Find

The :find method finds the first occurrence of a pattern in a string (or nil if there isn't one).

To get everything before a / we can just find the / and take the subtring of everything before and after it.

str = "hiy1o/meyo:939a:a"

slashPos = str:find("/")

if slashPos then
    -- Found a /
    before = str:sub(1, slashPos - 1)
    -- hiy1o
    after = str:sub(slashPos + 1)
    -- meyo:939a:a
else
    -- No "/" in str
    -- (slashPos is nil)
end

Match

Alternatively, we can use match. It returns the matching parts of patterns contained in parenthesis.

str = "hiy1o/meyo:939a:a"

notSlash = "[^/]*" -- a list of at least 0 not-slash characters
anything = ".*" -- a list of any characters
matcher = "(" .. notSlash ..  ")" .. "/" .. "(" .. anything.. ")"
-- Pattern separated for clarity. Equivalent to this:
-- "([^/]*)/(.*)"

before, after = str:match( matcher )
if before then
    -- There was a slash
    -- before:  "hiy1o"
    -- after: "meyo:939a:a"
else
    -- No slash
end

Note: because I use * instead of + in my patterns, this will match things without anything before & after the slash. If you want to require there be things before and after, change the corresponding * to a + (with +, if nothing was typed, the match will fail and instead only return nils)