src/vs/workbench/contrib/tasks/common/problemMatcher.ts/ProblemMatcherParser/createWatchingPattern
class ProblemMatcherParser:	private createWatchingPattern(external: string | Config.IWatchingPattern | undefined): IWatchingPattern | null {
		if (Types.isUndefinedOrNull(external)) {
			return null;
		}
		let regexp: RegExp | null;
		let file: number | undefined;
		if (Types.isString(external)) {
			regexp = this.createRegularExpression(external);
		} else {
			regexp = this.createRegularExpression(external.regexp);
			if (Types.isNumber(external.file)) {
				file = external.file;
			}
		}
		if (!regexp) {
			return null;
		}
		return file ? { regexp, file } : { regexp, file: 1 };
	}