Xor for two lines string.

# Summary
[Demo here](https://gchq.github.io/CyberChef/#recipe=Comment('Expected:%20PPPP')Fork('%5C%5Cn','%5C%5Cn',false)Conditional_Jump('abcd',false,'value',10)Subsection('1234',true,true,false)Register('(%5B%5C%5Cs%5C%5CS%5D*)',true,false,false)Return()Label('value')Find_/_Replace(%7B'option':'Regex','string':'a'%7D,'$R0',true,false,true,false)XOR(%7B'option':'UTF8','string':'$R0'%7D,'Standard',false/disabled)&input=YWJjZAoxMjM0)

I extracted `1234` to register, used $R0 to do xor failed, maybe some flow control error.
### Example Input
input:
```
abcd
1234
```

### Example Output
```
a ^ 1 => P
b ^ 2 => P
c ^ 3 => P
d ^ 4 => P
```
Expected: `PPPP`

Start with the linked CyberChef demo recipe and reproduce the XOR operation using the two-line input `abcd` and `1234`. Inspect how the extracted `$R0` value is passed into XOR and verify the result against the expected output `PPPP`.

Written by the indexing model from the issue text.