а теперь твоя задача, максимально подробно объяснить свой код, добавь комментарий на каждую строку кода. Так чтобы один раз прочитав я смог повторить это самостоятельно
#include <stdio.h>
#include <getopt.h>
#include <stdlib.h>
#include <string.h>
#include <regex.h>

#define MAX_PATTERN_LENGTH 9999999
#define MAX_LINE_LENGTH 10000

typedef struct flag {
    int e;
    int i;
    int v;
    int c;
    int l;
    int n;
    int filecount;
} flags;

typedef struct patterns {
    char **patterns;
    int count;
} patterns_t;

// Функции
int s21_parser(int argc, char *argv[], flags *flag, patterns_t *pat);
void process_file(const char *filename, flags flag, patterns_t pat);
int match_pattern(const char *line, patterns_t pat, flags flag);
void print_match(const char *filename, int line_num, const char *line, flags flag);
void free_patterns(patterns_t *pat);

int main (int argc, char *argv[]) {
    flags flag = {0, 0, 0, 0, 0, 0, 0};
    patterns_t pat = {NULL, 0};
    int ret = 0;

    int start = s21_parser(argc, argv, &flag, &pat);
    if (start == -1) {
        ret = 1;
    } else {
        if (flag.filecount == 0) {
            process_file("", flag, pat);
        } else {
            for (int i = start; i < argc; i++) {
                process_file(argv[i], flag, pat);
            }
        }
    }

    free_patterns(&pat);
    return ret;
}

int s21_parser(int argc, char *argv[], flags *flag, patterns_t *pat) {
    int option;
    int ret = 0;

    pat->patterns = malloc(sizeof(char*) * argc);
    if (!pat->patterns) {
        fprintf(stderr, "Memory allocation error\n");
        ret = -1;
    }

    if (ret == 0) {
        pat->count = 0;

        while ((option = getopt(argc, argv, "e:ivcln")) != -1) {
            switch (option) {
                case 'e':
                    flag->e = 1;
                    pat->patterns[pat->count++] = strdup(optarg);
                    break;
                case 'i':
                    flag->i = 1;
                    break;
                case 'v':
                    flag->v = 1;
                    break;
                case 'c':
                    flag->c = 1;
                    break;
                case 'l':
                    flag->l = 1;
                    break;
                case 'n':
                    flag->n = 1;
                    break;
                case '?':
                    fprintf(stderr, "Unknown option: -%c\n", optopt);
                    ret = -1;
                    break;
                default:
                    break;
            }
            if (ret == -1) {
                break;
            }
        }
    }

    if (ret == 0 && !flag->e) {
        if (optind < argc) {
            pat->patterns[pat->count++] = strdup(argv[optind++]);
        } else {
            fprintf(stderr, "No pattern provided.\n");
            ret = -1;
        }
    }

    if (ret == 0) {
        flag->filecount = argc - optind;
    } else {
        free_patterns(pat);
    }

    if (ret == -1) {
        return -1;
    } else {
        return optind;
    }
}

void free_patterns(patterns_t *pat) {
    if (pat->patterns) {
        for (int i = 0; i < pat->count; i++) {
            free(pat->patterns[i]);
        }
        free(pat->patterns);
        pat->patterns = NULL;
    }
}

int match_pattern(const char *line, patterns_t pat, flags flag) {
    int match = 0;
    regex_t regex;
    int reti;
    int cflags = REG_EXTENDED;
    if (flag.i) {
        cflags |= REG_ICASE;
    }

    for (int i = 0; i < pat.count; i++) {
        reti = regcomp(&regex, pat.patterns[i], cflags);
        if (reti) {
            fprintf(stderr, "Could not compile regex: %s\n", pat.patterns[i]);
            continue;
        }
        reti = regexec(&regex, line, 0, NULL, 0);
        if (!reti) {
            match = 1;
        } else if (reti == REG_NOMATCH) {
            // No match, ничего не делаем
        } else {
            char errbuf[100];
            regerror(reti, &regex, errbuf, sizeof(errbuf));
            fprintf(stderr, "Regex match failed: %s\n", errbuf);
        }
        regfree(&regex);
        if (match)
            break;
    }

    if (flag.v)
        return !match;
    else
        return match;
}

void print_match(const char *filename, int line_num, const char *line, flags flag) {
    if (flag.filecount > 1 && filename[0] != '\0') {
        printf("%s:", filename);
    }
    if (flag.n) {
        printf("%d:", line_num);
    }
    printf("%s", line);
    if (line[strlen(line) - 1] != '\n') {
        printf("\n");
    }
}

void process_file(const char *filename, flags flag, patterns_t pat) {
    FILE *file;
    int need_processing = 1;

    if (filename[0] == '\0') {
        file = stdin;
    } else {
        file = fopen(filename, "r");
        if (!file) {
            fprintf(stderr, "s21_grep: %s: Cannot open file\n", filename);
            need_processing = 0;
        }
    }

    if (need_processing) {
        char *line = NULL;
        size_t len = 0;
        ssize_t read;
        int count = 0;
        int matched = 0;
        int line_num = 0;

        while ((read = getline(&line, &len, file)) != -1) {
            line_num++;
            matched = match_pattern(line, pat, flag);
            if (matched) {
                count++;
                if (!flag.c && !flag.l) {
                    print_match(filename, line_num, line, flag);
                }
            }
        }

        if (flag.c) {
            if (flag.filecount > 1 && filename[0] != '\0') {
                printf("%s:%d\n", filename, count);
            } else {
                printf("%d\n", count);
            }
        }

        if (flag.l && count > 0 && filename[0] != '\0') {
            printf("%s\n", filename);
        }

        free(line);
    }

    if (file != stdin) {
        fclose(file);
    }
}