Using regular expressions

Using regular expressions

How to:

To use regular expressions in C, you’ll primarily be working with the POSIX regex library (<regex.h>). This example demonstrates basic pattern matching:

#include <stdio.h>
#include <stdlib.h>
#include <regex.h>

int main(){
    regex_t regex;
    int return_value;
    char *pattern = "^a[[:alnum:]]"; // Pattern to match strings starting with 'a' followed by alphanumeric characters
    char *test_string = "apple123";

    // Compile the regular expression
    return_value = regcomp(&regex, pattern, REG_EXTENDED);
    if (return_value) {
        printf("Could not compile regex\n");

    // Execute the regular expression
    return_value = regexec(&regex, test_string, 0, NULL, 0);
    if (!return_value) {
        printf("Match found\n");
    } else if (return_value == REG_NOMATCH) {
        printf("No match found\n");
    } else {
        printf("Regex match failed\n");

    // Free allocated memory used by the regex

    return 0;

Sample output for a matching string (“apple123”):

Match found

And for a non-matching string (“banana”):

No match found

Deep Dive:

Regular expressions in C, as part of the POSIX standard, offer a robust way to perform string matching and manipulation. However, the POSIX regex library’s API in C is considered more cumbersome than those found in languages designed with first-class string manipulation features like Python or Perl. The syntax for patterns is similar across languages, but C requires manual memory management and more boilerplate code to prepare, execute, and clean up after using regex patterns.

Despite these challenges, learning to use regex in C is rewarding because it deepens understanding of lower-level programming concepts. Additionally, it opens up possibilities for C programming in areas such as text processing and data extraction where regex is indispensable. For more complex patterns or regex operations, alternatives such as PCRE (Perl Compatible Regular Expressions) library might offer a more feature-rich and somewhat easier interface, though it requires integrating an external library into your C project.