Regex pattern check

matches-username-at-end.txt

@.*[a-zA-z0-9_]$

regex-check.swift

let pattern = "^([A-Za-z\\d_]+$)" // this checks for usernames matching alphanumeric + _
let regex = try! NSRegularExpression(pattern: pattern, options: [])
let range = NSMakeRange(0, updatedText.characters.count)
let num_matches = regex.numberOfMatches(in: updatedText, options: [], range: range)

guard updatedText.characters.count == 0 || num_matches == 1 else {
  return false
}