Fix GA4 Unassigned Traffic with Source/Medium Regex

An Unassigned row in your channel report means one thing: sessions arrived whose source/medium values matched none of the 19 default channel rules. You cannot edit the default rules — but you can write a custom channel group whose regex conditions capture exactly those values. This page walks the fix end to end: find the culprit values, test the pattern, paste it in.

Why GA4 shows Unassigned in the first place

GA4 classifies every session into a channel group using a fixed set of 19 default rules (Organic Search, Paid Search, Email, Social, Referral, and so on). Each rule matches specific source/medium values — google / cpc is Paid Search, newsletter / email is Email, and so on. When a session's values match none of the rules, GA4 puts it in Unassigned instead of guessing.

The usual suspects that fall through the rules:

• Email platforms sending with their own domain as source and (not set) as medium (Substack, beehiiv, Ghost — anything that isn't explicitly tagged)
• Custom mediums your team invented: fb_ads, ig_ads, pmax, newsletter, crm
• Mixed-case values like Email from older tags — actually fine, see the case-insensitivity note below
• AI assistants sending referrals: chatgpt.com, claude.ai, perplexity.ai — no default rule existed for these until recently, and most properties still have none
• Partner, affiliate and internal invite links with nonstandard sources

Step 1 — find the exact values causing Unassigned

Before writing any regex, read the data. In GA4:

1. Open Reports → Acquisition → Traffic acquisition
2. Set the date range large enough to matter (90 days gives the pattern statistically useful weight)
3. Add secondary dimension Session source / medium
4. Filter the table to Session default channel group | exactly matches | Unassigned
5. Sort by sessions descending

You now have the literal list of offending source / medium pairs with their volumes. Group them mentally: all the email platforms together, all the ad mediums together, the AI referrers together. One regex per group — not one giant pattern trying to catch everything, which nobody can debug six months later.

Step 2 — write the pattern against real values

Two GA4 rules govern every pattern you will write here — both are covered in depth on the GA4 regex match types page:

• Full-match semantics. facebook matches only the exact value facebook, not facebook.com. For contains-behavior, wrap: .*facebook.*
• RE2 engine. No lookahead, no backreferences. The patterns below are all RE2-safe — and channel-grouping regex is also case-insensitive, so email matches Email and EMAIL without any flag

Each pattern below has a Try link that loads it into the tester with a realistic sample value — paste your own offending values from step 1 and confirm the match before touching GA4.

6 ready-made Unassigned fix patterns

These go into a custom channel group (Reports → Library → edit the channel group, or Admin → Channel groups). Each row: the condition you set, the regex, and a sample value it recaptures.

Build the AI-referrer channel first if you have not touched channel groups in 2025–2026 — assistant-referred traffic is the fastest-growing unassigned category, and "AI Referrals" as its own channel turns a mystery row into a reportable segment. The full custom channel grouping regex guide covers rule ordering and the default 19 rules these patterns slot alongside.

Step 3 — where to apply it (and what reprocesses history)

This table is the whole reason Unassigned "comes back" every quarter for most teams: someone fixes the tags, sees Unassigned unchanged, and concludes the fix failed. The tags only govern the future. The channel group is the retroactive fix — and because it is applied at query time, editing its regex next month re-sorts history again with the new rules.

Step 4 — fix the tags too (the forward-looking half)

Channel groups bandage the reporting layer; the underlying tagging should still be corrected, or the gap keeps growing. For every group you captured above, work backwards to the tag:

• Newsletter platforms: append utm_source=<platform>&utm_medium=email to every outgoing link (most platforms have a default-parameters setting — set it once)
• Meta ads: let the Ads Manager auto-UTM do utm_medium=paid (which default Paid Social catches) rather than a custom fb_ads only your channel group understands
• Partner links: a shared link-builder with locked utm_source=partner beats polite requests in Slack

Then verify each new tag lands correctly — the tester with the UTM parameter regex patterns makes a fast QA sheet for link exports.

FAQ

Related on RegexLab: the custom channel grouping regex guide (all default rules + pattern templates), the GA4 match types guide (full vs partial match per surface), the GA4 regex cheat sheet, the UTM parameter regex and URL regex pattern pages, and RE2 vs PCRE.