Test suites from lua.org:
```
-- tests for gmatch
local a = 0
for i in string.gmatch('abcde', '()') do assert(i == a+1); a=i end
assert(a==6)
t = {n=0}
for w in string.gmatch("first second word", "%w+") do
t.n=t.n+1; t[t.n] = w
end
assert(t[1] == "first" and t[2] == "second" and t[3] == "word")
t = {3, 6, 9}
for i in string.gmatch ("xuxx uu ppar r", "()(.)%2") do
assert(i == table.remove(t, 1))
end
assert(#t == 0)
t = {}
for i,j in string.gmatch("13 14 10 = 11, 15= 16, 22=23", "(%d+)%s*=%s*(%d+)") do
t[tonumber(i)] = tonumber(j)
end
a = 0
for k,v in pairs(t) do assert(k+1 == v+0); a=a+1 end
assert(a == 3)
-- tests for `%f' (`frontiers')
assert(string.gsub("aaa aa a aaa a", "%f[%w]a", "x") == "xaa xa x xaa x")
assert(string.gsub("[[]] [][] [[[[", "%f[[].", "x") == "x[]] x]x] x[[[")
assert(string.gsub("01abc45de3", "%f[%d]", ".") == ".01abc.45de.3")
assert(string.gsub("01abc45 de3x", "%f[%D]%w", ".") == "01.bc45 de3.")
assert(string.gsub("function", "%f[\1-\255]%w", ".") == ".unction")
assert(string.gsub("function", "%f[^\1-\255]", ".") == "function.")
assert(string.find("a", "%f[a]") == 1)
assert(string.find("a", "%f[^%z]") == 1)
assert(string.find("a", "%f[^%l]") == 2)
assert(string.find("aba", "%f[a%z]") == 3)
assert(string.find("aba", "%f[%z]") == 4)
assert(not string.find("aba", "%f[%l%z]"))
assert(not string.find("aba", "%f[^%l%z]"))
local i, e = string.find(" alo aalo allo", "%f[%S].-%f[%s].-%f[%S]")
assert(i == 2 and e == 5)
local k = string.match(" alo aalo allo", "%f[%S](.-%f[%s].-%f[%S])")
assert(k == 'alo ')
local a = {1, 5, 9, 14, 17,}
for k in string.gmatch("alo alo th02 is 1hat", "()%f[%w%d]") do
assert(table.remove(a, 1) == k)
end
assert(#a == 0)
```
Code from lua:
https://www.lua.org/source/5.2/lstrlib.c.html
```
static int gmatch_aux (lua_State *L) {
MatchState ms;
size_t ls, lp;
const char *s = lua_tolstring(L, lua_upvalueindex(1), &ls);
const char *p = lua_tolstring(L, lua_upvalueindex(2), &lp);
const char *src;
ms.L = L;
ms.matchdepth = MAXCCALLS;
ms.src_init = s;
ms.src_end = s+ls;
ms.p_end = p + lp;
for (src = s + (size_t)lua_tointeger(L, lua_upvalueindex(3));
src <= ms.src_end;
src++) {
const char *e;
ms.level = 0;
lua_assert(ms.matchdepth == MAXCCALLS);
if ((e = match(&ms, src, p)) != NULL) {
lua_Integer newstart = e-s;
if (e == src) newstart++; /* empty match? go at least one position */
lua_pushinteger(L, newstart);
lua_replace(L, lua_upvalueindex(3));
return push_captures(&ms, src, e);
}
}
return 0; /* not found */
}
```
On empty match need increase string offset.
