The strstr() function finds the first occurrence of a substring within a larger string. It returns a pointer to the beginning of the found substring, or NULL if the substring is not found.

Table of Contents

Basic Syntax

#include <string.h>
char *strstr(const char *haystack, const char *needle);

Parameter	Description
`haystack`	The main string to search in
`needle`	The substring to search for
Return value	Pointer to first occurrence, or `NULL` if not found

Example 1: Basic Substring Search

#include <stdio.h>
#include <string.h>
int main() {
char text[] = "The quick brown fox jumps over the lazy dog";
char word[] = "fox";
char *result = strstr(text, word);
if(result != NULL) {
printf("Found '%s' at position: %ld\n", word, result - text);
printf("Remaining string: %s\n", result);
} else {
printf("Substring not found\n");
}
return 0;
}

Output:

Found 'fox' at position: 16
Remaining string: fox jumps over the lazy dog

Example 2: Substring Not Found

#include <stdio.h>
#include <string.h>
int main() {
char sentence[] = "Hello World";
char search[] = "xyz";
char *result = strstr(sentence, search);
if(result == NULL) {
printf("'%s' not found in '%s'\n", search, sentence);
}
return 0;
}

Output:

'xyz' not found in 'Hello World'

Example 3: Finding All Occurrences

#include <stdio.h>
#include <string.h>
int main() {
char text[] = "the cat in the hat sat on the mat";
char search[] = "the";
char *ptr = text;
int count = 0;
while((ptr = strstr(ptr, search)) != NULL) {
printf("Found '%s' at position: %ld\n", search, ptr - text);
ptr++;  // Move past this occurrence
count++;
}
printf("Total occurrences: %d\n", count);
return 0;
}

Output:

Found 'the' at position: 0
Found 'the' at position: 11
Found 'the' at position: 23
Total occurrences: 3

Example 4: Extracting Text Before/After Substring

#include <stdio.h>
#include <string.h>
int main() {
char data[] = "username=john123&password=secret";
char delimiter[] = "=";
char *equal_sign = strstr(data, delimiter);
if(equal_sign != NULL) {
// Extract key (before =)
int key_len = equal_sign - data;
char key[50];
strncpy(key, data, key_len);
key[key_len] = '\0';
// Extract value (after =)
char *value = equal_sign + 1;
char val[50];
strcpy(val, value);
printf("Key: %s\n", key);
printf("Value: %s\n", val);
}
return 0;
}

Output:

Key: username
Value: john123&password=secret

Example 5: Finding File Extension

#include <stdio.h>
#include <string.h>
int main() {
char filename[] = "document.pdf";
char *dot = strstr(filename, ".");
if(dot != NULL) {
printf("Filename: %.*s\n", (int)(dot - filename), filename);
printf("Extension: %s\n", dot + 1);
}
// Another example with multiple dots
char file2[] = "backup.tar.gz";
char *last_dot = strrchr(file2, '.');  // strrchr finds last occurrence
if(last_dot != NULL) {
printf("\nFile: %s\n", file2);
printf("Extension: %s\n", last_dot + 1);
}
return 0;
}

Output:

Filename: document
Extension: pdf
File: backup.tar.gz
Extension: gz

Example 6: Case-Sensitive vs Case-Insensitive

#include <stdio.h>
#include <string.h>
#include <ctype.h>
// Custom case-insensitive version
char *stristr(const char *haystack, const char *needle) {
if(!*needle) return (char*)haystack;
for(; *haystack; haystack++) {
if(tolower(*haystack) == tolower(*needle)) {
const char *h = haystack;
const char *n = needle;
while(*h && *n && tolower(*h) == tolower(*n)) {
h++;
n++;
}
if(!*n) return (char*)haystack;
}
}
return NULL;
}
int main() {
char text[] = "Hello WORLD, world is beautiful";
// strstr is case-sensitive
char *result1 = strstr(text, "world");
printf("strstr('world'): %s\n", result1 ? "Found" : "Not found");
char *result2 = strstr(text, "WORLD");
printf("strstr('WORLD'): %s\n", result2 ? result2 : "Not found");
// Case-insensitive version
char *result3 = stristr(text, "world");
printf("stristr('world'): %s\n", result3);
return 0;
}

Output:

strstr('world'): Found
strstr('WORLD'): WORLD, world is beautiful
stristr('world'): Hello WORLD, world is beautiful

Example 7: Replacing Substring

#include <stdio.h>
#include <string.h>
void replaceFirst(char *text, const char *find, const char *replace) {
char *pos = strstr(text, find);
if(pos != NULL) {
char temp[200];
int prefix_len = pos - text;
int find_len = strlen(find);
// Build new string
strncpy(temp, text, prefix_len);
temp[prefix_len] = '\0';
strcat(temp, replace);
strcat(temp, pos + find_len);
// Copy back
strcpy(text, temp);
}
}
int main() {
char message[200] = "I like apples and apples are tasty";
printf("Original: %s\n", message);
replaceFirst(message, "apples", "oranges");
printf("After replace: %s\n", message);
return 0;
}

Output:

Original: I like apples and apples are tasty
After replace: I like oranges and apples are tasty

Example 8: Validating String Format

#include <stdio.h>
#include <string.h>
int isValidEmail(const char *email) {
// Check for @ symbol
char *at = strstr(email, "@");
if(at == NULL) return 0;
// Check for dot after @
char *dot = strstr(at, ".");
if(dot == NULL) return 0;
// Check minimal length
if(at - email < 1) return 0;
if(strlen(dot + 1) < 2) return 0;
return 1;
}
int main() {
char emails[][50] = {
"[email protected]",
"invalid.email",
"test@domain",
"[email protected]"
};
for(int i = 0; i < 4; i++) {
printf("%s: %s\n", emails[i], 
isValidEmail(emails[i]) ? "Valid" : "Invalid");
}
return 0;
}

