fix mark expression scanner: search for backslash in string value, not entire input

changelog/14474.bugfix.rst

@@ -0,0 +1 @@
+Fixed a regression where ``-k`` and ``-m`` expressions containing both backslash characters in identifiers and string literal arguments would incorrectly raise a ``SyntaxError`` about escaping.

src/_pytest/mark/expression.py

@@ -102,10 +102,10 @@ def lex(self, input: str) -> Iterator[Token]:
                         (FILE_NAME, 1, pos + 1, input),
                     )
                 value = input[pos : end_quote_pos + 1]
-                if (backslash_pos := input.find("\\")) != -1:
+                if (backslash_pos := value.find("\\")) != -1:
                     raise SyntaxError(
                         r'escaping with "\" not supported in marker expression',
-                        (FILE_NAME, 1, backslash_pos + 1, input),
+                        (FILE_NAME, 1, pos + backslash_pos + 1, input),
                     )
                 yield Token(TokenType.STRING, value, pos)
                 pos += len(value)

testing/test_mark_expression.py

@@ -86,6 +86,20 @@ def matcher(name: str, /, **kwargs: str | int | bool | None) -> bool:
         evaluate("\nfoo\n", matcher)
 
 
+def test_backslash_in_identifier_with_string_literal() -> None:
+    r"""Backslashes in identifiers should not cause false rejections when the
+    expression also contains string literals. Regression test for a bug where
+    the scanner searched the entire input for backslashes instead of only the
+    current string literal value."""
+
+    def matcher(name: str, /, **kwargs: str | int | bool | None) -> bool:
+        return {r"\nfoo\n", r"test\case", "mark"}.__contains__(name)
+
+    assert evaluate(r'\nfoo\n and mark(x="y")', matcher)
+    assert evaluate(r'mark(x="y") and \nfoo\n', matcher)
+    assert evaluate(r'test\case and mark(x="y")', matcher)
+
+
 @pytest.mark.parametrize(
     ("expr", "column", "message"),
     (