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This article is about how to understand lex and yacc, the editor thinks it is very practical, so I share it with you to learn. I hope you can get something after reading this article.

I. background

Learn lex and yacc from scratch

1. Basics

Lex has only state and state transition, no stack, and is good at pattern matching; yacc can handle FSA (finite state machine) with stack, which is more suitable for more complex tasks.

Pattern matching primitive

Metacharacter

Match description

.

Any character (except newline)

\ n

New line

*

Repeat the previous expression 0 or more times

+

Repeat the previous expression one or more times

?

Repeat the previous expression 0 or 1 times

^

The beginning of the line

$

The end of the line

A | b

An or b

(ab) +

Repeat group ab one or more times

[...]

Any character that appears

Some examples of matching

Expression.

Match description

Abc

Abc

Abc*

Ab, abc, abcc, abccc,.

Abc+

Abc, abcc, baccc,.

A (bc) +

Abc, abcbc, abcbcbc,.

A (bc)?

A, abc

[abc]

A, b, c

[aMuz]

Any character from a to z

[a\-z]

A, -, z

[- az]

-, a, z

[a-zA-Z0-9] +

One or more of any numeric letters

[\ t\ n]

Witespace

[^ ab]

Any character except afort b

[a ^ b]

A, ^, b

[a | b]

A, |, b

A | b

An or b

Matching rules:

1. Greed: two patterns match the same string and match the longest pattern

two。 Order priority: two patterns of the same length that match the previously defined patterns

The format of the .l file content is divided into three parts by%%, as follows:

.... definitions.

%%

.rules....

%%

... subroutines...

Rules is required, and other parts are optional.

Some built-in functions and variables

Int yylex (void)

Call the parser and return token

Char * yytext

Specify a matching string

Yyleng

The length of the string on the match

Int yywrap (void)

If you return 1, it's over.

FILE * yyout

Output file, default stdout

FILE * yyin

Input file, default stdin

INITIAL

Initial start conditionBEGIN condition

Switch start condition

ECHO

Write mached string

# define ECHO fwrite (yytext, yyleng, 1, yyout)

Second, start with the first example

Environment description:

VMware Workstation 12 Pro, ubuntu17.04, lex2.6.1

Output the contents of the file and add the line number before it

Lineno.l

% {int yylineno;%}% ^ (. *)\ n printf ("% 4d\ t% s", + + yylineno, yytext);% int main (int argc, char * argv []) {FILE * fp = NULL; if (argc = 2) {fp = fopen (argv [1], "r"); if (NULL! = fp) {yyin = fp;}} yylex () If (NULL! = fp) {fclose (fp);} return 0;}

Use lex to convert lineno.l files to .c files

$lex lineno.l $ls lex.yy.c lineno.l

Use gcc to compile lex.yy.c into an executable file

$gcc lex.yy.c-o lineno

/ tmp/ccNgesbZ.o: in the function 'yylex':

Lex.yy.c: (.text + 0x55c): undefined reference to 'yywrap'

/ tmp/ccNgesbZ.o: in the function 'input':

Lex.yy.c: (.text + 0x116c): undefined reference to 'yywrap'

Collect2: error: ld returned 1 exit status

It is said that the online query is to implement the yywrap function in .l file.

After modification:

% {int yylineno;%}% ^ (. *)\ n printf ("% 4d\ t% s", + + yylineno, yytext);% int main (int argc, char * argv []) {FILE * fp = NULL; yylineno = 0; if (argc = = 2) {fp = fopen (argv [1], "r"); if (NULL! = fp) {yyin = fp }} yylex (); if (NULL! = fp) {fclose (fp);} return 0;} int yywrap () {return 1;}

Compile successfully again

$gcc lex.yy.c-o lineno$lineno lineno.l

1% {

2 int yylineno

3%}

four

5%

6 ^ (. *)\ nprintf ("% 4d\ t% s", + + yylineno, yytext)

7%

eight

9 int main (int argc, char * argv [])

10 {

11 FILE * fp = NULL

12 yylineno = 0

thirteen

14 if (argc = = 2) {

15 fp = fopen (argv [1], "r")

16 if (NULL! = fp) {

17 yyin = fp

18}

19}

twenty

21 yylex ()

twenty-two

23 if (NULL! = fp) {

24 fclose (fp)

25}

twenty-six

27 return 0

28}

twenty-nine

30 int yywrap ()

31 {

32 return 1

33}

The above is how to understand lex and yacc. The editor believes that there are some knowledge points that we may see or use in our daily work. I hope you can learn more from this article. For more details, please follow the industry information channel.

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