Be more POSIXly correct in the use of expr.
diff --git a/dx/etc/dx b/dx/etc/dx
index c2b3c9b..f773495 100644
--- a/dx/etc/dx
+++ b/dx/etc/dx
@@ -63,7 +63,7 @@
 javaOpts=""
 
 while expr "x$1" : 'x-J' >/dev/null; do
-    opt=`expr "$1" : '-J\(.*\)'`
+    opt=`expr "x$1" : 'x-J\(.*\)'`
     javaOpts="${javaOpts} -${opt}"
     if expr "x${opt}" : "xXmx[0-9]" >/dev/null; then
         defaultMx="no"