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How to Reverse a String in C?

How to Match a Pattern in a String in C?

Last Updated : 15 Dec, 2024

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Matching a pattern in a string involves searching for a specific sequence of characters within a larger string. In this article, we will learn different methods to efficiently match patterns in a string in C.

The most straightforward method to match a string is by using strstr() function. Let’s take a look at an example:

C

#include <stdio.h> #include <string.h>  int main() {     char s[] = "hello geeks";     char p[] = "geeks";      // Using strstr to find first occurrence     // of the pattern p in s     char *res = strstr(s, p);      if (res != NULL)         printf("Found at position: %ld", res - s);     else         printf("Not found.");      return 0; }

Output

Found at position: 6

Explanation: The strstr() function in C is used to find the first occurrence of a substring (pattern) within a larger string. It returns a pointer to the first occurrence of the pattern in the string, or NULL if the pattern is not found. The function performs a case-sensitive search, meaning it will only match the exact characters as specified in the pattern.

There are also a few other methods to match a pattern in a string in C. Some of them are as follows:

Using Regular Expressions

We can use the regcomp() function to compile a regular expression pattern and regexec() to search for it in the given string. If the pattern matches the string, the function returns 1, otherwise, it returns 0.

C

#include <stdio.h> #include <string.h> #include <regex.h>  int search(char *s, char *p) {     regex_t regex;     int ret;      // Compile the regular expression     ret = regcomp(&regex, p, 0);      // Execute the regular expression match     ret = regexec(&regex, s, 0, NULL, 0);     regfree(&regex);      // Return 1 if match found, 0 otherwise     return ret == 0;   }  int main() {     char s[] = "hello geeks";     char p[] = "geeks";      	// Matching pattern p in string s     int res = search(s, p);     if (res)         printf("Found");     else         printf("Not found.");      return 0; }

Output

Found

Using a Loop

In this method, we iterate through the string and compare each character in the string with the pattern. If we find a match, we return the position of the match.

C

#include <stdio.h>  int search(char *s, char *p) {     int i = 0, j = 0;      	// Traverse the string s     while (s[i] != '\0') {              	// Match the characters of p one by one         if (s[i] == p[j]) {             j++;                      	// If all characters are matched             if (p[j] == '\0') return 1;          } else j = 0;         i++;     }      	// If pattern is not found     return 0; }  int main() {     char s[] = "hello geeks";     char p[] = "geeks";      // Matching pattern p in string s     int res = search(s, p);     if (res)         printf("Found");     else         printf("Not found.");      return 0; }

Output

Found

How to Reverse a String in C?

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