How to extract a substring from a string in C?

Question

I tried using strncmp but it only works if I give it a specific number of bytes I want to extract.

char line[256] = This "is" an example. //I want to extract "is"
char line[256] = This is "also" an example. // I want to extract "also"
char line[256] = This is the final "example".  // I want to extract "example"
char substring[256]

How would I extract all the elements in between the ""? and put it in the variable substring?

Answer 1

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!

Answer 2

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);

Answer 3

Here is a long way to do this: Assuming string to be extracted will be in quotation marks (Fixed for error check suggested by kieth in comments below)

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

int main(){

    char input[100];
    char extract[100];
    int i=0,j=0,k=0,endFlag=0;

    printf("Input string: ");
    fgets(input,sizeof(input),stdin);
    input[strlen(input)-1] = '\0';

    for(i=0;i<strlen(input);i++){
        if(input[i] == '"'){

                j =i+1;
                while(input[j]!='"'){
                     if(input[j] == '\0'){
                         endFlag++;
                         break;
                     }
                     extract[k] = input[j];
                     k++;
                     j++;
                }
        }
    }
    extract[k] = '\0';

    if(endFlag==1){
        printf("1.Your code only had one quotation mark.\n");
        printf("2.So the code extracted everything after that quotation mark\n");
        printf("3.To make sure buffer overflow doesn't happen in this case:\n");
        printf("4.Modify the extract buffer size to be the same as input buffer size\n");

        printf("\nextracted string: %s\n",extract);
    }else{ 
       printf("Extract = %s\n",extract);
    }

    return 0;
}

Output(1):

$ ./test
Input string: extract "this" from this string
Extract = this

Output(2):

$ ./test
Input string: Another example to extract "this gibberish" from this string
Extract = this gibberish

Output(3):(Error check suggested by Kieth)

$ ./test

Input string: are you "happy now Kieth ?
1.Your code only had one quotation mark.
2.So the code extracted everything after that quotation mark
3.To make sure buffer overflow doesn't happen in this case:
4.Modify the extract buffer size to be the same as input buffer size

extracted string: happy now Kieth ?

--------------------------------------------------------------------------------------------------------------------------------

Although not asked for it -- The following code extracts multiple words from input string as long as they are in quotation marks:

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

int main(){

    char input[100];
    char extract[50];
    int i=0,j=0,k=0,endFlag=0;

    printf("Input string: ");
    fgets(input,sizeof(input),stdin);
    input[strlen(input)-1] = '\0';

    for(i=0;i<strlen(input);i++){
        if(input[i] == '"'){
            if(endFlag==0){
                j =i+1;
                while(input[j]!='"'){
                     extract[k] = input[j];
                     k++;
                     j++;
                }
                endFlag = 1;
            }else{
               endFlag =0;
            }

            //break;
        }
    }

    extract[k] = '\0';

    printf("Extract = %s\n",extract);

    return 0;
}

Output:

$ ./test
Input string: extract "multiple" words "from" this "string"
Extract = multiplefromstring

Answer 4

Have you tried looking at the strchr function? You should be able to call that function twice to get pointers to the first and second instances of the " character and use a combination of memcpy and pointer arithmetic to get what you want.

Answer 5

#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.