scala xml pattern match

Scala-2-12

Welcome to Scala 2.12.18 (OpenJDK 64-Bit Server VM, Java 11.0.20).
Type in expressions for evaluation. Or try :help.

scala> val <a>{ b }</a> = <a></a>
scala.MatchError: <a></a> (of class scala.xml.Elem)
  ... 55 elided

scala> val <a>{ _ }</a> = <a>c</a>

scala> val <a>{ _* }</a> = <a>c</a>

Scala-2-13

Welcome to Scala 2.13.12 (OpenJDK 64-Bit Server VM, Java 11.0.20).
Type in expressions for evaluation. Or try :help.

scala> val <a>{ b }</a> = <a></a>
scala.MatchError: <a></a> (of class scala.xml.Elem)
  ... 59 elided

scala> val <a>{ _ }</a> = <a>c</a>

scala> val <a>{ _* }</a> = <a>c</a>

Scala-3

Welcome to Scala 3.3.1 (11.0.20, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                                                                                             
scala> val <a>{ b }</a> = <a></a>
1 warning found
-- Warning: --------------------------------------------------------------------
1 |val <a>{ b }</a> = <a></a>
  |     ^^^^^^^^^^^^^^^^^^^^^
  |pattern binding uses refutable extractor `_root_.scala.xml.Elem`
  |
  |If this usage is intentional, this can be communicated by adding `: @unchecked` after the expression,
  |which may result in a MatchError at runtime.
scala.MatchError: <a></a> (of class scala.xml.Elem)
  ... 66 elided
                                                                                                                                             
scala> val <a>{ _ }</a> = <a>c</a>
1 warning found
-- Warning: --------------------------------------------------------------------
1 |val <a>{ _ }</a> = <a>c</a>
  |     ^^^^^^^^^^^^^^^^^^^^^^
  |pattern binding uses refutable extractor `_root_.scala.xml.Elem`
  |
  |If this usage is intentional, this can be communicated by adding `: @unchecked` after the expression,
  |which may result in a MatchError at runtime.
                                                                                                                                             
scala> val <a>{ _* }</a> = <a>c</a>
-- [E032] Syntax Error: --------------------------------------------------------
1 |val <a>{ _* }</a> = <a>c</a>
  |            ^
  |            pattern expected
  |
  | longer explanation available when compiling with `-explain`