message filters RegEx Error: “An error occurred during processing: \.”

I am trying to create a message filter that validates mail-from to be RFC5322 compliant formated.

Therefore I created with the help of regex101.com/r/3uvtNl/1 this valid paython Regex:

"^(?=.{1,64}@)[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,63}$"

When creating a new filter using that regex I receive the follwing error:

[]> NEW
Enter filter script. Enter '.' on its own line to end.
devRFCValidator: if (mail-from != "^(?=.{1,64}@)[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,63}$") {
log-entry("MAG: RFC5322 RegExValidator - notvalid:" $EnvelopeFrom);
}
else
{
log-entry("MAG: RFC5322 RegExValidator - valid:" $EnvelopeFrom);
}
.
An error occurred during processing: \.
Choose the operation you want to perform:
- NEW - Create a new filter.
...

 Does some know which version of paython regex is used inside AsyncOS 16.x?

Or any suggestins regarding this error? 

Based on www.cisco.com/c/en/us/td/docs/security/ces/ces_16-0-2/user_guide/b_ESA_Admin_Guide_ces_16-0-2/b_ESA_Admin_Guide_12_1_chapter_01000.html#con_1296763 I didnot find any restrictions regarding this regex.
Thanks for support