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|--)");
	}
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.