[BugFix] Fix unexpected shift of extraction for rex with nested capture groups in named groups

[RyanL1997]

Description

Fix unexpected shift of extraction for rex with nested capture groups in named groups

The rex command in PPL had a critical bug when using named capture groups that contained nested unnamed groups. This caused extracted field values to shift by one position, producing incorrect results.

• Root Cause: Code used sequential indices 1, 2, 3... but nested groups create non-sequential indices 1, 3, 5...
• Solution: Bypass index calculation entirely by using Java's native named group extraction (matcher.group(groupName))

Example of the Bug

Query:

curl -X POST "localhost:9200/_plugins/_ppl" \
    -H "Content-Type: application/json" \
    -d '{
      "query": "source=accounts | rex field=email \"(?<user>(amber|hattie|nanette)[a-z]*)@(?<domain>(pyrami|netagy|quility))\\.(?<tld>(com|org))\" | fields user, domain, tld | head 1"
    }'

Expected Result (correct):

["amberduke", "pyrami", "com"]

Actual Result (wrong):

["amberduke", "amber", "pyrami"]

Root Cause

When Java's regex engine processes the pattern (?<user>(amber|hattie))[a-z]*, it assigns group numbers to ALL capture groups (named and unnamed):

Pattern: (?<user>(amber|hattie))[a-z]*@(?<domain>(pyrami|netagy))\.(?<tld>(com|org))

Group Assignment:

• Group 0: Entire match
• Group 1: (?<user>...) ← Named group "user"
• Group 2: (amber|hattie) ← Unnamed nested group
• Group 3: (?<domain>...) ← Named group "domain"
• Group 4: (pyrami|netagy) ← Unnamed nested group
• Group 5: (?<tld>...) ← Named group "tld"
• Group 6: (com|org) ← Unnamed nested group

Before the fix, the bug is in CalciteRelNodeVisitor.java (lines 265-321). The code does:

  List<String> namedGroups = RegexCommonUtils.getNamedGroupCandidates(patternStr);
  // namedGroups = ["user", "domain", "tld"]

  for (int i = 0; i < namedGroups.size(); i++) {
      extractCall = PPLFuncImpTable.INSTANCE.resolve(
          context.rexBuilder,
          BuiltinFunctionName.REX_EXTRACT,
          fieldRex,
          context.rexBuilder.makeLiteral(patternStr),
          context.relBuilder.literal(i + 1));  // ← WRONG: Assumes sequential named groups
      // ...
  }

The code assumes named groups are at indices 1, 2, 3, ... but the actual indices are 1, 3, 5, ... due to the unnamed nested groups.

With the above buggy logic:

• REX_EXTRACT(field, pattern, 1) → Gets Group 1 (?<user>...) = "amberduke" → CORRECT
• REX_EXTRACT(field, pattern, 2) → Gets Group 2 (amber|hattie) = "amber" → WRONG
• REX_EXTRACT(field, pattern, 3) → Gets Group 3 (?<domain>...) = "pyrami" → WRONG

The second and third extractions are off by one group because they hit the unnamed nested groups.

LogicalProject(
user=[REX_EXTRACT($7, '(?<user>(amber|hattie))[a-z]*@(?<domain>(pyrami|netagy))\.(?<tld>(com|org))', 1)],
domain=[REX_EXTRACT($7, '(?<user>(amber|hattie))[a-z]*@(?<domain>(pyrami|netagy))\.(?<tld>(com|org))', 2)],  -- Wrong!
    tld=[REX_EXTRACT($7, '(?<user>(amber|hattie))[a-z]*@(?<domain>(pyrami|netagy))\.(?<tld>(com|org))', 3)]     -- Wrong!
)

Related Issues

• Resolve [BUG] rex command off-by-one error with nested capture groups in named groups #4466

Check List

• New functionality includes testing.
• New functionality has been documented.
• New functionality has javadoc added.
• New functionality has a user manual doc added.
• New PPL command checklist all confirmed.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff or -s.
• Public documentation issue/PR created.

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[RyanL1997]

I'm currently thinking about adding an error handling here

[dai-chen]

I found there is a namedGroups() API in Matcher (since JDK 20?). If we can get correct index here, we don't need to modify the UDFs below? Alternatively we can move capture name -> index logic here from UDFs?

[RyanL1997]

That is correct - the core issue is matching named groups to their correct indices, and Pattern.namedGroups() would be the perfect solution. However, I discovered that we're blocked by a
compatibility constraint:

• Pattern.namedGroups() was introduced in JDK 20
• We need backward compatibility with JDK 11/17 - for 2.19-dev

I agree that directly leveraging the Pattern.namedGroups() is the right architectural approach - we should definitely migrate to it when we fully upgrade to JDK 20+. At that point, it would be a simple one-line change in CalciteRelNodeVisitor.

[RyanL1997]

as for now, I added a TODO here @dai-chen