Digit right after regex reference? \1123

Hi,

I want to replace a domain by a specific IP address inside the response from backend.

How can I set a digit right after the \1 reference?

rspirep ^Location:\ (https?://)?www.domain.de(.*) Location:\ \1123.45.67.89\2

My suggestions \01123.45 and \g{1}123.45 seem not to work…

Thank You,
Marc