Github user gatorsmile commented on a diff in the pull request:

    https://github.com/apache/spark/pull/15398#discussion_r83537154
  
    --- Diff: 
sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/RegexpExpressionsSuite.scala
 ---
    @@ -74,6 +107,31 @@ class RegexpExpressionsSuite extends SparkFunSuite with 
ExpressionEvalHelper {
         checkEvaluation("a\nb" like regEx, true, create_row("a%b"))
     
         checkEvaluation(Literal.create(null, StringType) like regEx, null, 
create_row("bc%"))
    +
    +    checkEvaluation("" like regEx, true, create_row(""))
    +    checkEvaluation("a" like regEx, false, create_row(""))
    +    checkEvaluation("" like regEx, false, create_row("a"))
    +
    +    checkEvaluation("""\\\\""" like regEx, true, create_row("""%\\%"""))
    +    checkEvaluation("""%%""" like regEx, true, create_row("""%%"""))
    +    checkEvaluation("""\__""" like regEx, true, create_row("""\\\__"""))
    +    checkEvaluation("""\\\__""" like regEx, false, 
create_row("""%\\%\%"""))
    +    checkEvaluation("""_\\\%""" like regEx, false, create_row("""%\\"""))
    +
    +    // scalastyle:off nonascii
    +    checkEvaluation("a\u20ACa" like regEx, true, create_row("_\u20AC_"))
    +    checkEvaluation("a€a" like regEx, true, create_row("_€_"))
    +    checkEvaluation("a€a" like regEx, true, create_row("_\u20AC_"))
    +    checkEvaluation("a\u20ACa" like regEx, true, create_row("_€_"))
    +    // scalastyle:on nonascii
    +
    +    // TODO: should throw an exception?
    --- End diff --
    
    To answer your point 3, I did a try in DB2. 
    ```
    db2 => select actkwd from act where actkwd like '%A%\a' escape '\'
    SQL0130N  The ESCAPE clause is not a single character, or the pattern 
string 
    contains an invalid occurrence of the escape character.  SQLSTATE=22025
    ```
    
    In DB2, normally, our design is very conservative. If we think this could 
be a user error, we will stop it with an error. We do not want to give users 
any surprise. 


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