From 906bfecc72c9579fb150ae33059b190a2a387225 Mon Sep 17 00:00:00 2001
From: "linear-code[bot]" <222613912+linear-code[bot]@users.noreply.github.com>
Date: Tue, 3 Nov 2026 15:57:21 +0000
Subject: [PATCH] fix(Engine): Standardise regex matching in the Elixir
 server-side SDK

Anchor regex segment conditions at the start of the trait value, for every
alternative, without requiring a full-string match.

Generated with [Linear](https://linear.app/flagsmith/agent-session/58bae237-657f-42a7-a3f0-4a18638a498f)

Co-authored-by: linear-code[bot] <222613912+linear-code[bot]@users.noreply.github.com>
---
 lib/flagsmith_engine.ex               | 2 +-
 test/unit/segment_conditions_test.exs | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/lib/flagsmith_engine.ex b/lib/flagsmith_engine.ex
index 388103e..c8eac7b 100644
--- a/lib/flagsmith_engine.ex
+++ b/lib/flagsmith_engine.ex
@@ -510,7 +510,7 @@ defmodule Flagsmith.Engine do
         type: :string,
         value: t_value
       }) do
-    case Regex.compile(value) do
+    case Regex.compile(value, [:anchored]) do
       {:ok, regex} ->
         String.match?(t_value, regex)
 
diff --git a/test/unit/segment_conditions_test.exs b/test/unit/segment_conditions_test.exs
index ae49d27..27ef0fc 100644
--- a/test/unit/segment_conditions_test.exs
+++ b/test/unit/segment_conditions_test.exs
@@ -56,6 +56,13 @@ defmodule Flagsmith.Engine.SegmentConditionsTest do
     {:REGEX, "foo", "[a-z]+", true},
     {:REGEX, "FOO", "[a-z]+", false},
     {:REGEX, "1.2.3", "\\d", true},
+    {:REGEX, "production", "prod", true},
+    {:REGEX, "nonproduction", "prod", false},
+    {:REGEX, "development", "prod|dev", true},
+    {:REGEX, "nondevelopment", "prod|dev", false},
+    {:REGEX, "jane@example.com", "@example\\.com$", false},
+    {:REGEX, "jane@example.com", "^[^@]+@example\\.com$", true},
+    {:REGEX, "foo", "[", false},
     {:MODULO, 1, "2|0", false},
     {:MODULO, 2, "2|0", true},
     {:MODULO, 3, "2|0", false},