feat(engine): unlock variable-width lookbehind under engine=regex; annotate the re-engine failure with an actionable error

1 files changed, 59 insertions, 0 deletions

diff --git a/tests/builder/lookbehind.test.py b/tests/builder/lookbehind.test.py
new file mode 100644
index 0000000..f16a6cc
--- /dev/null
+++ b/tests/builder/lookbehind.test.py
@@ -0,0 +1,59 @@
+"""Tests for lookbehind width behavior across the ``re`` and ``regex`` engines."""
+
+import pytest
+
+from edify import RegexBuilder
+from edify.errors.backend import VariableWidthLookbehindNotSupportedError
+
+
+def _variable_width_lookbehind_builder():
+    return RegexBuilder().assert_behind().between(1, 3).string("foo").end().string("bar")
+
+
+def _fixed_width_lookbehind_builder():
+    return RegexBuilder().assert_behind().string("foo").end().string("bar")
+
+
+def test_variable_width_lookbehind_under_re_raises_annotated_error():
+    with pytest.raises(VariableWidthLookbehindNotSupportedError) as excinfo:
+        _variable_width_lookbehind_builder().to_regex(engine="re")
+    text = str(excinfo.value)
+    assert "assert_behind" in text
+    assert "engine='regex'" in text
+    assert "= note:" in text
+    assert "help:" in text
+
+
+def test_variable_width_lookbehind_under_regex_compiles_and_matches():
+    compiled = _variable_width_lookbehind_builder().to_regex(engine="regex")
+    assert compiled.engine == "regex"
+    assert compiled.search("foobar") is not None
+    assert compiled.search("foofoobar") is not None
+    assert compiled.search("foofoofoobar") is not None
+    assert compiled.search("bar") is None
+
+
+def test_fixed_width_lookbehind_still_works_under_re():
+    compiled = _fixed_width_lookbehind_builder().to_regex(engine="re")
+    assert compiled.search("foobar") is not None
+    assert compiled.search("bar") is None
+
+
+def test_variable_width_lookbehind_error_chains_the_underlying_pattern_error():
+    with pytest.raises(VariableWidthLookbehindNotSupportedError) as excinfo:
+        _variable_width_lookbehind_builder().to_regex(engine="re")
+    import re
+
+    assert isinstance(excinfo.value.__cause__, re.error)
+
+
+def test_re_engine_still_surfaces_other_pattern_errors_unchanged(monkeypatch):
+    import re
+
+    def raise_other_error(_pattern, flags=0):
+        raise re.error("some other syntax error")
+
+    monkeypatch.setattr(re, "compile", raise_other_error)
+    with pytest.raises(re.error) as excinfo:
+        RegexBuilder().digit().to_regex(engine="re")
+    assert "some other syntax error" in str(excinfo.value)