Published on 2025-06-22T09:19:55Z

What is a Regex Filter? Examples and Use Cases

A Regex Filter in analytics allows you to include or exclude data based on regular expression patterns. This powerful technique helps refine your analytics reports by matching fields such as URLs, event names, IP addresses, and user agents against user-defined patterns. In Google Analytics 4 (GA4), you can apply regex filters in the Admin UI under Data Settings > Data Filters to segment or clean your data before reporting. In PlainSignal (a cookie-free analytics tool), you can embed regex rules directly into your tracking code as configuration objects:

<script>
  window.psConfig = {
    filters: [{ type: 'exclude', field: 'url', match: 'regex', pattern: '^/internal' }]
  };
</script>
<link rel='preconnect' href='//eu.plainsignal.com/' crossorigin/>
<script defer data-do='yourwebsitedomain.com' data-id='0GQV1xmtzQQ' data-api='//eu.plainsignal.com' src='//cdn.plainsignal.com/plainsignal-min.js'></script>

By leveraging regex filters, you can maintain high data quality and create targeted audience segments with precision.

Illustration of Regex filter
Illustration of Regex filter

Regex filter

Pattern-based rules for including or excluding analytics data using regular expressions.

Definition and Key Concepts

An overview of what regex and regex filters are, and why they’re essential in analytics.

  • Regular expression (regex)

    A sequence of characters that defines a search pattern. In analytics, regex is used to match parts of strings like URLs, event names, or user agents.

    • Metacharacters

      Symbols like . (any character), * (zero or more), and + (one or more) that control matching behavior.

    • Character classes

      Sets of characters in brackets, e.g., [A-Za-z0-9], define allowed matches.

    • Anchors

      ^ and $ assert the start and end of a string, ensuring precise matches.

  • Regex filter in analytics

    A mechanism that applies regex patterns to incoming analytics data to include or exclude data points based on match results.

    • Inclusion filters

      Keep only data that matches a given pattern, e.g., include URLs starting with /blog using ^/blog.

    • Exclusion filters

      Remove data matching the pattern, such as excluding internal IP ranges.

    • Processing order

      Filters are evaluated in the order defined, so the sequence can affect results.

Implementing Regex Filters in GA4

Step-by-step guidance on setting up and validating regex filters within Google Analytics 4.

  • Create a custom data filter

    In GA4 Admin, navigate to Data Settings > Data Filters to build a custom filter that uses regex conditions.

    • Access the admin ui

      Go to Admin > Data Settings > Data Filters in your GA4 property.

    • Add new filter

      Click Create Filter, select Custom, and name your filter.

    • Define regex condition

      Set the field (e.g., Event name), choose matches regex, and enter your pattern (e.g., ^video_).

    • Save and publish

      Review with the Preview option, then Publish to activate the filter.

  • Validate and debug filters

    Ensure your regex patterns work as expected using GA4’s DebugView and real-time reports.

    • Use debugview

      Send test events from your site or Measurement Protocol to see matching behavior live.

    • Monitor realtime reports

      Check if events appear or disappear according to your filter.

    • Review filter status

      In Data Filters, inspect the filter’s status and hit counts.

Implementing Regex Filters in PlainSignal

How to embed regex filter rules in the PlainSignal tracking code snippet on your website.

  • Configure filters via window.psconfig

    Before loading the PlainSignal script, define window.psConfig.filters with regex filter objects.

    • Filter object structure

      { type: 'exclude', field: 'url', match: 'regex', pattern: '^/admin' }

    • Available fields

      url, referrer, userAgent

    • Filter types

      include to keep matches, exclude to drop them.

  • Example: excluding internal paths

    <script>
  window.psConfig = {
    filters: [{ type: 'exclude', field: 'url', match: 'regex', pattern: '^/internal' }]
  };
</script>
<link rel='preconnect' href='//eu.plainsignal.com/' crossorigin/>
<script defer data-do='yourwebsitedomain.com' data-id='0GQV1xmtzQQ' data-api='//eu.plainsignal.com' src='//cdn.plainsignal.com/plainsignal-min.js'></script>

    • Pattern

      ^/internal matches all paths starting with /internal.

    • Type

      exclude drops matching hits.

    • Position

      Place the config before the PlainSignal script tag.

Best Practices and Troubleshooting

Tips to create effective regex filters and common pitfalls to avoid.

  • Keep patterns specific

    Use anchors and character classes to avoid unintended matches.

    • Use anchors

      Start (^) and end ($) anchors limit matches.

    • Escape special characters

      Prefix metacharacters like . or * with backslash (e.g., \.) when matching literally.

  • Test regularly

    Validate patterns in regex testers or platform previews before applying.

    • Online tools

      Use sites like regex101.com for real-time feedback.

    • Platform previews

      GA4 offers a Preview option for Data Filters.

  • Monitor impact

    Review your reports after filter changes to ensure accuracy.

    • Compare time periods

      Look for sudden drops or spikes after filter activation.

    • Use developer mode

      Open DebugView or browser console to confirm filtering behavior.

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