Flex备忘

1 提出问题

例如, 有这样一段字符串:

VAR ics142: INTEGER; // variable declaration

假设我们想把它变成:

VAR  ID(ics142)  COLON  ID(INTEGER)  SEMICOLON

我们可以使用gnu flex.

2 准备试验环境

Fedora 30

$ sudo dnf install flex
$ sudo dnf install flex-devel

3 设置规则

    // An example of using the flex C++ scanner class.

%option C++ noyywrap

%{
%}

alpha   [A-Za-z]
dig [0-9]
colon   :
semicolon   ;
variable  VAR
comment   \/\/({alpha}|{ws})*
ws  [ \t]+
name    ({alpha}|{dig}|\$)({alpha}|{dig}|\_|\.|\-|\/|\$)*

%%
{variable}  std::cout << YYText() << " ";
{ws}            /* skip blanks and tabs */
{comment}   /* skip blanks and tabs */
{name}      std::cout << "ID(" << YYText() << ") ";
{colon}         std::cout << "COLON ";
{semicolon}     std::cout << "SEMICOLON ";
%%

int main( int /* argc */, char** /* argv */ )
{
    FlexLexer* lexer = new yyFlexLexer;
    lexer->yylex();
    return 0;
}

4 编译

g++ lex.yy.cc -lfl

5 执行

echo "VAR ics142: INTEGER; // variable declaration" | ./a.out

6 总结

image.png

参考:
http://alumni.cs.ucr.edu/~lgao/teaching/flex.html
https://github.com/westes/flex/blob/master/examples/testxxLexer.l