code: function getBoundaryRegExp(sContentType) {
  		var sBatchBoundary = getHeaderParameterValue(sContentType, "boundary"),
  			iMultiPartTypeIndex = sContentType.trim().indexOf("multipart/mixed");
  
  		if (iMultiPartTypeIndex !== 0 || !sBatchBoundary) {
  			throw new Error('Invalid $batch response header "Content-Type": ' + sContentType);
  		}
  
  		// escape RegExp-related characters
  		sBatchBoundary = escapeRegExp(sBatchBoundary);
  		return new RegExp('--' + sBatchBoundary + '(?:[ \t]*\r\n|--)');
  	}
docs: Create regular expression based on boundary parameter of the given "multipart/mixed"
  Content-Type header value.
  @param {string} sContentType
  Value of the "multipart/mixed" Content-Type header value
  @returns {object} Regular expression which will be used to parse the $batch request body
  @throws {Error} If the specified Content-Type header value does not represent
  "multipart/mixed" MIME type with "boundary" parameter.