diff --git a/include/ruby/re.h b/include/ruby/re.h
index 41b3e49..39acee9 100644
--- a/include/ruby/re.h
+++ b/include/ruby/re.h
@@ -52,6 +52,7 @@ struct RMatch {
 #define RMATCH_REGS(obj)  (&(R_CAST(RMatch)(obj))->rmatch->regs)
 
 VALUE rb_reg_regcomp(VALUE);
+long rb_reg_search0(VALUE, VALUE, long, int, int);
 long rb_reg_search(VALUE, VALUE, long, int);
 VALUE rb_reg_regsub(VALUE, VALUE, struct re_registers *, VALUE);
 long rb_reg_adjust_startpos(VALUE, VALUE, long, int);
diff --git a/re.c b/re.c
index 106cf59..1ceb6ea 100644
--- a/re.c
+++ b/re.c
@@ -1375,7 +1375,7 @@
 
 /* returns byte offset */
 long
-rb_reg_search(VALUE re, VALUE str, long pos, int reverse)
+rb_reg_search0(VALUE re, VALUE str, long pos, int reverse, int set_backref_str)
 {
     long result;
     VALUE match;
@@ -1450,17 +1450,26 @@
 	    FL_UNSET(match, FL_TAINT);
     }
 
-    RMATCH(match)->str = rb_str_new4(str);
+    if (set_backref_str) {
+	RMATCH(match)->str = rb_str_new4(str);
+	OBJ_INFECT(match, str);
+    }
+
     RMATCH(match)->regexp = re;
     RMATCH(match)->rmatch->char_offset_updated = 0;
     rb_backref_set(match);
 
     OBJ_INFECT(match, re);
-    OBJ_INFECT(match, str);
 
     return result;
 }
 
+long
+rb_reg_search(VALUE re, VALUE str, long pos, int reverse)
+{
+	return rb_reg_search0(re, str, pos, reverse, 1);
+}
+
 VALUE
 rb_reg_nth_defined(int nth, VALUE match)
 {
diff --git a/string.c b/string.c
index 075876f..3eda81c 100644
--- a/string.c
+++ b/string.c
@@ -4021,6 +4021,7 @@ enum neighbor_char {
     int iter = 0;
     char *sp, *cp;
     int tainted = 0;
+    int str_replace;
     rb_encoding *str_enc;
 
     switch (argc) {
@@ -4041,7 +4042,8 @@ enum neighbor_char {
     }
 
     pat = get_pat(argv[0], 1);
-    beg = rb_reg_search(pat, str, 0, 0);
+    str_replace = !iter && NIL_P(hash);
+    beg = rb_reg_search0(pat, str, 0, 0, !str_replace);
     if (beg < 0) {
 	if (bang) return Qnil;	/* no match, no substitution */
 	return rb_str_dup(str);
@@ -4064,7 +4066,7 @@ enum neighbor_char {
 	regs = RMATCH_REGS(match);
 	beg0 = BEG(0);
 	end0 = END(0);
-	if (iter || !NIL_P(hash)) {
+	if (!str_replace) {
             if (iter) {
                 val = rb_obj_as_string(rb_yield(rb_reg_nth_match(0, match)));
             }
@@ -4104,7 +4106,7 @@ enum neighbor_char {
 	}
 	cp = RSTRING_PTR(str) + offset;
 	if (offset > RSTRING_LEN(str)) break;
-	beg = rb_reg_search(pat, str, offset, 0);
+	beg = rb_reg_search0(pat, str, offset, 0, !str_replace);
     } while (beg >= 0);
     if (RSTRING_LEN(str) > offset) {
         rb_enc_str_buf_cat(dest, cp, RSTRING_LEN(str) - offset, str_enc);