Question

Q1.I want to match “&JOY” in a user input string, for example, “bla bla &JOY bla bla &JOY blabla”
How can I write the regex expression for this, and are there any C regex functions could return all the positions where the matches happen?
Q2.If I want to substitute &JOY with another string is there a convenient C function to do this?

Answer 1

C standard library does not provide regex functions. However, as your example looks for a fixed string, you don’t need regex. Substring search is enough. It can be accomplished with strstr function:

#include <string.h>
..........

char* text = "bla bla &JOY bla bla &JOY blabla";
char* pattern = "&JOY";
char* substr_ptr;
int pos;

while ( substr_ptr = strstr(pattern, text) )
{
    pos = substr_ptr - text;
    printf("found at %d\n", pos);
}

As to your second question — no, there is no such function. You have to do it manually:

• if replacing string has the same length as the substring you want to replace, just overwrite it in-place (memcpy);
• otherwise, you will have to allocate memory for resulting string and copy three parts there (original string before substring occurrence, replacing string, and the remainder of the original string).