LEX–YACC Lab · CSL411 Compiler Lab (KTU S7)

1 · Tokens: what LEX reads and what it passes on

One row per kind of lexeme, in order: on a tie LEX picks the earlier row, otherwise the longest match wins. In a LEX+YACC program each useful row ends with return NAME; — that NAME is the token YACC sees.

Definitions (optional, e.g. digit [0-9], used as {digit})

#Pattern (regular expression)Action (C code)

Quick add:

C declarations: counters and variables (go inside %{ … %}) After yylex(); in main() (print the counts, etc.)

See the token stream for a sample input

2 · Grammar: the YACC productions

Write productions the YACC way: S : A S B on one line, | alternative on the next, ; to end. An empty alternative is just | with nothing after it. Characters go in single quotes: '+'. Put actions at the end: { $$=$1+$3; }.

Declarations: precedence, lowest first (optional) Productions (the first head is the start symbol)

Tokens from step 1:

yyerror() message (printed when the input is not in the language)

main() body (default: yyparse();)

3 · Try it

The board runs your rules and grammar right here in the browser, the way LEX and YACC would. One input per box; the newline at the end counts.

4 · Your program, in the lab-manual format

Your rows and productions, wrapped in the same skeleton as the lab manual. Copy them to the lab machine (or an online LEX/YACC runner) and build with the commands below.

Build & run

✍ Write:

0 · The question

1 · Tokens: what LEX reads and what it passes on

2 · Grammar: the YACC productions

3 · Try it

4 · Your program, in the lab-manual format

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