PlaceholderDifferenceEvaluator: capturing groups in custom delimiter regexes break placeholder parsing #350

PlaceholderDifferenceEvaluator builds its placeholder and argument patterns by concatenating the user-supplied delimiter regexes with its own capturing groups, then reads the captured parts with hardcoded group indices (group(2) and group(3)). When a custom delimiter regex itself contains a capturing group, those indices shift and parsing breaks.

For example, an args opening delimiter regex of (\\() is functionally identical to the default \\( (it just wraps the escaped parenthesis in a capturing group), but:

Diff diff = DiffBuilder.compare("<a>${xmlunit.matchesRegex([a-z]+)}</a>").withTest("<a>abc</a>")
    .withDifferenceEvaluator(new PlaceholderDifferenceEvaluator(null, null, "(\\()", "(\\))", null))
    .build();

reads "(" as the argument list instead of "[a-z]+" and fails with an XMLUnitException caused by a PatternSyntaxException ("Unclosed group").

Likewise, a placeholder opening delimiter regex of (\\$\\{) stops ${xmlunit.ignore} from being recognised at all, so the comparison reports a difference.

This was noticed downstream in WireMock's equalToXml placeholder support (wiremock/wiremock#3094).

The fix I have ready: give the captured parts of the combined patterns named groups (placeholderContent, placeholderKeyword, placeholderArgs) and read them by name, so custom delimiter regexes may contain any number of capturing groups without affecting parsing. Includes regression tests. Happy to open a PR.