Output:

[email protected]: Valid
invalid.email: Invalid
test@domain: Invalid
[email protected]: Valid

Common Mistakes

#include <stdio.h>
#include <string.h>
int main() {
char text[] = "Hello World";
// ❌ WRONG - Checking wrong way
if(strstr(text, "World") > 0) {  // Comparing pointer to int
printf("This comparison is wrong\n");
}
// ✅ CORRECT - Check against NULL
if(strstr(text, "World") != NULL) {
printf("Found correctly\n");
}
// ❌ WRONG - Empty needle (returns haystack always)
char *result = strstr(text, "");
printf("Empty needle returns: %s\n", result);  // Returns whole string
// ❌ WRONG - Modifying string literal with result
char *literal = "Constant string";
char *found = strstr(literal, "stant");
// found[0] = 'X';  // CRASH - String literal is read-only
// ✅ CORRECT - Use modifiable array
char modifiable[] = "Constant string";
char *found2 = strstr(modifiable, "stant");
if(found2) found2[0] = 'X';  // Works fine
printf("Modified: %s\n", modifiable);
return 0;
}

Output:

Found correctly
Empty needle returns: Hello World
Modified: ConXtant string

strstr vs Other Search Functions

Function	Description	Return Value
`strstr(haystack, needle)`	Find first substring	Pointer or NULL
`strchr(str, char)`	Find first character	Pointer or NULL
`strrchr(str, char)`	Find last character	Pointer or NULL
`strpbrk(str, chars)`	Find any character from set	Pointer or NULL
`strspn(str, chars)`	Length of initial segment	Length (size_t)
`strcspn(str, chars)`	Length before any match	Length (size_t)

Performance Considerations

#include <stdio.h>
#include <string.h>
#include <time.h>
// Simple naive search (similar to strstr)
char *naive_search(const char *haystack, const char *needle) {
size_t hlen = strlen(haystack);
size_t nlen = strlen(needle);
if(nlen == 0) return (char*)haystack;
for(size_t i = 0; i <= hlen - nlen; i++) {
if(memcmp(haystack + i, needle, nlen) == 0) {
return (char*)(haystack + i);
}
}
return NULL;
}
int main() {
char text[1000] = "This is a test string for searching patterns";
char pattern[] = "searching";
// strstr is highly optimized (often uses advanced algorithms)
clock_t start = clock();
for(int i = 0; i < 1000000; i++) {
strstr(text, pattern);
}
clock_t end = clock();
printf("strstr time: %f seconds\n", (double)(end - start) / CLOCKS_PER_SEC);
return 0;
}

Practical Example: Simple HTML Parser

#include <stdio.h>
#include <string.h>
void extractLinks(const char *html) {
const char *ptr = html;
char *link_start, *link_end;
while((link_start = strstr(ptr, "href=\"")) != NULL) {
link_start += 6;  // Skip 'href="'
link_end = strstr(link_start, "\"");
if(link_end != NULL) {
int len = link_end - link_start;
printf("Found link: %.*s\n", len, link_start);
ptr = link_end;
} else {
break;
}
}
}
int main() {
char html[] = 
"<a href=\"https://example.com\">Example</a> "
"<a href=\"/page.html\">Page</a> "
"<img src=\"image.jpg\">";
extractLinks(html);
return 0;
}

Output:

Found link: https://example.com
Found link: /page.html

Quick Reference

Operation	Code	Result
Find substring	`strstr(text, "word")`	Pointer or NULL
Get position	`pos = strstr(text, "word") - text`	Index or -1 if not found
Check exists	`if(strstr(text, "word"))`	True if found
Find after position	`strstr(ptr + 1, "word")`	Search after current
Extract before	`strncpy(buf, text, pos - text)`	Text before match
Extract after	`strcpy(buf, pos + len)`	Text after match

Best Practices

Always check if result is NULL before using the pointer
Don't modify string literals returned by strstr
Use pointer arithmetic to get position indices
Remember it's case-sensitive - convert both strings to same case if needed
Empty needle returns haystack - handle this case if problematic
For single characters, use strchr (more efficient than strstr)

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Explains substring search inside strings in C.

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Basic Syntax

Example 1: Basic Substring Search

Example 2: Substring Not Found

Example 3: Finding All Occurrences

Example 4: Extracting Text Before/After Substring

Example 5: Finding File Extension

Example 6: Case-Sensitive vs Case-Insensitive

Example 7: Replacing Substring

Example 8: Validating String Format

Common Mistakes

strstr vs Other Search Functions

Performance Considerations

Practical Example: Simple HTML Parser

Quick Reference

Best Practices

Complete C Programming Guide + Compilers Collection

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2. C rand() Function Mechanics and Limitations

3. C log() Function

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5. Mastering time_t Type in C

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7. C log() Function (Alternate Guide)

8. C log10() Function

9. C tan() Function

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11. Free Online C Compiler

C Functions, Arguments, Parameters & Flow

Mastering Functions in C – Complete Guide

Function Arguments in C

Function Parameters in C

Function Declarations in C

Function Calls in C

Void Functions in C

Return Values in C

Pass-by-Value in C

Pass-by-Reference in C

C strstr() Function

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C Structures, Memory, Scope & Linkage

C Scope, Storage Classes & Typedef

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Online Compilers

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