Allow negative numbers in conditional part of format string

Closes #229

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1887802 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/poi/ss/format/CellFormatPart.java

@@ -108,7 +108,7 @@ public class CellFormatPart {
     static {
         // A condition specification
         String condition = "([<>=]=?|!=|<>)    # The operator\n" +
-                "  \\s*([0-9]+(?:\\.[0-9]*)?)\\s*  # The constant to test against\n";
+                "  \\s*(-?[0-9]+(?:\\.[0-9]*)?)\\s*  # The constant to test against\n";
 
         // A currency symbol / string, in a specific locale
         String currency = "(\\[\\$.{0,3}-[0-9a-f]{3}\\])";