I can't enter RE in the formula window.

When I type RE in the formula window, RE changes to R which is a differnt shape of R. So, I can’t make formulas like x_RE or just RE. How can I enter those formulas? My LibreOffice version is 6.1.4.

Please note that your question is not about Writer, but is about Math (that is the application in LibreOffice that creates formulas).