fix(strings): don't halve backslashes in interpolated string literals

String literals in expressions have their escapes decoded once by the lexer
(processEscapeCharacters), then interpolate() decoded every backslash pair a
second time, halving runs of backslashes (\\ -> \, \\\\ -> \\). Output
from an expression diverged from the same string read as input, and from jq.

interpolate() only needs to treat a backslash pair as an escape when it guards
an interpolation paren (\\( -> literal \(); a standalone pair must pass
through unchanged. Guard the skip on a following '(' so plain backslash runs
survive, mirroring the lexer's own \\( special case.

Fixes #2561

pkg/yqlib/operator_strings.go

@@ -62,8 +62,14 @@ func interpolate(d *dataTreeNavigator, context Context, str string) (string, err
 					i++
 					continue
 				case '\\':
-					// skip the escaped backslash
+					// A backslash pair is only an interpolation escape when it
-					i++
+					// guards an opening paren ("\\(" -> literal "\("). The lexer
+					// (processEscapeCharacters) has already decoded string escapes,
+					// so a standalone pair must pass through unchanged rather than
+					// be halved a second time (#2561).
+					if i+2 < len(runes) && runes[i+2] == '(' {
+						i++ // skip the second backslash; '(' is emitted as literal text next
+					}
 				default:
 					log.Debugf("Ignoring non-escaping backslash @ %v[%d]", str, i)
 				}

pkg/yqlib/operator_strings_test.go

@@ -29,6 +29,22 @@ var stringsOperatorScenarios = []expressionScenario{
 			"D0, P[], (!!str)::\\\n",
 		},
 	},
+	{
+		skipDoc:     true,
+		description: "Interpolation - backslashes are not halved (#2561)",
+		expression:  `"\\\\"`,
+		expected: []string{
+			"D0, P[], (!!str)::\\\\\n",
+		},
+	},
+	{
+		skipDoc:     true,
+		description: "Interpolation - odd backslash run preserved (#2561)",
+		expression:  `"\\\\\\"`,
+		expected: []string{
+			"D0, P[], (!!str)::\\\\\\\n",
+		},
+	},
 	{
 		skipDoc:     true,
 		description: "Interpolation - nested",