fix(python): fix regex lookbehind patterns being incorrectly converted (#4560)

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Dag Brattli <dag@brattli.net>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 4 people

src/fable-library-py/fable_library/reg_exp.py

@@ -71,7 +71,10 @@ def _options_to_flags(options: int) -> int:
 def create(pattern: str, options: int = 0) -> Pattern[str]:
     # raw_pattern = pattern.encode("unicode_escape").decode("utf-8")
     flags = _options_to_flags(options)
-    pattern = pattern.replace("?<", "?P<")
+    # Convert .NET named group syntax (?<name>...) to Python (?P<name>...).
+    # Only replace ?< when followed by a word character (name start), not by
+    # = or ! which are part of lookbehind assertions (?<=...) and (?<!...).
+    pattern = re.sub(r"\?<(?=[A-Za-z_])", "?P<", pattern)
     return re.compile(pattern, flags=flags)
 
 

tests/Python/TestRegex.fs

@@ -636,3 +636,17 @@ let ``test doesn't succeed when not existing group without any named groups`` ()
     let actual = r.Replace(text, replace)
 
     actual |> equal expected
+
+[<Fact>]
+let ``test positive lookbehind works`` () =
+    let r = Regex @"(?<=A)\w+"
+    let text = "AB AC AD BD"
+    let ms = r.Matches(text) |> Seq.map (fun m -> m.Value) |> Seq.toList
+    ms |> equal [ "B"; "C"; "D" ]
+
+[<Fact>]
+let ``test negative lookbehind works`` () =
+    let r = Regex @"(?<!A)\d"
+    let text = "A1 B2 A3 C4"
+    let ms = r.Matches(text) |> Seq.map (fun m -> m.Value) |> Seq.toList
+    ms |> equal [ "2"; "4" ]