diff --git a/stockfish/models.py b/stockfish/models.py
index 05c8157..e47ee62 100644
--- a/stockfish/models.py
+++ b/stockfish/models.py
@@ -35,6 +35,8 @@ class Stockfish:
         "10.0": "2018-11-29",
     }
 
+    _PIECE_CHARS = ("P", "N", "B", "R", "Q", "K", "p", "n", "b", "r", "q", "k")
+
     # _PARAM_RESTRICTIONS stores the types of each of the params, and any applicable min and max values, based
     # off the Stockfish source code: https://github.com/official-stockfish/Stockfish/blob/65ece7d985291cc787d6c804a33f1dd82b75736d/src/ucioption.cpp#L58-L82
     _PARAM_RESTRICTIONS: Dict[str, Tuple[type, Optional[int], Optional[int]]] = {
@@ -629,25 +631,35 @@ def _get_sf_go_command_output(self) -> List[str]:
     def _is_fen_syntax_valid(fen: str) -> bool:
         # Code for this function taken from: https://gist.github.com/Dani4kor/e1e8b439115878f8c6dcf127a4ed5d3e
         # Some small changes have been made to the code.
-        regexMatch = re.match(
+        if not re.match(
             r"\s*^(((?:[rnbqkpRNBQKP1-8]+\/){7})[rnbqkpRNBQKP1-8]+)\s([b|w])\s(-|[K|Q|k|q]{1,4})\s(-|[a-h][1-8])\s(\d+\s\d+)$",
             fen,
-        )
-        if not regexMatch:
+        ):
             return False
-        regexList = regexMatch.groups()
-        if len(regexList[0].split("/")) != 8:
-            return False  # 8 rows not present.
-        for fenPart in regexList[0].split("/"):
+
+        fen_fields = fen.split()
+
+        if any(
+            (
+                len(fen_fields) != 6,
+                len(fen_fields[0].split("/")) != 8,
+                any(x not in fen_fields[0] for x in "Kk"),
+                any(not fen_fields[x].isdigit() for x in (4, 5)),
+                int(fen_fields[4]) >= int(fen_fields[5]) * 2,
+            )
+        ):
+            return False
+
+        for fenPart in fen_fields[0].split("/"):
             field_sum: int = 0
             previous_was_digit: bool = False
             for c in fenPart:
-                if c in ["1", "2", "3", "4", "5", "6", "7", "8"]:
+                if "1" <= c <= "8":
                     if previous_was_digit:
                         return False  # Two digits next to each other.
                     field_sum += int(c)
                     previous_was_digit = True
-                elif c.lower() in ["p", "n", "b", "r", "q", "k"]:
+                elif c in Stockfish._PIECE_CHARS:
                     field_sum += 1
                     previous_was_digit = False
                 else:
diff --git a/tests/stockfish/test_models.py b/tests/stockfish/test_models.py
index bc505eb..93a7467 100644
--- a/tests/stockfish/test_models.py
+++ b/tests/stockfish/test_models.py
@@ -1149,25 +1149,46 @@ def test_is_fen_valid(self, stockfish: Stockfish):
         old_info = stockfish.info
         old_depth = stockfish._depth
         old_fen = stockfish.get_fen_position()
-        correct_fens = [
+        correct_fens: List[Optional[str]] = [
             "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1",
             "r1bQkb1r/ppp2ppp/2p5/4Pn2/8/5N2/PPP2PPP/RNB2RK1 b kq - 0 8",
             "4k3/8/4K3/8/8/8/8/8 w - - 10 50",
             "r1b1kb1r/ppp2ppp/3q4/8/P2Q4/8/1PP2PPP/RNB2RK1 w kq - 8 15",
+            "4k3/8/4K3/8/8/8/8/8 w - - 99 50",
         ]
         invalid_syntax_fens = [
             "r1bQkb1r/ppp2ppp/2p5/4Pn2/8/5N2/PPP2PPP/RNB2RK b kq - 0 8",
             "rnbqkb1r/pppp1ppp/4pn2/8/2PP4/8/PP2PPPP/RNBQKBNR w KQkq - 3",
             "rn1q1rk1/pbppbppp/1p2pn2/8/2PP4/5NP1/PP2PPBP/RNBQ1RK1 w w - 5 7",
             "4k3/8/4K3/71/8/8/8/8 w - - 10 50",
+            "r1bQkb1r/ppp2ppp/2p5/4Pn2/8/5N2/PPP2PPP/RNB2R2 b kq - 0 8",
+            "r1bQ1b1r/ppp2ppp/2p5/4Pn2/8/5N2/PPP2PPP/RNB2RK1 b kq - 0 8",
+            "4k3/8/4K3/8/8/8/8/8 w - - 100 50",
+            "4k3/8/4K3/8/8/8/8/8 w - - 101 50",
+            "4k3/8/4K3/8/8/8/8/8 w - - -1 50",
+            "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 0",
+            "r1b1kb1r/ppp2ppp/3q4/8/P2Q4/8/1PP2PPP/RNB2RK1 w kq - - 8 15",
+            "r1b1kb1r/ppp2ppp/3q4/8/P2Q4/8/1PP2PPP/RNB2RK1 w kq 8 15",
+            "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR W KQkq - 0 1",
+            "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR - KQkq - 0 1",
+            "r1bQkb1r/ppp2ppp/2p5/4Pn2/8/5N2/PPP2PPP/RNB2RK1 b kq - - 8",
+            "r1bQkb1r/ppp2ppp/2p5/4Pn2/8/5N2/PPP2PPP/RNB2RK1 b kq - 0 -",
+            "r1bQkb1r/ppp2ppp/2p5/4Pn2/8/5N2/PPP2PPP/RNB2RK1 b kq - -1 8",
+            "4k3/8/4K3/8/8/8/8/8 w - - 99 e",
+            "4k3/8/4K3/8/8/8/8/8 w - - 99 ee",
         ]
+        correct_fens.extend([None] * (len(invalid_syntax_fens) - len(correct_fens)))
+        assert len(correct_fens) == len(invalid_syntax_fens)
         for correct_fen, invalid_syntax_fen in zip(correct_fens, invalid_syntax_fens):
             old_del_counter = Stockfish._del_counter
-            assert stockfish.is_fen_valid(correct_fen)
+            if correct_fen is not None:
+                assert stockfish.is_fen_valid(correct_fen)
+                assert stockfish._is_fen_syntax_valid(correct_fen)
             assert not stockfish.is_fen_valid(invalid_syntax_fen)
-            assert stockfish._is_fen_syntax_valid(correct_fen)
             assert not stockfish._is_fen_syntax_valid(invalid_syntax_fen)
-            assert Stockfish._del_counter == old_del_counter + 2
+            assert Stockfish._del_counter == old_del_counter + (
+                2 if correct_fen is not None else 0
+            )
 
         time.sleep(2.0)
         assert stockfish._stockfish.poll() is None