c - Store text between two characters in a array

Question

I am having this char array char txt[80] = "Some text before $11/01/2017$"; and need to copy the content between the two $ into a string which would be 11/01/2017. How can I do this with the <string.h> functions?

question from:https://stackoverflow.com/questions/65832486/store-text-between-two-characters-in-a-array

Answer 1

A simple way to obtain a single token that does not modify the original string is to use two calls to strcspn() establishing a start-pointer to the first delimiter ('$' in this case) and an end-pointer to the last character in the token (the character before the second '$' or end-of-string if no second '$' is present). You then validate that characters exist between the start-pointer and end-pointer and use memcpy() to copy the token.

A short example is:

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

int main (void) {
    
    char txt[80] = "Some text before $11/01/2017$",
        *sp = txt + strcspn (txt, "$"),                 /* start ptr to 1st '$' */
        *ep = sp + strcspn (*sp ? sp + 1 : sp, "$
"),  /* end ptr to last c in token */
        result[sizeof txt] = "";                        /* storage for result */
    
    if (ep > sp) {                                      /* if chars in token */
        memcpy (result, sp + 1, ep - sp);               /* copy token to result */
        result[ep - sp] = 0;                            /* nul-termiante result */
        printf ("%s
", result);                        /* output result */
    }
    else
        fputs ("no characters in token
", stderr);
}

(note: the ternary simply handles the case txt is the empty-string. The ' ' is added as part of the 2nd delimiter to handle strings past from fgets() or POSIX getline() where no second '$' is present and ' ' is the last character in the string.)

Works also with any combination of empty-string, zero, one or two '$' and does not modify original so is safe for use with String-Literals.

Example Use/Output

$ ./bin/single_token
11/01/2017

Let me know if you have additional questions.

---

Variation Allowing Valid Empty-String as Result

A neat improvement provided by @chqrlie providing a test of (*sp == '$') instead of (ep > sp) would allow the empty-string (no characters in token) to be a valid result -- I agree). The change would be:

    if (*sp == '$') {                                   /* if chars in token */
        memcpy (result, sp + 1, ep - sp);               /* copy token to result */
        result[ep - sp] = 0;                            /* nul-termiante result */
        printf ("%s
", result);                        /* output result */
    }

So if you want to consider an empty token (like an empty field in a .csv, e.g. "one,,three,four") to be a valid token, use this alternative.