Dave

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

static void php_ldap_do_escape(const unsigned char *value, const int valuelen, const unsigned char *map, unsigned char **result, int *resultlen)
{
        char hex[] = "0123456789abcdef";
        int i, p;
        size_t len;

        for (i = len = 0; i < valuelen; i++) {
            len += map[value[i]] ? 3 : 1;
        }

        (*result) = (unsigned char *) malloc(len + 1);
        (*resultlen) = (int) len;

        for (i = p = 0; i < valuelen; i++) {
                if (map[value[i]]) {
                        (*result)[p++] = 92;
                        (*result)[p++] = hex[value[i] >> 4];
                        (*result)[p++] = hex[value[i] & 0x0f];
                } else {
                        (*result)[p++] = value[i];
                }
        }

        (*result)[p] = '\0';
}

static void php_ldap_make_escape_map(unsigned char **map, const unsigned char *ignores, const int ignoreslen, const unsigned char *chars, const int charslen)
{
        int i;

        if (!*map) {
                (*map) = (unsigned char *)calloc(256, sizeof(char));

                if (!chars) {
                        for (i = 0; i < 256; i++) {
                                (*map)[i] = 1;
                        }
                        return;
                }
        }

        if (chars) {
                for (i = 0; i < charslen; i++) {
                        (*map)[chars[i]] = 1;
                }
        }

        if (ignores) {
                for (i = 0; i < ignoreslen; i++) {
                        (*map)[ignores[i]] = 0;
                }
        }
}

int main(char **argv) {
    unsigned char *value, *ignores, *map, *result;
    int valuelen, ignoreslen, resultlen;
    long flags;

    value = "#()*+=<foo>=+*()#";
    valuelen = strlen(value)+1;

    ignores = "()<>=";
    ignoreslen = strlen(ignores)+1;

    map = NULL;

    php_ldap_make_escape_map(&map, ignores, ignoreslen - 1, "\\*()\0", sizeof("\\*()\0") - 1);
    php_ldap_make_escape_map(&map, ignores, ignoreslen - 1, "\\,=+<>;\"#", sizeof("\\,=+<>;\"#") - 1);
    php_ldap_make_escape_map(&map, ignores, ignoreslen - 1, NULL, 0);
    php_ldap_do_escape(
        value, valuelen - 1, map, &result, &resultlen);

    printf("result: %s\n", result);

    free(map);
    free(result);
}