Flex Specification yytext

I'm trying to build a flex specification for the k notation, for instance: 3k5 = 3500. I have the following:

[0-9]+{LETTER}      { yyless(yyleng-1); yy_push_state(X_REAL); aux = 开发者_StackOverflow中文版atof(yytext); }
<X_REAL>"k"[0-9]+   { yytext[0] = "." ; printf("%f", ((atof(yytext) * 10^(3)) + aux * 10^(3))); }

However I get an error when trying to put "." in the first char of yytext:

In member function ‘virtual int AtScanner::yylex()’: error: invalid conversion from ‘const char*’ to ‘char’ error: invalid operands of types ‘double’ and ‘int’ to binary ‘operator^’

How can I manipulate the yytext?

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You cannot modify yytext, it's const.

You should go another way: perhaps just allocate a new string using strdup?

Other problem: yytext[0] is a char, so should be the rhs. So we need '.' instead of ".".

Yet another problem: 10^3 is not producing 1000, in C it's bitwise exclusive OR operator (thanks to @Chris for pointing this out). So just put plain 1000 instead.

So, the final code should look like:

<X_REAL>"k"[0-9]+   { char* number = strdup(yytext);
                      number[0] = '.';
                      printf("%f", ((atof(number) * 1000) + aux * 1000));
                      free(number); }

Note that I didn't check the correctness of calculations.

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Per @rici's comment, modifying yytext should be ok, so the code can be simplified as follows:

<X_REAL>"k"[0-9]+   { yytext[0] = '.';
                      printf("%f", ((atof(yytext) * 1000) + aux * 1000)); }

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Modifying yytext (or a copy) seems like a bad way of going about this. Why not just extract the numbers directly?

[0-9]+k[0-9]+    { char *p;
                   long first = strtol(yytext, &p, 10);
                   long second = strtol(p+1, 0, 10);
                   double value = first*1000.0 + second*pow(10.0, p-yytext-yyleng+4.0);
                   printf("%f", value); }