Fix shared regex state across repeated matcher use (#749)

[chiliec]

Fixes #749.

Problem

A regex bound to a variable and reused across multiple $match/$replace/$split calls produces different results on repeated use, breaking JSONata's immutability guarantee. It worked in 1.8.7 / 2.0.1 and regressed by 2.0.6.

(
  $input := "a,b,c";
  $pattern := /[,-]/;
  {
    "2x": [$replace($input, $pattern, "|"), $replace($input, $pattern, "|")],
    "3x": [$replace($input, $pattern, "|"), $replace($input, $pattern, "|"), $replace($input, $pattern, "|")]
  }
)

Expected every element to be "a|b|c". On master it returns corrupted values such as "a|b|,b,|,|b,c".

Root cause

evaluateRegex builds one RegExp and closes over it:

var re = new environment.base.RegexEngine(expr.value);
var closure = function(str, fromIndex) {
    re.lastIndex = fromIndex || 0;
    ...

A regex literal bound to a variable is evaluated once, so the returned closure — and that single stateful re with its mutable lastIndex — is shared by every subsequent matcher call. $replace walks matches via the next() chain, which depends on re.lastIndex persisting; when another call reuses the same closure it resets/advances the same lastIndex, so independent calls interfere and corrupt each other's results.

Fix

Create a fresh RegExp for each top-level match walk (when the closure is entered without an existing instance), and thread that walk's own instance through the next() chain so multi-match iteration within a single walk is unchanged. Independent calls no longer share mutable regex state.

Test

Adds test/test-suite/groups/regex/case039.json, the repeated-use scenario from the issue (asserts every $replace yields "a|b|c").

Verification (local, Node 20.18.1)

• RED→GREEN: with the fix reverted, case039 fails (1799 passing / 1 failing); with the fix it passes (1800 passing / 0 failing).
• npm test — full suite green, and the strict gate holds: Statements/Branches/Functions/Lines all 100% (nyc check-coverage --branches 100 exits 0). The new branch is covered by case039.
• ESLint clean.

Signed-off-by: Vladimir Babin vovababin@gmail.com