Question 1

What regex flavor does GA4 use?

Accepted Answer

GA4 and BigQuery both use Google's RE2 engine — a subset of PCRE. RE2 does not support lookaheads, lookbehinds, or back-references. Patterns that work in PCRE-based playgrounds (most online regex testers) can silently fail in production GA4 filters, audiences, and lookup tables. This tester enforces RE2 syntax so you catch incompatibilities before shipping.

Question 2

Where can I use regex inside GA4?

Accepted Answer

GA4 accepts regex in: (1) Internal Traffic filters (matches IP addresses), (2) Unwanted Referrals (matches referral domains), (3) Custom Definitions (regex-based dimension extraction), (4) Audience builder conditions, (5) Explorations segment definitions, (6) Event modify/create rules. All six surfaces use RE2 — case-sensitive by default unless prefixed with (?i).

Question 3

Why does my regex match in this tester but not in GA4?

Accepted Answer

The most common cause is anchor mismatch. GA4's filter UI labels are misleading: "matches regex" treats the pattern as fully anchored (^ and $ implicit), while "contains regex" does not. If your pattern works here but not in GA4, check whether you need to add .* to the start/end, or switch the filter type. The second-most-common cause is case sensitivity — RE2 is case-sensitive by default.

Question 4

Is this regex tester actually private?

Accepted Answer

Yes. The tester runs entirely in your browser using JavaScript's built-in regex engine constrained to RE2 syntax. Your patterns and test strings never leave the page, are not logged, and are not sent to any server. Safe to test patterns that contain real URLs, internal IP addresses, or PII you are trying to filter out.

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