Don't create exceptions in RegexExtractor on failed extractions

[ashrafiucse]

Description

RegexExtractor now parses its internal group-count and match-count variables (<refname>_g, <refname>_matchNr) without relying on exceptions, via a small parseIntOrDefault helper. Values that are present but not numbers still log the same warning as before; absent variables yield the previous default values, so behavior is unchanged.

Motivation and Context

Fixes #6240

Every failed extraction calls RegexExtractor#removeGroups, which parsed the (still absent) group-count variable with Integer#parseInt. Parsing null always throws a NumberFormatException, which was caught and silently discarded:

try {
    groups = Integer.parseInt(vars.get(buf.toString())); // null when no match succeeded yet
} catch (NumberFormatException e) {
    groups = 0; // silent
}

So each failing extraction paid for creating and discarding an exception. In failure-heavy test runs (which are exactly the scenario a load test exercises against a struggling system), failed extractions are the hot path.

I verified this against a locally built distribution with -Xlog:exceptions=info (which reports every thrown exception, including caught ones): a test plan with a non-matching extractor over 200 iterations produced exactly 200 NumberFormatExceptions, all in RegexExtractor.removeGroups (one per failed extraction, and nothing visible in jmeter.log). A matching-extractor control run produced zero. With this change the failing run produces zero exceptions.

For scale: the exception costs roughly 0.5 µs per failed extraction, so this is a modest but free win, and it also removes the pointless churn. The remaining parseInt call inside the helper only triggers for well-formed digit strings.

How Has This Been Tested?

• New tests in TestRegexExtractor:
  • testNoMatchOnFreshVariablesAppliesDefault: the previously exception-throwing path (no match, no prior group variables) applies the default and leaves no group variables behind
  • testNoMatchCleansUpPreviousGroupVariables: group variables from an earlier successful extraction are cleaned up on failure
  • testNoMatchWithTamperedGroupCountVariable: a corrupted <refname>_g value is handled (warn + default) without breaking the extraction
  • testAllMatchesWithTamperedMatchNumberVariable: a corrupted <refname>_matchNr value does not break all-matches mode
• All 29 TestRegexExtractor tests and the full :src:components:test suite (553 tests) pass
• ./gradlew classes style passes
• End-to-end check with a locally built distribution as described above: 200 failing iterations → 0 exceptions (previously 200), all samples successful, extracted/default values unchanged

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly. (release notes entry added to xdocs/changes.xml)