Fix result if pattern is empty

mrblib/string.rb

@@ -124,15 +124,31 @@ def scan(reg, &block)
   #
   # ISO 15.2.10.5.36
   def sub(*args, &block)
-    if args.size == 2
-      pre, post = split(args[0], 2)
-      return self unless post # The sub target wasn't found in the string
-      pre + args[1].__sub_replace(pre, args[0], post) + post
-    elsif args.size == 1 && block
-      split(args[0], 2).join(block.call(args[0]))
+    unless (1..2).include?(args.length)
+      raise ArgumentError, "wrong number of arguments (given #{args.length}, expected 2)"
+    end
+
+    pattern, replace = *args
+    pattern = pattern.to_str
+    if args.length == 2 && block
+      block = nil
+    end
+    if !block
+      replace = replace.to_str
+    end
+    result = []
+    this = dup
+    found = index(pattern)
+    return this unless found
+    result << this[0, found]
+    offset = found + pattern.length
+    result << if block
+      block.call(pattern).to_s
     else
-      raise ArgumentError, "wrong number of arguments"
+      replace.__sub_replace(this[0, found], pattern, this[offset..-1] || "")
     end
+    result << this[offset..-1] if offset < length
+    result.join
   end
 
   ##

test/t/string.rb

@@ -585,6 +585,16 @@ def to_str
   assert_equal 'aBcabc', 'abcabc'.sub('b', 'B')
   assert_equal 'aBcabc', 'abcabc'.sub('b') { |w| w.capitalize }
   assert_equal 'aa$', 'aa#'.sub('#', '$')
+  assert_equal '.abc', "abc".sub("", ".")
+
+  str = "abc"
+  miss = str.sub("X", "Z")
+  assert_equal str, miss
+  assert_not_equal str.object_id, miss.object_id
+
+  a = []
+  assert_equal '.abc', "abc".sub("") { |i| a << i; "." }
+  assert_equal [""], a
 end
 
 assert('String#sub with backslash') do