Create a regex from parts using a tagged template.

regexSupplant.js

function regexSupplant (strings, ...values) {
  let parts = []
  while (strings.length || values.length) {
    if (strings.length) {
      parts.push(strings.shift()
        .split('\n')
        .map((val) => val.trim())
        .join('')
      )
    }
    if (values.length) {
      parts.push(values.shift()
        .toString()
        .replace(/(^\/)|(\/$)/g, '')
      )
    }
  }
  const regexpish = parts.join('')
  const pattern = regexpish.replace(/^\/|\/\w*$/g, '')
  const flags = (/\/(\w*)$/g).exec(regexpish)[1]
  return new RegExp(pattern, flags)
}

const validMentionPrecedingChars = /(?:^|[^a-zA-Z0-9_!#$%&*@＠]|(?:^|[^a-zA-Z0-9_+~.-])(?:rt|RT|rT|Rt):?)/;
const atSigns = /[@＠]/;
const screenName = /[a-zA-Z0-9_]{1,20}/;
const list = /\/[a-zA-Z][a-zA-Z0-9_\-]{0,24}/;

const validMentionOrList = regexSupplant`/ 
  (${validMentionPrecedingChars})
  (${atSigns})
  (${screenName})
  (${list})?
/g`;

console.log(validMentionOrList);