diff --git a/lex.py b/lex.py
index bd41af8..93f4c0a 100755
--- a/lex.py
+++ b/lex.py
@@ -351,8 +351,8 @@ class RegionRule(Rule):
             if self.pairs[i][1]:
                 try:
                     stopre = re.compile(self.pairs[i][1] % matchd, self.reflags)
-                except:
-                    raise Exception, "%r\n%r\n%r" % (self.pairs[i][1], matchd, self.reflags)
+                except Exception, e:
+                    raise Exception, "%r\n%r\n%r\n%r" % (e, self.pairs[i][1], matchd, self.reflags)
             else:
                 stopre = None
             if i == len(self.pairs) - 1:
diff --git a/mode/nasm.py b/mode/nasm.py
index 73faf78..c7060de 100644
--- a/mode/nasm.py
+++ b/mode/nasm.py
@@ -11,15 +11,19 @@ word = chr1 + chr2 + '*'
 
 class NasmGrammar(Grammar):
     rules = [
+        PatternRule('comment', ';.*$'),
+        RegionRule('comment', '%if +0', Grammar, r'%%endif'),
+
         # for use with ndisasm files
-        PatternRule('nasm.address', '^[0-9a-fA-F]+ +[0-9a-fA-F]+'),
+        PatternMatchRule('x', '^([0-9a-fA-F]{8})(  )([0-9a-fA-F]+)', 'nasm.integer', 'spaces', 'nasm.address'),
+        PatternRule('nasm.address', '^[0-9a-fA-F]{8}  [0-9a-fA-F]+'),
 
         PatternRule('spaces', ' +'),
         PatternRule("nasm.integer", "(0[xX][0-9a-fA-F]+|0|[1-9][0-9]*|0[0-7]+)[lL]?"),
         PatternRule('nasm.register', "(?:eax|ebx|ecx|edx|ax|ah|al|bx|bh|bl|bp|cx|ch|cl|cmpsb|dh|di|dl|ds|dx|esi|es|edi|esp|ebp|si|sp|stosd)(?!" + chr2 + ")"),
         PatternRule('nasm.pseudo', "(?:a16|a32|byte|db|dd|do|dq|dt|dword|qword|tword|fs|gs|near|o16|o32|short)(?!" + chr2 + ")"),
         PatternRule('nasm.instruction', "(?:jg|jeq|jecxz|jne|ja|jmp|push|pushad|pushfd|call|ret|sub|adc|add|popad|popaw|popa|popfd|pop|call|and|arpl|bound|cwd|cdq|clc|cld|cli|cmp|cmpxchg|cpuid|das|dec|divpd|div|enter|leave|fadd|fld|fmul|fsqrt|fsub|fs|hlt|imul|inc|insb|insd|insw|ins|int|int3|in|jcxz|jc|jna|jnc|jnz|jo|js|jz|lea|lldt|loadsb|lock|loopne|movzx|movd|mov|mul|neg|not|nop|or|outsb|outsd|outsw|repe|retf|rol|sal|sar|sbb|scasw|shl|shr|shld|shrd|sldt|syscall|sysenter|sysexit|test|xchg|xadd|xor)(?!" + chr2 + ")"),
-        PatternRule('nasm.macro', "%(?:define|undef|assign|strlen|macro|endmacro|if|elif|else|endif|ifdef|ifndef|include|push|pop|stacksize)(?!" + chr2 + ")"),
+        PatternRule('nasm.macro', "\%(?:define|undef|assign|strlen|macro|endmacro|if|elif|else|endif|ifdef|ifndef|include|push|pop|stacksize)(?!" + chr2 + ")"),
         PatternRule('nasm.keyword', "(?:section|global|extern)(?!" + chr2 + ")"),
         PatternRule('nasm.prefix', "(?:dword|word|lock)(?!" + chr2 + ")"),
         PatternMatchRule('x', '(' + word + ')(:)', 'nasm.label', 'delimiter'),
@@ -30,7 +34,6 @@ class NasmGrammar(Grammar):
         RegionRule('string', "'''", StringGrammar3, "'''"),
         RegionRule('string', '"', StringGrammar2, '"'),
         RegionRule('string', "'", StringGrammar1, "'"),
-        PatternRule('comment', ';.*$'),
         PatternRule('eol', '\n'),
         PatternRule('continuation', r'\\\n$'),
     ]