regex.c


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int match(char r, char t)
{
    return r == '.' || r == t;
}
int matchstr(const char *r, const char *t)
{
    int state = 0, i = (r[0] == '^' ? 1 : 0), j = 0;
    while (1)
    {
        if (r[i] == '\0')
            return 1;
        if (r[i] == '$' && r[i + 1] == '\0')
            return t[j] == '\0';
        if (t[j] == '\0')
            return 0;

        if (state == 0) // base case
        {
            if (r[i + 1] == '*')
                state = 1;
            else if (match(r[i], t[j]))
                i++, j++;
            else if (r[0] == '^')
                return 0;
            else
                i = 0;
        }
        if (state == 1) // star
        {
            if (!match(r[i], t[j]) || match(r[i + 2], t[j]))
                state = 0, i += 2;
            else
                j++;
        }
    }
    return 0;
}

#include <stdio.h>

int main(int argc, char **argv)
{
    if (argc != 3)
    {
        printf("Usage: %s <regex> <text>\n", argv[0]);
        return 2;
    }
    return !matchstr(argv[1], argv[2]);
}