FEEL engine replaces regex expressions in output

[mboskamp]

Describe the bug

To Reproduce
Steps to reproduce the behavior:
Using Camunda Platform 7:

1. Deploy a simple DMN with a regex string (e.g., test\s\r\ntest in the output column.
2. Execute an instance (e.g., through BPMN or REST API) triggering the row with the regex output.
3. Check the output in Cockpit. The displayed regex string was changed. The escaped characters are transformed.

Example DMN

<?xml version="1.0" encoding="UTF-8"?>
<definitions xmlns="https://www.omg.org/spec/DMN/20191111/MODEL/" xmlns:dmndi="https://www.omg.org/spec/DMN/20191111/DMNDI/" xmlns:dc="http://www.omg.org/spec/DMN/20180521/DC/" xmlns:modeler="http://camunda.org/schema/modeler/1.0" xmlns:camunda="http://camunda.org/schema/1.0/dmn" id="Definitions_0g2ba0p" name="DRD" namespace="http://camunda.org/schema/1.0/dmn" exporter="Camunda Modeler" exporterVersion="5.21.0" modeler:executionPlatform="Camunda Platform" modeler:executionPlatformVersion="7.20.0">
  <decision id="regex" name="regex" camunda:historyTimeToLive="90">
    <decisionTable id="DecisionTable_1ac01ar">
      <input id="Input_1" label="Student Class" camunda:inputVariable="student_class">
        <inputExpression id="InputExpression_1" typeRef="integer">
          <text>student_class</text>
        </inputExpression>
      </input>
      <output id="Output_1" label="Regex" name="regex" typeRef="string" />
      <rule id="DecisionRule_0pa34gk">
        <inputEntry id="UnaryTests_1o9hf59">
          <text>1</text>
        </inputEntry>
        <outputEntry id="LiteralExpression_0oxhb51">
          <text>"\s\r\n"</text>
        </outputEntry>
      </rule>
    </decisionTable>
  </decision>
  <dmndi:DMNDI>
    <dmndi:DMNDiagram>
      <dmndi:DMNShape dmnElementRef="regex">
        <dc:Bounds height="80" width="180" x="160" y="100" />
      </dmndi:DMNShape>
    </dmndi:DMNDiagram>
  </dmndi:DMNDI>
</definitions>

Expected behavior
The output of the triggered DMN row is not changed. The escaped characters are not transformed. The original regex string is returned as output.

Solution idea

• FEEL engine can handle escape and regex characters properly.
• \s should not be replaced with a whitespace but handled the same way as \n and \r.

Environment

• FEEL engine version: [1.17.5], but newer versions should have the same behavior
• Affects:
  • Camunda Automation Platform 7: [7.22.0] Regex in DMN replaced in output camunda-bpm-platform#4530
• Support case: https://jira.camunda.com/browse/SUPPORT-23111