Not a valid regular expression file – How to solve this Elasticsearch error

Opster Team

July-20, Version: 1.7-8.0

Before you begin reading this guide, we recommend you try running the Elasticsearch Check-Up which can resolve issues that cause many errors.

This guide will help you check for common problems that cause the log ” not a valid regular expression file ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: Node.

Advanced users might want to skip right to the common problems section in each concept or try running the Check-Up to analyze Elasticsearch configuration and help resolve this error.

Log Context

Log “not a valid regular expression file”classname  is We extracted the following from Elasticsearch source code for those seeking an in-depth context :

 if (uaPatterns.isEmpty() && osPatterns.isEmpty() && devicePatterns.isEmpty()) {
 throw new ElasticsearchParseException("not a valid regular expression file");
 private Pattern compilePattern(String regex; String regex_flag) {
 // Only flag present in the current default regexes.yaml


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