Replace number expression with flex - flex-lexer

Question

I use Flex for replace number expression in code source:
For instance:
Input string: ... echo "test"; if ($isReady) $variable = 2 * 5; ...
Desired result string: ... echo "test"; if ($isReady) $variable = 10; ...
My code:
%{
#include <stdio.h>
#include <stdlib.h>
%}
MYEXP [0-9]+[ \t\n\r]*\+[ \t\n\r]*[0-9]+
%%
{MYEXP} {
printf("multiplication ");
// code for processing
}
%%
void main()
{
yylex();
}
How can I process multiplication with Flex? Or I have to process with C language?

Answer 1

Some of the answers are in the comments, but the question has not yet been closed with an answer in two years. I thought some notes, for the purposes of completion, would be useful for people who are thinking of things like this in the future.
Simple arithmetic expression, in the form exemplified in the question can be recognised by a tool like flex, which matches regular expressions using an FSA (Finite State Automaton - or FSM Finite State Machine). This works when the syntax is simple id + id, but fails when the expressions become more complex. The handling of the operator precedence in id + id * id and the nested parenthesis in something like ((id + id) * (id + id)) means that a Regular Grammar can no longer work. This requires a context-free grammar. (Computer Science students should know this from Chomsky Language Theory). So the operations can only be performed in flex for the simplest forms of expression.
The replacement of simple expressions, which only contain constants, is an optimisation called constant folding and is performed by most compilers as standard. Performing this as a form of pre-processing on most code will not produce any improvement. So when proposing to write tools to do a job like this you have to reflect on whether it is essential or not!
Now down to the actual details of the question, which have been picked up in the comments; yes, a rule will be needed for each operator, addition and multiplication; and when matched a substring will be needed to pick up the operands. It will look something like this:
MYplusEXP [0-9]+[ \t\n\r]*\+[ \t\n\r]*[0-9]+
MYmultEXP [0-9]+[ \t\n\r]*\*[ \t\n\r]*[0-9]+
%%
char [20] left; char * right;
{MYplusEXP} {right = strstr(yytext,"+"); /* yytext is already terminated with \0 */
strncopy(left,yytext,right-yytext+1);
printf("%d",atoi(left)+atoi(right));
}
{MYmultEXP} {right = strstr(yytext,"*");
strncopy(left,yytext,right-yytext+1);
printf("%d",atoi(left)*atoi(right));
}
However I feel a bit dirty after doing that pointer arithmetic
In summary, it might be better done with other tools or not at all!