num-replace.c

#include <errno.h>
#include <ctype.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#if defined(TEST_1)
const char *input = "1str, 123 gdg1(75); test32 0";
#elif defined(TEST_2)
const char *input = "1 str, 123 gdg1(75); test32 0";
#elif defined(TEST_3)
const char *input = "str, 123 gdg1(75); test32 0s";
#elif defined(TEST_4)
const char *input = "str, 123 gdg1(75); test32 0 s";
#elif defined(TEST_5)
const char *input = "str, 123 gdg1(75); test32 1d1 0";
#elif defined(TEST_6)
const char *input = "str, 123 gdg1(75); test32 d1d 0";
#elif defined(TEST_7)
const char *input = "1 123 1 75 3 1";
#elif defined(TEST_8)
const char *input = "a bcd e fg h j";
#elif defined(TEST_9)
const char *input = "a";
#elif defined(TEST_10)
const char *input = "0";
#else
const char *input = "str, 123 gdg1(75); test32 0";
#endif

const char *RE = "NUM";

static char *put_replacement(char *out, const char *r, size_t len)
{
    memcpy(out, r, len);
    return out + len;
}

static void replace(const char *in, char *out, size_t outlen)
{
    const char *q;
    int flag = 0;
    char p;

    while ((p = *in++)) {
        if (flag == 0) {
            if (isdigit(p)) {
                q = in - 1;
                flag = 1;
                continue;
            }
            if (isalpha(p))
                flag = 2;
        }

        if (flag == 1) {
            if (isdigit(p))
                continue;
            if (isalpha(p)) {
                out = put_replacement(out, q, in - 1 - q);
                flag = 2;
            } else {
                out = put_replacement(out, RE, strlen(RE));
                flag = 0;
            }
        }

        if (flag == 2 && !(isalpha(p) || isdigit(p))) {
            flag = 0;
        }

        *out++ = p;
    }

    if (flag == 1)
        out = put_replacement(out, RE, strlen(RE));

    *out++ = '\0';
}

int main(int argc, char *argv[])
{
    const char *in = input;
    size_t outlen;
    char *out;

    /*
     * New length is counted as old * strlen(RE) / 2 + strlen(RE) since we
     * have to have delimiters. Additional strlen(RE) for the case, when
     * initial length is too small. And 1 for '\0' terminator.
     */
    outlen = strlen(in) * strlen(RE) / 2 + strlen(RE) + 1;
    out = malloc(outlen);
    if (!out)
        return ENOMEM;

    replace(in, out, outlen);

    printf("in: %s\n", in);
    printf("out: %s\n", out);

    free(out);
    return 0;
}