diff --git a/regcomp.c b/regcomp.c
index de44cfe037..69a63011d1 100644
--- a/regcomp.c
+++ b/regcomp.c
@@ -3950,17 +3950,22 @@ optimize_node_left(Node* node, NodeOptInfo* opt, OptEnv* env)
 	}
       }
 
-      if (! ONIGENC_IS_SINGLEBYTE(env->enc)) {
-        if (! IS_NULL(cc->mbuf) ||
-            (cc->not != 0 && found != 0)) {
-          for (i = 0; i < SINGLE_BYTE_SIZE; i++) {
-            z = ONIGENC_IS_MBC_HEAD(env->enc, i);
-            if (z) {
-              mb_found = 1;
-              add_char_opt_map_info(&opt->map, i);
-            }
-          }
-        }
+      if (IS_NULL(cc->mbuf)) {
+	if (cc->not) {
+	  for (i = 0; i < SINGLE_BYTE_SIZE; i++) {
+	    add_char_opt_map_info(&opt->map, i);
+	  }
+	  mb_found = 1;
+	}
+      }
+      else {
+	for (i = 0; i < SINGLE_BYTE_SIZE; i++) {
+	  z = ONIGENC_IS_MBC_HEAD(env->enc, i);
+	  if (z) {
+	    mb_found = 1;
+	    add_char_opt_map_info(&opt->map, i);
+	  }
+	}
       }
 
       if (mb_found) {
diff --git a/regexec.c b/regexec.c
index 1bae0d9516..581fe64b69 100644
--- a/regexec.c
+++ b/regexec.c
@@ -362,26 +362,11 @@ typedef struct {
   };\
 } while(0)
 
-static unsigned int MatchStackLimitSize = DEFAULT_MATCH_STACK_LIMIT_SIZE;
-
-extern unsigned int
-onig_get_match_stack_limit_size(void)
-{
-  return MatchStackLimitSize;
-}
-
-extern int
-onig_set_match_stack_limit_size(unsigned int size)
-{
-  MatchStackLimitSize = size;
-  return 0;
-}
-
 static int
 stack_double(StackType** arg_stk_base, StackType** arg_stk_end,
 	     StackType** arg_stk, StackType* stk_alloc, MatchArg* msa)
 {
-  unsigned int n;
+  int n;
   StackType *x, *stk_base, *stk_end, *stk;
 
   stk_base = *arg_stk_base;
@@ -400,12 +385,7 @@ stack_double(StackType** arg_stk_base, StackType** arg_stk_end,
   }
   else {
     n *= 2;
-    if (MatchStackLimitSize != 0 && n > MatchStackLimitSize) {
-      if ((unsigned int )(stk_end - stk_base) == MatchStackLimitSize)
-        return ONIGERR_MATCH_STACK_LIMIT_OVER;
-      else
-        n = MatchStackLimitSize;
-    }
+    if (n > MATCH_STACK_LIMIT_SIZE) return ONIGERR_MATCH_STACK_LIMIT_OVER;
     x = (StackType* )xrealloc(stk_base, sizeof(StackType) * n);
     if (IS_NULL(x)) {
       STACK_SAVE;
@@ -2593,13 +2573,11 @@ bm_search_notrev(regex_t* reg, UChar* target, UChar* target_end,
       if (t < target) return p + 1;
 
       skip = reg->map[*s];
-      p = s + 1;
-      if (p >= text_end) return (UChar* )NULL;
+      p++;
       t = p;
-      do {
+      while ((p - t) < skip) {
 	p += enc_len(reg->enc, *p);
-      } while ((p - t) < skip && p < text_end);
-
+      }
       s += (p - t);
     }
   }
@@ -2613,13 +2591,11 @@ bm_search_notrev(regex_t* reg, UChar* target, UChar* target_end,
       if (t < target) return p + 1;
 
       skip = reg->int_map[*s];
-      p = s + 1;
-      if (p >= text_end) return (UChar* )NULL;
+      p++;
       t = p;
-      do {
+      while ((p - t) < skip) {
 	p += enc_len(reg->enc, *p);
-      } while ((p - t) < skip && p < text_end);
-
+      }
       s += (p - t);
     }
   }
diff --git a/regint.h b/regint.h
index bcc5fa5fc4..ba201f1a28 100644
--- a/regint.h
+++ b/regint.h
@@ -46,7 +46,7 @@
 #define USE_QUALIFIER_PEEK_NEXT
 
 #define INIT_MATCH_STACK_SIZE                     160
-#define DEFAULT_MATCH_STACK_LIMIT_SIZE              0 /* unlimited */
+#define MATCH_STACK_LIMIT_SIZE                 500000
 
 /* interface to external system */
 #ifdef NOT_RUBY      /* gived from Makefile */