diff --git a/src/net/m14m/romannumeralkata/ExpressionParser.java b/src/net/m14m/romannumeralkata/ExpressionParser.java
index 5ed70ea..6fdd8ac 100644
--- a/src/net/m14m/romannumeralkata/ExpressionParser.java
+++ b/src/net/m14m/romannumeralkata/ExpressionParser.java
@@ -1,5 +1,7 @@
 package net.m14m.romannumeralkata;
 
+import java.util.regex.*;
+
 public class ExpressionParser {
     private final NumeralParser romanNumeralParser;
     private final Formatter romanNumeralFormatter;
@@ -16,7 +18,9 @@ public ExpressionParser(RomanNumeralParser romanNumeralParser,
         this.arabicNumeralFormatter = arabicNumeralFormatter;
     }
 
-    public Expression parse(String input) {
+    public Expression parse(String input) throws IllegalExpressionError {
+        Matcher matcher = Pattern.compile("([mdclxviMDCLXVI]+|\\d+)").matcher(input);
+        if (!matcher.matches()) throw new IllegalExpressionError();
         Expression expression;
         if (input.matches("[mdclxviMDCLXVI]+")) {
             expression = new Numeral(arabicNumeralFormatter, romanNumeralParser, input);
diff --git a/src/net/m14m/romannumeralkata/Main.java b/src/net/m14m/romannumeralkata/Main.java
index 0d9c68c..22186a2 100644
--- a/src/net/m14m/romannumeralkata/Main.java
+++ b/src/net/m14m/romannumeralkata/Main.java
@@ -21,9 +21,13 @@ public Main(Reader input, Writer output) {
     public void execute() throws IOException {
         String input;
         while ((input = this.input.readLine()) != null) {
-            Expression expression = expressionParser.parse(input);
-            String displayValue = expression.getFormatter().format(expression.getValue());
-            output.println(displayValue);
+            try {
+                Expression expression = expressionParser.parse(input);
+                String displayValue = expression.getFormatter().format(expression.getValue());
+                output.println(displayValue);
+            } catch (IllegalExpressionError illegalExpressionError) {
+                throw new RuntimeException(illegalExpressionError);
+            }
         }
     }
 }
diff --git a/test/net/m14m/romannumeralkata/ExpressionParserTest.java b/test/net/m14m/romannumeralkata/ExpressionParserTest.java
index c3ae677..72580b3 100644
--- a/test/net/m14m/romannumeralkata/ExpressionParserTest.java
+++ b/test/net/m14m/romannumeralkata/ExpressionParserTest.java
@@ -11,18 +11,18 @@ public class ExpressionParserTest {
             new ArabicNumeralParser(), new ArabicNumeralFormatter()
     );
 
-    @Test public void shouldParseRomanNumeralExpressions() {
+    @Test public void shouldParseRomanNumeralExpressions() throws IllegalExpressionError {
         assertThat(parser.parse("iii").getValue(), is(3));
         assertThat(parser.parse("IX").getValue(), is(9));
     }
 
-    @Test public void shouldParseArabicNumeralExpressions() {
+    @Test public void shouldParseArabicNumeralExpressions() throws IllegalExpressionError {
         assertThat(parser.parse("3").getValue(), is(3));
         assertThat(parser.parse("9").getValue(), is(9));
     }
 
     @Test(expected = IllegalExpressionError.class)
-    public void shouldDisallowBadExpressions() {
+    public void shouldDisallowBadExpressions() throws IllegalExpressionError {
         parser.parse("abc");
     }
 }