(Mr|Mrs|Ms|Dr|Er)\.[a-zA-Z]+

Explanation:

• (Mr|Mrs|Ms|Dr|Er) - uses grouping and alternation to match any of the following: Mr, Mrs, Ms, Dr, Er

• \. - matches the literal . character

• [a-zA-Z]+ - matches one or more occurence of an uppercase/lowercase letter.

const regex = /(Mr|Mrs|Ms|Dr|Er)\.[a-zA-Z]+/;

function validate(str){
  console.log(`${str} matches regex? ${regex.test(str)}`)
}

validate("Mr.John")
validate("Mrs.John")
validate("Ms.John")
validate("Dr.John")
validate("Er.John")
validate("Spectric.Spectric")

To match all variants you can write the pattern as:

^(?:Mrs?|Ms|[DE]r)\.

Regex demo

If there has to be a char A-Za-z following, you could also match optional whitespace chars preceding the character class

^(?:Mrs?|Ms|[DE]r)\.\s*[A-Za-z]

Regex demo