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This example comes from a short article series in the Linux
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Gazette by Richard A. Sevenich and Christopher Lopes, titled
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"Compiler Construction Tools". The article series starts at
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http://www.linuxgazette.com/issue39/sevenich.html
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Small changes and updates to newest JFlex+Cup versions
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Commented By: Christopher Lopes
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To Create: > jflex lcalc.flex
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and then after the parser is created
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/* --------------------------Usercode Section------------------------ */
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import java_cup.runtime.*;
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/* -----------------Options and Declarations Section----------------- */
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The name of the class JFlex will create will be Lexer.
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Will write the code to the file Lexer.java.
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The current line number can be accessed with the variable yyline
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and the current column number with the variable yycolumn.
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Will switch to a CUP compatibility mode to interface with a CUP
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Code between %{ and %}, both of which must be at the beginning of a
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line, will be copied letter to letter into the lexer class source.
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Here you declare member variables and functions that are used inside
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/* To create a new java_cup.runtime.Symbol with information about
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the current token, the token will have no value in this
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private Symbol symbol(int type) {
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return new Symbol(type, yyline, yycolumn);
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/* Also creates a new java_cup.runtime.Symbol with information
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about the current token, but this object has a value. */
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private Symbol symbol(int type, Object value) {
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return new Symbol(type, yyline, yycolumn, value);
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These declarations are regular expressions that will be used latter
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in the Lexical Rules Section.
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/* A line terminator is a \r (carriage return), \n (line feed), or
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LineTerminator = \r|\n|\r\n
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/* White space is a line terminator, space, tab, or line feed. */
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WhiteSpace = {LineTerminator} | [ \t\f]
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/* A literal integer is is a number beginning with a number between
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one and nine followed by zero or more numbers between zero and nine
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dec_int_lit = 0 | [1-9][0-9]*
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/* A identifier integer is a word beginning a letter between A and
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Z, a and z, or an underscore followed by zero or more letters
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between A and Z, a and z, zero and nine, or an underscore. */
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dec_int_id = [A-Za-z_][A-Za-z_0-9]*
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/* ------------------------Lexical Rules Section---------------------- */
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This section contains regular expressions and actions, i.e. Java
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code, that will be executed when the scanner matches the associated
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regular expression. */
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/* YYINITIAL is the state at which the lexer begins scanning. So
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these regular expressions will only be matched if the scanner is in
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the start state YYINITIAL. */
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/* Return the token SEMI declared in the class sym that was found. */
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";" { return symbol(sym.SEMI); }
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/* Print the token found that was declared in the class sym and then
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"+" { System.out.print(" + "); return symbol(sym.PLUS); }
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"-" { System.out.print(" - "); return symbol(sym.MINUS); }
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"*" { System.out.print(" * "); return symbol(sym.TIMES); }
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"/" { System.out.print(" / "); return symbol(sym.DIVIDE); }
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"(" { System.out.print(" ( "); return symbol(sym.LPAREN); }
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")" { System.out.print(" ) "); return symbol(sym.RPAREN); }
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/* If an integer is found print it out, return the token NUMBER
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that represents an integer and the value of the integer that is
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held in the string yytext which will get turned into an integer
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{dec_int_lit} { System.out.print(yytext());
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return symbol(sym.NUMBER, new Integer(yytext())); }
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/* If an identifier is found print it out, return the token ID
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that represents an identifier and the default value one that is
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given to all identifiers. */
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{dec_int_id} { System.out.print(yytext());
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return symbol(sym.ID, new Integer(1));}
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/* Don't do anything if whitespace is found */
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{WhiteSpace} { /* just skip what was found, do nothing */ }
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/* No token was found for the input so through an error. Print out an
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Illegal character message with the illegal character that was found. */
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[^] { throw new Error("Illegal character <"+yytext()+">"); }
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removed
examples/cup/lcalc.flex
