adding function to extract usernames from full matrix ids

matrix-sdk-android/src/main/java/org/matrix/android/sdk/api/MatrixPatterns.kt

@@ -156,6 +156,20 @@ object MatrixPatterns {
         return matrixId?.substringAfter(":", missingDelimiterValue = "")?.takeIf { it.isNotEmpty() }
     }
 
+    /**
+     * Extract user name from a matrix id.
+     *
+     * @param matrixId
+     * @return null if the input is not a valid matrixId
+     */
+    fun extractUserNameFromId(matrixId: String): String? {
+        return if (isUserId(matrixId)) {
+            matrixId.removePrefix("@").substringBefore(":", missingDelimiterValue = "")
+        } else {
+            null
+        }
+    }
+
     /**
      * Orders which are not strings, or do not consist solely of ascii characters in the range \x20 (space) to \x7E (~),
      * or consist of more than 50 characters, are forbidden and the field should be ignored if received.

matrix-sdk-android/src/test/java/org/matrix/android/sdk/api/MatrixPatternsTest.kt

@@ -35,6 +35,23 @@ class MatrixPatternsTest {
             MatrixPatterns.isUserId(input) shouldBeEqualTo expected
         }
     }
+
+    @Test
+    fun `given matrix id cases, when extracting userName, then returns expected`() {
+        val cases = listOf(
+                MatrixIdCase("foobar", userName = null),
+                MatrixIdCase("@foobar", userName = null),
+                MatrixIdCase("foobar@matrix.org", userName = null),
+                MatrixIdCase("@foobar: matrix.org", userName = null),
+                MatrixIdCase("foobar:matrix.org", userName = null),
+                MatrixIdCase("@foobar:matrix.org", userName = "foobar"),
+        )
+
+        cases.forEach { (input, expected) ->
+            MatrixPatterns.extractUserNameFromId(input) shouldBeEqualTo expected
+        }
+    }
 }
 
 private data class UserIdCase(val input: String, val isUserId: Boolean)
+private data class MatrixIdCase(val input: String, val userName: String?)