Playground.swift

import UIKit

let string = "Lorem ipsum dolor sit amet\n #test# #ganothertest# not a tag"
let textView = UITextView(frame: CGRect(x: 0, y: 0, width: 200, height: 100))
textView.text = string
textView.font = UIFont(name: "Helvetica", size: 15)

func frameOfTextInRange(range:NSRange, inTextView textView:UITextView) -> CGRect {
    let beginning = textView.beginningOfDocument
    let start = textView.position(from: beginning, offset: range.location)
    let end = textView.position(from: start!, offset: range.length)
    let textRange = textView.textRange(from: start!, to: end!)
    let rect = textView.firstRect(for: textRange!)
    return textView.convert(rect, from: textView)
}

//let textViewBG = UIView(frame: textView.bounds)
//textView.backgroundColor = UIColor.clear
//textViewBG.backgroundColor = UIColor.white


let pattern = "(\\#[a-zA-Z]+\\b\\#)(?!;)"
let regex = try! NSRegularExpression(pattern: pattern, options: [])
let matches = regex.matches(in: string, options: [], range: NSMakeRange(0, string.count))
let attributedString = NSMutableAttributedString(string: string)

let myAttr = [NSAttributedString.Key.foregroundColor: UIColor.white]
let myString = textView.attributedText.mutableCopy() as! NSMutableAttributedString

let paragraphStyle = NSMutableParagraphStyle()
paragraphStyle.lineHeightMultiple = 1.3

let textAttributes = [NSMutableAttributedString.Key.paragraphStyle: paragraphStyle,
                      NSAttributedString.Key.font: UIFont(name: "Helvetica", size: 15)]


myString.addAttributes(textAttributes as [NSAttributedString.Key : Any], range: NSMakeRange(0, textView.text.count))
textView.attributedText = myString.copy() as? NSAttributedString

for m in matches {
//        myString.addAttributes(myAttr, range: m.range)
    myString.addAttributes(myAttr, range: m.range)
    let range = m.range
    var frame = frameOfTextInRange(range: range, inTextView: textView)
    frame = frame.insetBy(dx: -1.6, dy: 2)
    frame = frame.offsetBy(dx: 0, dy: 2)
    
    let v = UIView(frame: frame)
    v.layer.cornerRadius = v.frame.height / 2
    v.backgroundColor = #colorLiteral(red: 0.8078431487, green: 0.02745098062, blue: 0.3333333433, alpha: 1)
    textView.insertSubview(v, at: 0)
}


textView.attributedText = myString.copy() as? NSAttributedString
//textViewBG.addSubview(textView)