1. Compiler Design 3. Lexical Analyzer, Flex
Kanat Bolazar
January 26, 2010

2. Lexical Analyzer
The main task of the lexical analyzer is to read the input source program, scanning the characters, and produce a sequence of tokens that the parser can use for syntactic analysis.
The interface may be to be called by the parser to produce one token at a time
Maintain internal state of reading the input program (with lines)
Have a function “getNextToken” that will read some characters at the current state of the input and return a token to the parser
Other tasks of the lexical analyzer include
Skipping or hiding whitespace and comments
Keeping track of line numbers for error reporting
Sometimes it can also produce the annotated lines for error reports
Produce the value of the token
Optional: Insert identifiers into the symbol table

3. Character Level Scanning
The lexical analyzer needs to have a well-defined valid character set
Produce invalid character errors
Delete invalid characters from token stream so as not to be used in the parser analysis
E.g. don’t want invisible characters in error messages
For every end-of-line, keep track of line numbers for error reporting
Skip over or hide whitespace and comments
If comments are nested (not common), must keep track of nesting to find end of comments
May produce hidden tokens, for convenience of scanner structure
Always produce an end-of-file token
Important that quoted strings and comments don’t get stuck if an unexpected end of file occurs

4. Tokens, Token Types and Values
The set of tokens is typically something like the following table
Or may have separate token types for different operators or reserved words
May want to keep line number with each token
Token Type
Token Value
Informal Description
Integer constant
Numeric value
Numbers like 3, -5, 12 without decimal pts.
Floating constant
Numbers like 3.0, -5.1,
Reserved word
Word string
Words like if, then, class, …
Identifiers
Symbol table index
Words not reserved starting with letter or _ and containing only letters, _, and digits
Relations
Operator string
<, <=, ==, …
Operators
=, +, - , ++, …
Char constant
Char value
‘A’, …
String
“this is a string”, …
Hidden: end-of-line
Hidden: comment

5. Token Actions Each token recognized can have an action function
Many token types produce a value
In the case of numeric values, make sure property numeric errors produced, e.g. integer overflow
Put identifiers in the symbol table
Note that at this time, no effort is made to distinguish scope; there will be one symbol table entry for each identifier
Later, separate scope instances will be produced
Other types of actions
End-of-line (can be treated as a token type that doesn’t output to the parser)
Increment line number
Get next line of input to scan

6. Testing
Execute lexical analyzer with test cases and compare results with expected results
Test cases
Exercise every part of lexical analyzer code
Produce every error message
Don’t have to be valid programs – just valid sequence of tokens

7. Lex and Yacc Two classical tools for compilers:
Lex: A Lexical Analyzer Generator
Yacc: “Yet Another Compiler Compiler”
Lex creates programs that scan your tokens one by one.
Yacc takes a grammar (sentence structure) and generates a parser.
Lexical Rules
Grammar Rules
Lex
Yacc
Input
yylex()
yyparse()
Parsed Input

8. Flex: A Fast Scanner Generator
Often, instead of the standard Lex and Yacc, Flex and Bison are used:
Flex: A fast lexical analyzer
(GNU) Bison: A drop-in replacement for (backwards compatible with) Yacc
Resources:
(the Lex & Yacc Page)

9. Flex Example 1: Delete This
Shortest Flex example, “deletethis.l”: %%
deletethis
This scanner will match and not echo (default behavior) the word “deletethis”.
Compile and run it:
$ flex deletethis.l # creates lex.yy.c
$ gcc -o scan lex.yy.c -lfl # fl: flex library
$ ./scan
This deletethis is not deletethis useful.
This is not useful. ^D

10. Flex Example 2: Replace This
Another very short Flex example, “replacer.l”: %%
replacethis printf(“replaced”);
This scanner will match “replacethis” and replace it with “replaced”.
Compile and run it:
$ flex -o replacer.yy.c replacer.l
$ gcc -o replacer replacer.yy.c -lfl
$ ./replacer
This replacethis is not very replacethis useful.
This replaced is not very replaced useful.
Please dontreplacethisatall.
Please dontreplacedatall.

11. Flex Example 3: Common Errors
Let's replace “the the” with “the”: %%
the the printf(“the”);
uhh
Unfortunately, this does not work: The second “the” is considered part of C code:
the the printf(“the”);
Also, the open and close matching double quotes used in documents will give errors, so you must always replace:
“the” → "the"

12. Flex Example 3: Common Errors, cont'd
You discover such errors when you compile the C code, not when you use flex:
$ flex -o errors.yy.c errors.l
$ gcc -o errors errors.yy.c -lfl
errors.l: In function ‘yylex’:
errors.l:2: error: ‘the’ undeclared
...
The error is reported back in our errors.l file, but we can also find it in errors.yy.c:
case 1:
YY_RULE_SETUP
#line 2 "errors.l" <-- For error reporting
the printf("the"); <-- the ? not C code
YY_BREAK
case 2:

13. Flex Example 4: Replace Duplicate
Let's replace “the the” with “the”: %%
"the the" printf("the");
This time, it works:
$ flex -o duplicate.yy.c duplicate.l
$ gcc -o duplicate duplicate.yy.c -lfl
$ ./duplicate
This is the the file.
This is the file.
This is the the the file.
Lathe theory
Latheory

14. Flex Example 4: Replace And Delete
Let's replace “the the” with “the” and delete “uhh”: %%
"the the" printf("the");
uhh
Run as before:
This uhh is the the uhhh file.
This is the h file.
Generally, lexical rules are pattern-action pairs:
pattern1 action1 (C code)
pattern2 action2
...
Tokens almost never go across space chars as in "the the" above.
Regular expressions are often needed and used.

15. Flex File Structure
In Lex and Flex, the general rule file structure is:
definitions %%
rules
user code
Definitions:
DIGIT [0-9]
ID [a-z][a-z0-9]*
can be used later in rules with {DIGIT}, {ID}, etc:
{DIGIT}+"."{DIGIT}*
This is the same as:
([0-9])+"."([0-9])*

16. Flex Example 5: Count Lines
int num_lines = 0, num_chars = 0; %%
\n ++num_lines; ++num_chars;
num_chars;
main() {
yylex();
printf( "# of lines = %d, # of chars = %d\n",
num_lines, num_chars ); }

17. Some Regular Expressions for Flex
\"[^"]*\" string
"\t"|"\n"\" " whitespace (most common forms)
[a-zA-Z]
[a-zA-Z_][a-zA-Z0-9_]* identifier: allows a, aX, a45__
[0-9]*"."[0-9]+ allows .5 but not 5.
[0-9]+"."[0-9]* allows 5. but not .5
[0-9]*"."[0-9]* allows . by itself !!

18. Resources
Aho, Lam, Sethi, and Ullman, Compilers: Principles, Techniques, and Tools, 2nd ed. Addison-Wesley, (The “purple dragon book”)
Flex Manual. Available as single postscript file at the Lex and Yacc page online: