Fix: is_fully_escaped does not handle consecutive backslashes correctly

changelog/14392.bugfix.rst

@@ -0,0 +1 @@
+Fixed ``is_fully_escaped`` not handling consecutive backslashes correctly: an escaped backslash before a metacharacter (e.g. ``\\\\.``) was incorrectly treated as escaping the metacharacter itself, causing ``pytest.raises(match=...)`` to skip the regex diff display when it should have shown one.

src/_pytest/raises.py

@@ -345,9 +345,20 @@ def _check_raw_type(
 def is_fully_escaped(s: str) -> bool:
     # we know we won't compile with re.VERBOSE, so whitespace doesn't need to be escaped
     metacharacters = "{}()+.*?^$[]|"
-    return not any(
-        c in metacharacters and (i == 0 or s[i - 1] != "\\") for (i, c) in enumerate(s)
-    )
+    for i, c in enumerate(s):
+        if c in metacharacters:
+            # Count consecutive backslashes preceding this metacharacter.
+            # An odd number of backslashes means the metacharacter is escaped
+            # (the last backslash does the escaping); an even number means
+            # it is not escaped (backslashes escape each other in pairs).
+            n_backslashes = 0
+            j = i - 1
+            while j >= 0 and s[j] == "\\":
+                n_backslashes += 1
+                j -= 1
+            if n_backslashes % 2 == 0:
+                return False
+    return True
 
 
 def unescape(s: str) -> str:

testing/python/raises.py

@@ -444,3 +444,19 @@ def test_pipe_is_treated_as_regex_metacharacter(self) -> None:
         assert not is_fully_escaped("foo|bar")
         assert is_fully_escaped(r"foo\|bar")
         assert unescape(r"foo\|bar") == "foo|bar"
+
+    def test_consecutive_backslashes_in_escape_check(self) -> None:
+        """Consecutive backslashes escape each other, leaving the metachar unescaped."""
+        from _pytest.raises import is_fully_escaped
+
+        # r"\."  -> one backslash escapes the dot -> fully escaped
+        assert is_fully_escaped(r"\.")
+        # r"\\." -> two backslashes: the first escapes the second, dot is unescaped
+        assert not is_fully_escaped(r"\\.")
+        # r"\\\." -> three backslashes: pair escapes pair, last escapes dot -> fully escaped
+        assert is_fully_escaped(r"\\\.")
+        # Same idea with pipe metachar
+        # "\\\\|" is the string \\| (2 backslashes + pipe): even count, pipe is unescaped
+        assert not is_fully_escaped("\\\\|")
+        # r"\\\\|" is the string \\\\| (4 backslashes + pipe): even count, pipe is unescaped
+        assert not is_fully_escaped(r"\\\\|")