How to Match a Pattern in a String in C?

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:

#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.

#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.

#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 Match a Pattern in a String in C?

Using Regular Expressions

Using a Loop

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