book: beautiful code, chapter 1

bcc1.cpp

/*
 * Character | Meaning
 * c         | Matches any literal character c
 * .         | Matches any single character
 * ^         | Matches the beginning of the input string
 * $         | Matches the end of the input string
 * *         | Matches zero or more occurrences of the previous character
 */

#include <cstdio>

int match(char const* regexp, char const* text);
int matchhere(char const* regexp, char const* text);
int matchstar(char c, char const* regexp, char const* text) ;

int match(char const* regexp, char const* text) {
    if (regexp[0] == '^') {
        return matchhere(regexp + 1, text);
    }
    do {
        if (matchhere(regexp, text)) {
            return 1;
        }
    } while (*text++ != '\0');
    return 0;
}

int matchhere(char const* regexp, char const* text) {
    if (regexp[0] == '\0') {
        return 1;
    }
    if (regexp[1] == '*') {
        return matchstar(regexp[0], regexp + 2, text);
    }
    if (regexp[0] == '$' && regexp[1] == '\0') {
        return *text == '\0';
    }
    if (*text != '\0' && (regexp[0] == '.' || regexp[0] == *text)) {
        return matchhere(regexp + 1, text + 1);
    }
    return 0;
}

int matchstar(char c, char const* regexp, char const* text) {
    do {
        if (matchhere(regexp, text)) {
            return 1;
        }
    } while (*text != '\0' && (*text++ == c || c == '.'));
    return 0;
}

void test(char const* regexp, char const* text) {
    printf("regexp [%s] %s [%s]\n", regexp, (match(regexp, text) == 1 ? "matches" : "doesn't match"), text);
}

int main(int ac, char** av) {
    if (ac == 1) {
        test("a.c", "abc");
        test("a*c", "abc");
        return 0;
    }

    if (match(av[1], av[2])) {
        printf("regexp [%s] matches [%s]\n", av[1], av[2]);
    } else {
        printf("No match\n");
    }
}