How to extract a substring from a string in C?

Note: I edited this answer after I realized that as written the code would cause a problem as strtok doesn't like to operate on const char* variables. This was more an artifact of how I wrote the example than a problem with the underlying principle - but apparently it deserved a double downvote. So I fixed it.

The following works (tested on Mac OS 10.7 using gcc):

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

int main(void) {
const char* lineConst = "This \"is\" an example"; // the "input string"
char line[256];  // where we will put a copy of the input
char *subString; // the "result"

strcpy(line, lineConst);

subString = strtok(line,"\""); // find the first double quote
subString=strtok(NULL,"\"");   // find the second double quote

printf("the thing in between quotes is '%s'\n", subString);
}

Here is how it works: strtok looks for "delimiters" (second argument) - in this case, the first ". Internally, it knows "how far it got", and if you call it again with NULL as the first argument (instead of a char*), it will start again from there. Thus, on the second call it returns "exactly the string between the first and second double quote". Which is what you wanted.

Warning: strtok typically replaces delimiters with '\0' as it "eats" the input. You must therefore count on your input string getting modified by this approach. If that is not acceptable you have to make a local copy first. In essence I do that in the above when I copy the string constant to a variable. It would be cleaner to do this with a call to line=malloc(strlen(lineConst)+1); and a free(line); afterwards - but if you intend to wrap this inside a function you have to consider that the return value has to remain valid after the function returns... Because strtok returns a pointer to the right place inside the string, it doesn't make a copy of the token. Passing a pointer to the space where you want the result to end up, and creating that space inside the function (with the correct size), then copying the result into it, would be the right thing to do. All this is quite subtle. Let me know if this is not clear!

#include <string.h>
...        
substring[0] = '\0';
const char *start = strchr(line, '"') + 1;
strncat(substring, start, strcspn(start, "\""));

Bounds and error checking omitted. Avoid strtok because it has side effects.

if you want to do it with no library support...

void extract_between_quotes(char* s, char* dest)
{
   int in_quotes = 0;
   *dest = 0;
   while(*s != 0)
   {
      if(in_quotes)
      {
         if(*s == '"') return;
         dest[0]=*s;
         dest[1]=0;
         dest++;
      }
      else if(*s == '"') in_quotes=1;
      s++;
   }
}

then call it

extract_between_quotes(line, substring);