# make config parser (yy018)
      # source code for 018.l is below
      flex -8iCrfa 018.l;
      cc -Wall -std=c89 -static -o yy018 lex.yy.c;
                                                                                                                                                 
      # create a config (ua.txt)
      # format is name: "value"                                                                                                                  
      # no indent, must be left-justified
      cat > ua.txt
      httpVersion="1.0"
      Host: ""
      Connection: "close"
      User-Agent: "CERN-LineMode/2.15 libwww/2.17b3"
      ^D

   save as "018.l"

      int setenv (const char *, const char *, int);
      int fileno(FILE *);
      #define jmp (yy_start) = 1 + 2 *
      #define X(x) fputs(x,stdout)
      #define Y(x,y) fprintf(stdout,x,y)
      #define Y2(x,y) fprintf(stderr,x,y)
      int xxyy()
      {
       setenv("value",yytext,1);
       Y("\40\40\40if(0==setenv(\"%s\",",getenv("name"));
       Y("%s,0))",getenv("value"));
       Y("Y(\"%s: %%s\\r\\n\",",getenv("name"));
       Y("getenv(\"%s\"));\n",getenv("name"));
       return 0;
      }
      int lineno=1;
     %option noinput nounput noyywrap
     %s xa xc 
     %%
     \n lineno++;
     ^[\40\11]+ { 
      Y2("yy018: error: remove %d ",yyleng);
      Y2("leading spaces/tabs from line %d\n",lineno);
      yyterminate();
      } 
     ^"Host"[: ] setenv("name","Host",1);jmp xc; 
     ^[^\40\11\":\n]* setenv("name",yytext,1);jmp xa;
     <xa>[\"][^\"]*[\"] { 
      if(yyleng>2)xxyy();
      else if(yyleng==2&&yytext[0]=='\"'&&yytext[1]=='\"') { 
      setenv("value",yytext,1);
      Y("\40\40\40if(getenv(\"%s\"))",getenv("name"));
      Y("Y(\"%s: %%s\\r\\n\",",getenv("name"));
      Y("getenv(\"%s\"));\n",getenv("name"));
      };}
     <xc>[\"][^\"]*[\"] xxyy(); 
     \n|.
     %%
     int main(){yylex();return 0;}