[SPARK-18952] Regex strings not properly escaped in codegen for aggregations

## What changes were proposed in this pull request?

If I use the function regexp_extract, and then in my regex string, use `\`, i.e. escape character, this fails codegen, because the `\` character is not properly escaped when codegen'd.

Example stack trace:
```
/* 059 */     private int maxSteps = 2;
/* 060 */     private int numRows = 0;
/* 061 */     private org.apache.spark.sql.types.StructType keySchema = new org.apache.spark.sql.types.StructType().add("date_format(window#325.start, yyyy-MM-dd HH:mm)", org.apache.spark.sql.types.DataTypes.StringType)
/* 062 */     .add("regexp_extract(source#310.description, ([a-zA-Z]+)\[.*, 1)", org.apache.spark.sql.types.DataTypes.StringType);
/* 063 */     private org.apache.spark.sql.types.StructType valueSchema = new org.apache.spark.sql.types.StructType().add("sum", org.apache.spark.sql.types.DataTypes.LongType);
/* 064 */     private Object emptyVBase;

...

org.codehaus.commons.compiler.CompileException: File 'generated.java', Line 62, Column 58: Invalid escape sequence
	at org.codehaus.janino.Scanner.scanLiteralCharacter(Scanner.java:918)
	at org.codehaus.janino.Scanner.produce(Scanner.java:604)
	at org.codehaus.janino.Parser.peekRead(Parser.java:3239)
	at org.codehaus.janino.Parser.parseArguments(Parser.java:3055)
	at org.codehaus.janino.Parser.parseSelector(Parser.java:2914)
	at org.codehaus.janino.Parser.parseUnaryExpression(Parser.java:2617)
	at org.codehaus.janino.Parser.parseMultiplicativeExpression(Parser.java:2573)
	at org.codehaus.janino.Parser.parseAdditiveExpression(Parser.java:2552)
```

In the codegend expression, the literal should use `\\` instead of `\`

A similar problem was solved here: https://github.com/apache/spark/pull/15156.

## How was this patch tested?

Regression test in `DataFrameAggregationSuite`

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #16361 from brkyvz/reg-break.

0 files changed, 0 insertions, 0 deletions