diff --git a/src/Engine/Engine.php b/src/Engine/Engine.php
index f092b78..d867c86 100644
--- a/src/Engine/Engine.php
+++ b/src/Engine/Engine.php
@@ -351,7 +351,7 @@ private static function _contextMatchesCondition(
                     && !str_contains($contextValue, $condition->value);
 
             case SegmentConditionOperator::REGEX:
-                return (bool) preg_match("/{$condition->value}/", (string) $contextValue);
+                return (bool) preg_match("/{$condition->value}/A", (string) $contextValue); // A: anchored at start, not full match
         }
 
         if ($contextValue === null) {
diff --git a/tests/Engine/Unit/Segments/RegexConditionTest.php b/tests/Engine/Unit/Segments/RegexConditionTest.php
new file mode 100644
index 0000000..3a61240
--- /dev/null
+++ b/tests/Engine/Unit/Segments/RegexConditionTest.php
@@ -0,0 +1,73 @@
+<?php
+
+namespace FlagsmithTest\Engine\Unit\Segments;
+
+use Flagsmith\Engine\Engine;
+use Flagsmith\Engine\Utils\Types\Context\EvaluationContext;
+use PHPUnit\Framework\TestCase;
+
+class RegexConditionTest extends TestCase
+{
+    /** @return array<string, array{string, string, bool}> */
+    public function traitValuesAndPatterns(): array
+    {
+        return [
+            'value starting with the pattern' => ['production', 'prod', true],
+            'pattern found only mid-value' => ['nonproduction', 'prod', false],
+            'value starting with any alternative' => ['development', 'prod|dev', true],
+            'alternative found only mid-value' => ['nondev', 'prod|dev', false],
+            'end-anchored pattern found only mid-value' => ['jane@example.com', '@example\.com$', false],
+            'fully anchored email domain' => ['jane@example.com', '^[^@]+@example\.com$', true],
+            'fully anchored environment prefix' => ['prod-api', '^(prod|uat|dev)-.+$', true],
+            'fully anchored fixed-format ID' => ['user-0001', '^user-[0-9]{4}$', true],
+            'fully anchored ID with trailing characters' => ['user-00012', '^user-[0-9]{4}$', false],
+            'value differing in case' => ['Production', 'prod', false],
+            'numeric trait value' => ['123', '^\d+$', true],
+        ];
+    }
+
+    /**
+     * @dataProvider traitValuesAndPatterns
+     */
+    public function testIdentityIsInSegmentWhenTraitMatchesRegexFromTheStart(
+        string $traitValue,
+        string $pattern,
+        bool $expectedInSegment,
+    ): void {
+        // Given
+        $context = EvaluationContext::fromJsonObject((object) [
+            'environment' => (object) ['key' => 'api-key', 'name' => 'Test'],
+            'identity' => (object) [
+                'identifier' => 'jane',
+                'traits' => (object) ['environment' => $traitValue],
+            ],
+            'segments' => [
+                (object) [
+                    'key' => '1',
+                    'name' => 'regex_segment',
+                    'rules' => [
+                        (object) [
+                            'type' => 'ALL',
+                            'conditions' => [
+                                (object) [
+                                    'property' => 'environment',
+                                    'operator' => 'REGEX',
+                                    'value' => $pattern,
+                                ],
+                            ],
+                        ],
+                    ],
+                ],
+            ],
+        ]);
+
+        // When
+        $result = Engine::getEvaluationResult($context);
+
+        // Then
+        $this->assertSame(
+            $expectedInSegment ? ['regex_segment'] : [],
+            array_map(fn ($segment) => $segment->name, $result->segments),
+        );
+    }
+}