Rework the way we read a userID or repoID from the environment

- TIL : mkRegexWithOpts documentation is misleading. The first boolean
option is described as enforcing beginning of line (`^`) and end of
line (`$`) matching, along with not matching newline characters with
`.`. From my testing, I discovered this is not true, and in order to
enforce that regex behavior, I needed to explicitly add `^…$` to my
regex. Because the `mkRegexWithOpts` function was no longer doing what
I wanted, I opted for the simpler `mkRegex` function.

- readInt now returns a `Maybe Int` and expects a single `Just [s]`
match  supplied to attempt reading. This function is not general
purpose enough to be safe to use elsewhere, but is safe to use within
the context of `readIntFromEnv` because of the way the regex enforces
matches.

src/GitmonClient.hs

@@ -135,12 +135,17 @@ reportGitmon' SocketFactory{..} program gitCommand = do
 
         readIntFromEnv :: Maybe String -> Maybe Int
         readIntFromEnv Nothing = Nothing
-        readIntFromEnv (Just s) = Just (readInt $ subRegex regex s "")
-          where regex :: Regex
-                regex = mkRegexWithOpts "uint:" True False
+        readIntFromEnv (Just s) = readInt $ matchRegex regex s
+          where
+            -- | Expected format for userID and repoID is: "uint:123",
+            -- | where "uint:" indicates an unsigned integer followed by an integer value.
+            regex :: Regex
+            regex = mkRegex "^uint:([0-9]+)$"
+
+            readInt :: Maybe [String] -> Maybe Int
+            readInt (Just [s]) = Just (read s :: Int)
+            readInt _ = Nothing
 
-                readInt :: String -> Int
-                readInt s = read s :: Int
 
 withGitmonSocket :: (Socket -> IO c) -> IO c
 withGitmonSocket = bracket connectSocket close