FPC 3.3.1 Bug Report: fpjson Fails to Parse Backslashes Correctly
lazarus / 64-FPC development

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EsequiasTorres · 2025-12-29 18:18

FPC 3.3.1 Bug Report: fpjson Fails to Parse Backslashes Correctly

Date: December 29, 2025
Last Verified: December 29, 2025 (Bug still present in FPC
3.3.1-19122-ge8d90a3042)
Affected Version: Free Pascal Compiler 3.3.1 (Trunk)
IDE: Lazarus 4.5
Operating System: Windows 10
Component: FCL-JSON (fpjson, jsonscanner)
Severity: Critical (Breaks compatibility with standard JSON,
especially Windows paths)


Description

The fpjson unit in recent FPC 3.3.1 trunk builds fails to parse JSON
strings containing escaped backslashes (e.g., "\\") or backslashes
used in file paths (e.g., "C:\\Windows").

When parsing a string literal in JSON that contains an escaped
backslash, the parser eliminates all content before the last backslash
in the string. This behavior represents a regression from FPC
3.2.2/3.2.3, where JSON parsing works as expected.

Verified Bug Pattern (December 2025): The parser removes everything
before the final backslash character:
Input: "C:\\Windows" → Output: \Windows (Expected: C:\Windows)Input:
"A\\B" → Output: \B (Expected: A\B)

Steps to Reproduce

Compile and run the following Pascal program using FPC 3.3.1:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i program
TestJSONBackslash; {$mode objfpc}{$H+} uses  SysUtils, fpjson,
jsonparser; var  JObject: TJSONObject;  JParser: TJSONParser; 
JSONString: String;  ParsedPath: String;begin  // A simple JSON object
with a Windows path: {"path": "C:\Windows"}  // In JSON, backslashes
must be escaped: "C:\\Windows"  JSONString := '{"path":
"C:\\Windows"}';   WriteLn('Input JSON: ', JSONString);   JParser :=
TJSONParser.Create(JSONString, [joUTF8]);  try    try      JObject :=
JParser.Parse as TJSONObject;      ParsedPath :=
JObject.Strings['path'];            WriteLn('Parsed Path: ',
ParsedPath);            if ParsedPath = 'C:\Windows' then       
WriteLn('RESULT: PASS')      else        WriteLn('RESULT: FAIL
(Expected "C:\Windows", got "', ParsedPath, '")');            except 
   on E: Exception do        WriteLn('RESULT: EXCEPTION - ',
E.Message);    end;  finally    JParser.Free;    if Assigned(JObject)
then JObject.Free;  end;end. 
Expected Behavior (FPC 3.2.x)

Input JSON: {"path": "C:\\Windows"}
Parsed Path: C:\Windows
RESULT: PASS


Actual Behavior (FPC 3.3.1)
The output varies depending on the exact revision, but typically:

Input JSON: {"path": "C:\\Windows"}
Parsed Path: C:Windows  <-- Missing backslash
RESULT: FAIL

Or in some cases, it may corrupt adjacent characters.


Analysis

The issue appears to be located in jsonscanner.pp or jsonreader.pp
within the tokenization logic for string literals. Accessing the raw
source code of 3.3.1 reveals changes in how escape sequences are
handled compared to 3.2.x.


Workaround

We have confirmed that replacing the FPC 3.3.1 fcl-json units
(fpjson.pp, jsonparser.pp, jsonscanner.pp, jsonreader.pp) with the
stable versions from FPC 3.2.3 resolves the issue immediately,
allowing the application to compile and run correctly under FPC 3.3.1
(with minor interface patches for TJSONOption).


Recommendation

Please review the recent changes to jsonscanner.pp regarding backslash
escape handling logic.


Verification History

December 29, 2025 - Bug Still Present

Verified with FPC 3.3.1-19122-ge8d90a3042 that the bug persists. All
tests failed:
Windows Path test: FAIL (got \Windows instead of C:\Windows)Simple
Backslash test: FAIL (got \B instead of A\B)Double Backslash test:
FAIL (got \More instead of Test\\More)Path with Spaces test: FAIL (got
\Test instead of C:\Program Files\Test)

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dsiders · 2025-12-29 18:51

Quote from: EsequiasTorres on December 29, 2025, 07:18:16 pmFPC 3.3.1
Bug Report: fpjson Fails to Parse Backslashes Correctly
...

This forum is not the bug tracker.That is located at
https://gitlab.com/freepascal.org/fpc/source/-/issues.

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EsequiasTorres · 2025-12-29 19:07

Thank you so much.

https://gitlab.com/freepascal.org/fpc/source/-/issues/41563