378.2 Porting from lex/flex
This section tries to give an overview of activities and possible problems when porting a lexical
specification from the C/C++ tools lex and flex [13] 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 very much work.
This section is by far not complete and is based mainly on a survey of the flex man page and
very little personal experience. If you do engage in any porting activity from lex/flex to JFlex
and encounter problems, have better solutions for points presented here or have just some tips
you would like to share, please do contact me via email: Gerwin Klein <lsf@jflex.de>. I
will incorporate your experiences in this manual (with all due credit to you, of course).
8.2.1 Basic structure
A lexical specification for flex has the following basic structure:
definitions
%%
rules
%%
user code
The user code section usually contains some C code that is used in actions of the rules part
of the specification. For JFlex most of this code will have to be included in the class code
%{..%} directive in the options and declarations section (after translating the C code to
Java, of course).
8.2.2 Macros and Regular Expression Syntax
The definitions section of a flex specification is quite similar to the options and declara-
tions 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.
A little attention is needed for some escape sequences present in flex (such as \a) that are
not supported in JFlex. These escape sequences should be transformed into their octal or
hexadecimal equivalent.
Another point are predefined character classes. Flex offers the ones directly supported by
C, JFlex offers the ones supported by Java. These classes will sometimes have to be listed
manually (if there is need for this feature, it may be implemented in a future JFlex version).
388.2.3 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.
9 Working together
9.1 JFlex and CUP
One of the main design goals of JFlex was to make interfacing with the parser generators
CUP [8] and CUP2 [9] 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.
9.1.1 CUP2
Please refer to the CUP2 [9] 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.
9.1.2 CUP version 0.10j and above
Since CUP version 0.10j, this 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) {
...
9.1.3 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
39code 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
%eofval{
return mysym.EOF;
%eofval}
in the macro/directives section of the spec, or it would be
<<EOF>>
{ return mysym.EOF; }
in the rules section of your spec.
9.1.4 Using existing JFlex/CUP specifications with CUP 0.10j
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 the current JFlex
version 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 do not mind to change 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 as well,
40of course). The calling main procedure would then construct the parser as shown in the section
above.
The JFlex specification does not need to be changed.
9.2 JFlex and BYacc/J
JFlex has built-in support for the Java extension BYacc/J [10] 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:
%%
%byaccj
%{
/* 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(r);
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
41terminal 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 that bit.
A matching BYacc/J parser specification could look like this:
%{
import java.io.*;
%}
%token NL
%token <dval> NUM
/* newline */
/* a number */
%type <dval> exp
%left ’-’ ’+’
..
%right ’^’
/* exponentiation */
%%
..
exp:
NUM
| exp ’+’ exp
..
| exp ’^’ exp
| ’(’ exp ’)’
;
{ $$ = $1; }
{ $$ = $1 + $3; }
{ $$ = Math.pow($1, $3); }
{ $$ = $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);
}
42/* 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]));
yyparser.yyparse();
}
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.
9.3 JFlex and Jay
Combining JFlex with the Jay Parser Generator Jay[11] 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
• advance() which should return a boolean that is true if there is more work to do and
false if the end of input has been reached,
• token() which returns the last scanned token, and
• value() which returns an Object that contains the (optional) value of the last read
token.
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
43{
%}
// Token declarations, and types of non-terminals
%token DASH COLON
%token <Integer> NUMBER
%token <String> NAME
%type <Gameresult> game
%type <Vector<Gameresult>> gamelist
// start symbol
%start gamelist
%%
gamelist: game
{ $$ = new Vector<Gameresult>();
$<Vector<Gameresult>>$.add($1);
}
| gamelist game
{ $1.add($2); }
game: NAME DASH NAME NUMBER COLON NUMBER {
$$ = 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;
44String 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
%%
%public
%class MiniScanner
%implements MiniParser.yyInput
%integer
%line
%column
%unicode
%{
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;
45}
%}
nl =
[\n\r]+
ws =
[ \t\b\015]+
number = [0-9]+
name =
[a-zA-Z]+
dash =
"-"
colon =
":"
%%
{nl}
{ws}
{name}
{dash}
{colon}
{number}
{
{
{
{
{
{
/* do nothing */ }
/* happy meal */ }
value = yytext(); return MiniParser.NAME; }
return MiniParser.DASH; }
return MiniParser.COLON; }
try {
value = new Integer(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
10 Bugs and Deficiencies
10.1 Deficiencies
JFlex 1.6.1 conforms with Unicode Regular Expressions UTS#18 [5] Basic Unicode Support
Level 1, with a few exceptions - for details see UTS#18 Conformance.
10.2 Bugs
As of March 16, 2015, no major open problems are known for JFlex version 1.6.1.
Please use the bugs section of the JFlex web site 3 to check for open issues.
3
http://www.jflex.de/
4611 Copying and License
JFlex is free software, published under a BSD-style license.
There is absolutely NO WARRANTY for JFlex, its code and its documentation.
See the file COPYRIGHT for more information.
47References
[1] A. Aho, R. Sethi, J. Ullman, Compilers: Principles, Techniques, and Tools, 1986
[2] A. W. Appel, Modern Compiler Implementation in Java: basic techniques, 1997
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http://www.cs.princeton.edu/~appel/modern/java/JLex/
[4] K. Brouwer, W. Gellerich,E. Ploedereder, Myths and Facts about the Efficient Implemen-
tation of Finite Automata and Lexical Analysis, in: Proceedings of the 7th International
Conference on Compiler Construction (CC ’98), 1998
[5] M. Davis, Unicode Regular Expressions, Unicode Technical Standard #18, version 17
http://www.unicode.org/reports/tr18/tr18-17.html
[6] P. Dencker, K. Dürre, J. Henft, Optimization of Parser Tables for portable Compilers,
in: ACM Transactions on Programming Languages and Systems 6(4), 1984
[7] J. Gosling, B. Joy, G. Steele, The Java Language Specifcation, 1996,
http://java.sun.com/docs/books/jls/
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http://www2.cs.tum.edu/projects/cup/
[9] TU Munich, CUP2 LALR Parser Generator for Java,
http://www2.in.tum.de/cup2
[10] B. Jamison, BYacc/J,
http://byaccj.sourceforge.net/
[11] A.T. Schreiner, Jay Parser Generator,
http://www.cs.rit.edu/ ats/projects/lp/doc/jay/package-summary.html
[12] T. Lindholm, F. Yellin, The Java Virtual Machine Specification, 1996,
http://java.sun.com/docs/books/vmspec/
[13] V. Paxson, flex - The fast lexical analyzer generator, 1995
[14] R. E. Tarjan, A. Yao, Storing a Sparse Table, in: Communications of the ACM 22(11),
1979
[15] R. Wilhelm, D. Maurer, Übersetzerbau, Berlin 1997 2
[16] Unicode Utilities: UnicodeSet
http://unicode.org/cldr/utility/list-unicodeset.jsp
48