Deleting characters matching a pattern

Deleting characters matching a pattern

How to:

C doesn’t come with a built-in function for directly deleting characters from a string based on a pattern, unlike some higher-level languages. However, you can easily accomplish this task by manually iterating over the string and building a new one that excludes the unwanted characters. For instance, let’s assume you want to remove all digits from a string. You can do so as follows:

#include <stdio.h>
#include <ctype.h>

void remove_digits(char *str) {
    char *src = str, *dst = str;
    while (*src) {
        if (!isdigit((unsigned char)*src)) {
            *dst++ = *src;
    *dst = '\0';

int main() {
    char str[] = "C Programming 101: The Basics!";
    printf("Result: %s\n", str);
    return 0;

Sample output:

Result: C Programming : The Basics!

This example leverages isdigit from ctype.h to identify digits, shifting nondigit characters to the beginning of the string and terminating the string once all characters have been evaluated.

Deep Dive

The solution presented uses a two-pointer approach within the same array to effectively filter out unwanted characters, a technique emblematic of C’s hands-on memory management philosophy. This method is efficient because it operates in-place, avoiding the need for additional memory allocation and thus minimizing overhead.

Historically, the absence of high-level string manipulation functions in C has forced programmers to develop a deep understanding of string handling at the memory level, leading to innovative approaches like the one above. While this has the advantage of greater control and efficiency, it comes with a higher risk of errors, such as buffer overflows and off-by-one mistakes.

In modern development contexts, especially those emphasizing safety and security, languages that abstract away such low-level operations might be preferred for string manipulation tasks. Nevertheless, understanding and utilizing these C techniques remains invaluable for scenarios demanding fine-grained performance optimization or for working within environments where C’s minimalism and speed are paramount.