diff --git a/src/main/java/dev/personnummer/Personnummer.java b/src/main/java/dev/personnummer/Personnummer.java
index 9cd407b..1bc856f 100644
--- a/src/main/java/dev/personnummer/Personnummer.java
+++ b/src/main/java/dev/personnummer/Personnummer.java
@@ -14,9 +14,12 @@
  */
 public final class Personnummer implements Comparable<Personnummer> {
     private static final Pattern regexPattern;
+    private static final Pattern interimPatternTest;
+    private static final String interimTestStr = "(?![-+])\\D";
 
     static {
-        regexPattern = Pattern.compile("^(\\d{2})?(\\d{2})(\\d{2})(\\d{2})([-+]?)?((?!000)\\d{3})(\\d?)$");
+        regexPattern = Pattern.compile("^(\\d{2})?(\\d{2})(\\d{2})(\\d{2})([-+]?)?((?!000)\\d{3}|[TRSUWXJKLMN]\\d{2})(\\d?)$");
+        interimPatternTest = Pattern.compile(interimTestStr);
     }
 
     /**
@@ -112,6 +115,13 @@ public Personnummer(String personnummer, Options options) throws PersonnummerExc
             throw new PersonnummerException("Failed to parse personal identity number. Invalid input.");
         }
 
+        if (!options.allowInterimNumbers && interimPatternTest.matcher(personnummer).find()) {
+            throw new PersonnummerException(
+                    personnummer +
+                    " contains non-integer characters and options are set to not allow interim numbers"
+            );
+        }
+
         Matcher matches = regexPattern.matcher(personnummer);
         if (!matches.find()) {
             throw new PersonnummerException("Failed to parse personal identity number. Invalid input.");
@@ -154,9 +164,14 @@ public Personnummer(String personnummer, Options options) throws PersonnummerExc
 
         this.isMale =  Integer.parseInt(Character.toString(this.numbers.charAt(2))) % 2 == 1;
 
+        String nums = matches.group(6);
+        if (options.allowInterimNumbers) {
+            nums = nums.replaceFirst(interimTestStr, "1");
+        }
+
         // The format passed to Luhn method is supposed to be YYmmDDNNN
         // Hence all numbers that are less than 10 (or in last case 100) will have leading 0's added.
-        if (luhn(String.format("%s%s%s%s", this.year, this.month, this.day, matches.group(6))) != Integer.parseInt(this.controlNumber)) {
+        if (luhn(String.format("%s%s%s%s", this.year, this.month, this.day, nums)) != Integer.parseInt(this.controlNumber)) {
             throw new PersonnummerException("Invalid personal identity number.");
         }
     }
diff --git a/src/test/java/DataProvider.java b/src/test/java/DataProvider.java
index ba4dca4..cb14006 100644
--- a/src/test/java/DataProvider.java
+++ b/src/test/java/DataProvider.java
@@ -71,11 +71,16 @@ public static void initialize() throws IOException {
             JSONObject current = rootObject.getJSONObject(i);
             interimNr.add(new PersonnummerData(
                     current.getLong("integer"),
+                    current.getString("long_format"),
                     current.getString("short_format"),
                     current.getString("separated_format"),
+                    current.getString("separated_long"),
                     current.getBoolean("valid"),
-                    current.getString("type")
-            ));
+                    current.getString("type"),
+                    false, // ignore
+                    false // ignore
+                )
+            );
         }
     }
 
diff --git a/src/test/java/InterimnummerTest.java b/src/test/java/InterimnummerTest.java
index 91089a1..c0bac6d 100644
--- a/src/test/java/InterimnummerTest.java
+++ b/src/test/java/InterimnummerTest.java
@@ -34,8 +34,8 @@ public void testValidateInvalidInterim(PersonnummerData ssn) {
     public void testFormatLongInterim(PersonnummerData ssn) throws PersonnummerException {
         Personnummer pnr = Personnummer.parse(ssn.longFormat, opts);
 
-        assertEquals(pnr.format(false), ssn.longFormat);
-        assertEquals(pnr.format(), ssn.separatedFormat);
+        assertEquals(pnr.format(false), ssn.separatedFormat);
+        assertEquals(pnr.format(true), ssn.longFormat);
     }
 
     @ParameterizedTest
@@ -43,8 +43,8 @@ public void testFormatLongInterim(PersonnummerData ssn) throws PersonnummerExcep
     public void testFormatShortInterim(PersonnummerData ssn) throws PersonnummerException {
         Personnummer pnr = Personnummer.parse(ssn.shortFormat, opts);
 
-        assertEquals(pnr.format(false), ssn.longFormat);
-        assertEquals(pnr.format(), ssn.separatedFormat);
+        assertEquals(pnr.format(false), ssn.separatedFormat);
+        assertEquals(pnr.format(true), ssn.longFormat);
     }
 
     @ParameterizedTest