// replace any ${ENV} values with the appropriate environ. const envExpr = /(?<!\\)(\\*)\$\{([^${}]+)\}/g module.exports = (f, env) => f.replace(envExpr, (orig, esc, name) => { const val = env[name] !== undefined ? env[name] : `$\{${name}}` // consume the escape chars that are relevant. if (esc.length % 2) { return orig.slice((esc.length + 1) / 2) } return (esc.slice(esc.length / 2)) + val })