notes/node_modules/stylelint/lib/utils/isStandardSyntaxUrl.js

/* @flow */
"use strict"

const hasLessInterpolation = require("../utils/hasLessInterpolation")
const hasPsvInterpolation = require("../utils/hasPsvInterpolation")
const hasScssInterpolation = require("../utils/hasScssInterpolation")

/**
 * Check whether a URL is standard
 */
module.exports = function (url/*: string*/)/*: boolean*/ {
  if (url.length === 0) {
    return true
  }

  // Sass interpolation works anywhere
  if (hasScssInterpolation(url) || hasPsvInterpolation(url)) {
    return false
  }

  // Inside `'` and `"` work only LESS interpolation
  if (url[0] === "'" && url[url.length - 1] === "'" || url[0] === "\"" && url[url.length - 1] === "\"") {
    if (hasLessInterpolation(url)) {
      return false
    }

    return true
  }

  // Less variable works only at the beginning
  // Check is less variable, allow use '@url/some/path'
  // https://github.com/less/less.js/blob/3.x/lib/less/parser/parser.js#L547
  if (url[0] === "@" && /^@@?[\w-]+$/.test(url)) {
    return false
  }

  // In url without quotes scss variable can be everywhere
  // But in this case it is allowed to use only specific characters
  // Also forbidden "/" at the end of url
  if (url.indexOf("$") !== -1 && /^[\$\sA-Za-z0-9+-/*_'"\/]+$/.test(url) && url[url.length - 1] !== "/") {
    return false
  }

  return true
}