flex-lexer - Replace number expression with flex

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?