strstr() in C: Find a Substring and Its Position

strstr() searches a string for the first occurrence of another string. It returns a pointer to the start of the matching substring, or NULL when the substring is not found.

Syntax Top ↑

char *match = strstr(main_string, search_string);

Find a Substring and Its Position Top ↑

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

int main(void) {
    char text[] = "Welcome to C section of plus2net";
    char search[] = "to C";
    char *match = strstr(text, search);

    if (match != NULL) {
        printf("Zero-based position: %zu\n", (size_t)(match - text));
        printf("From the match: %s\n", match);
    } else {
        printf("Substring not found.\n");
    }

    return 0;
}

Check Whether a Word Exists Top ↑

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

int main(void) {
    char text[] = "C programming is fun";
    char word[] = "fun";

    if (strstr(text, word) != NULL) {
        printf("The word %s was found.\n", word);
    } else {
        printf("The word was not found.\n");
    }

    return 0;
}

Search User Input Top ↑

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

int main(void) {
    char text[100];
    char search[50];

    printf("Enter a string: ");
    if (fgets(text, sizeof(text), stdin) == NULL) {
        return 1;
    }
    text[strcspn(text, "\n")] = '\0';

    printf("Enter a substring: ");
    if (fgets(search, sizeof(search), stdin) == NULL) {
        return 1;
    }
    search[strcspn(search, "\n")] = '\0';

    char *match = strstr(text, search);
    if (match != NULL) {
        printf("Found at zero-based position %zu.\n", (size_t)(match - text));
    } else {
        printf("Substring not found.\n");
    }

    return 0;
}

Do Not Pass a Character to strstr() Top ↑

The second argument to strstr() must be a string. To search for one character, use strchr().

Preserved Example: Replace the First Matching Substring Top ↑

The original page included a search-and-replacement example. This safer version builds the result in a separate destination buffer instead of overwriting an unknown amount of storage at the match position.

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

int main(void) {
    const char text[] = "Welcome to C section of plus2net";
    const char search[] = "to C section";
    const char replacement[] = "to PHP section";
    char result[100];

    const char *match = strstr(text, search);
    if (match == NULL) {
        printf("No match found.\n");
        return 0;
    }

    size_t prefix = (size_t)(match - text);
    int written = snprintf(result, sizeof result,
                           "%.*s%s%s",
                           (int)prefix, text,
                           replacement,
                           match + strlen(search));

    if (written < 0 || (size_t)written >= sizeof result) {
        printf("Replacement did not fit.\n");
        return 1;
    }

    printf("%s\n", result);
    return 0;
}

strstr() in C: Find a Substring