Something a bit different:

function regexVar() {
    const re = new RegExp(/^[MEDR][rs]{1,2}\.[a-zA-Z]+$/)
    return re;
}

• ‘^’ match at beginning of string
• ‘[MEDR]’ any of these letters
• ‘[rs]’ followed by one of these letters
• \{1,2}’ repeated between 1 and two times.
• ‘.’ followed by period (escaped char.)
• ‘[a-zA-Z]+’ then one or more upper or lowercase letters
• ‘$” at then end of the string (nothing after)