how to get accurate results using callback functions after hs_scan

Hi, i want to know hot to get substring that matched with pattern. For example, if i have a string "hello 2019, by 2017", i want to use '[0-9]+' to get {2019,2018}, and i can get this result with boost::regex_search(). but if i run it in hyperscan, i can't get the same result.

Here is my code files that modified according to simplegrep.c, what's wrong?

`
static int eventHandler(unsigned int id, unsigned long long from, unsigned long long to, unsigned int flags, void *ctx) {

//printf("Match for pattern \"%s\" at offset %llu\n", (char *)ctx, to);
printf("matched!\n");
int length = to - from;
char* a = (char*)malloc(length*sizeof(char));
printf("from is %llu, end is %llu, length is %d \n", from, to, length);
strncpy(a, (char *)ctx + from, length);
printf("Match result is %s \n", a);
printf("test is %s \n", (char*)ctx);
free(a);
//size_t *matches = (size_t *)ctx;
//(*matches)++;
return 0;
}`

`
hs_scan(database, inputData, length, 0, scratch, eventHandler, inputData)
`

my test case is ./simplegrep [0-9]+ test.txt, and test.txt just have one string "hello 2018, bye 2007"

and my result is

Scanning 21 bytes with Hyperscan
matched!
from is 6, end is 7, length is 1
Match result is 2

test is hello 2018, bye 2017

matched!
from is 6, end is 8, length is 2
Match result is 20

test is hello 2018, bye 2017

matched!
from is 6, end is 9, length is 3
Match result is 201

test is hello 2018, bye 2017

matched!
from is 6, end is 10, length is 4
Match result is 2018
test is hello 2018, bye 2017

matched!
from is 16, end is 17, length is 1
Match result is 2

test is hello 2018, bye 2017

matched!
from is 16, end is 18, length is 2
Match result is 20

test is hello 2018, bye 2017

matched!
from is 16, end is 19, length is 3
Match result is 201

test is hello 2018, bye 2017

matched!
from is 16, end is 20, length is 4
Match result is 2017
test is hello 2018, bye 2017