Why a GA4-specific regex tester?
GA4 and BigQuery use Google's RE2engine — a subset of PCRE that doesn't support lookaheads, lookbehinds, or back-references. Patterns that work in online regex playgrounds (which mostly use PCRE) can silently fail in production.
This tester enforces RE2 syntax so you catch incompatibilities before shipping them to a filter, audience, or lookup table.
Regex in GTM is most commonly used in Custom Event triggers and Lookup Table variables. For a full walkthrough of GTM's variable types (including when Lookup Tables with regex beat a Matches RegEx condition), read GTM Variables: A Guide to the 6 Most Useful Custom Types.