tailwindcss/src/util/prefixSelector.js

import parser from 'postcss-selector-parser'
import tap from 'lodash/tap'

export default function(prefix, selector) {
  const getPrefix = typeof prefix === 'function' ? prefix : () => prefix

  return parser(selectors => {
    selectors.walkClasses(classSelector => {
      tap(classSelector.value, baseClass => {
        classSelector.value = `${getPrefix('.' + baseClass)}${baseClass}`
      })
    })
  }).processSync(selector)
}