raises: is_fully_escaped does not handle consecutive backslashes correctly (#14393)

Author: EternalRights

changelog/14392.bugfix.rst

@@ -0,0 +1 @@
+Fixed a bug in :func:`pytest.raises(match=...) <pytest.raises>` "fully escaped" detection, causing the regex diff display to be shown in some instances when the raw string diff display should be shown instead.

src/_pytest/raises.py

@@ -345,9 +345,10 @@ 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)
-    )
+    # Strip all escape sequences (backslash + any char), then check if any
+    # metacharacter remains unescaped in the resulting string.
+    stripped = re.sub(r"\\.", "", s)
+    return not any(c in metacharacters for c in stripped)
 
 
 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"\\\\|")