gsub/sub supports back references in substitutes. fixes #2816.

This implementation is compatible with CRuby's String#gsub/sub
except \1 ... \9 and \+.  They are useless without Regexp library.

mrblib/string.rb

@@ -20,6 +20,30 @@ class String
     self
   end
 
+  # private method for gsub/sub
+  def __sub_replace(pre, m, post)
+    s = ""
+    i = 0
+    while j = index("\\", i)
+      break if j == length-1
+      t = case self[j+1]
+          when "\\"
+            "\\"
+          when "`"
+            pre
+          when "&", "0"
+            m
+          when "'"
+            post
+          else
+            self[j, 2]
+          end
+      s += self[i, j-i] + t
+      i = j + 2
+    end
+    s + self[i, length-i]
+  end
+
   ##
   # Replace all matches of +pattern+ with +replacement+.
   # Call block (if given) for each match and replace
@@ -29,7 +53,17 @@ class String
   # ISO 15.2.10.5.18
   def gsub(*args, &block)
     if args.size == 2
-      split(args[0], -1).join(args[1])
+      s = ""
+      i = 0
+      while j = index(args[0], i)
+        seplen = args[0].length
+        k = j + seplen
+        pre = self[0, j]
+        post = self[k, length-k]
+        s += self[i, j-i] + args[1].__sub_replace(pre, args[0], post)
+        i = k
+      end
+      s + self[i, length-i]
     elsif args.size == 1 && block
       split(args[0], -1).join(block.call(args[0]))
     else
@@ -76,7 +110,8 @@ class String
   # ISO 15.2.10.5.36
   def sub(*args, &block)
     if args.size == 2
-      split(args[0], 2).join(args[1])
+      pre, post = split(args[0], 2)
+      pre + args[1].__sub_replace(pre, args[0], post) + post
     elsif args.size == 1 && block
       split(args[0], 2).join(block.call(args[0]))
     else

test/t/string.rb

@@ -254,6 +254,15 @@ assert('String#gsub', '15.2.10.5.18') do
   assert_equal('A',      'a'.gsub('a'){|w| w.capitalize })
 end
 
+assert('String#gsub with backslash') do
+  s = 'abXcdXef'
+  assert_equal 'ab<\\>cd<\\>ef',    s.gsub('X', '<\\\\>')
+  assert_equal 'ab<X>cd<X>ef',      s.gsub('X', '<\\&>')
+  assert_equal 'ab<X>cd<X>ef',      s.gsub('X', '<\\0>')
+  assert_equal 'ab<ab>cd<abXcd>ef', s.gsub('X', '<\\`>')
+  assert_equal 'ab<cdXef>cd<ef>ef', s.gsub('X', '<\\\'>')
+end
+
 assert('String#gsub!', '15.2.10.5.19') do
   a = 'abcabc'
   a.gsub!('b', 'B')
@@ -416,6 +425,15 @@ assert('String#sub', '15.2.10.5.36') do
   assert_equal 'aa$', 'aa#'.sub('#', '$')
 end
 
+assert('String#sub with backslash') do
+  s = 'abXcdXef'
+  assert_equal 'ab<\\>cdXef',    s.sub('X', '<\\\\>')
+  assert_equal 'ab<X>cdXef',     s.sub('X', '<\\&>')
+  assert_equal 'ab<X>cdXef',     s.sub('X', '<\\0>')
+  assert_equal 'ab<ab>cdXef',    s.sub('X', '<\\`>')
+  assert_equal 'ab<cdXef>cdXef', s.sub('X', '<\\\'>')
+end
+
 assert('String#sub!', '15.2.10.5.37') do
   a = 'abcabc'
   a.sub!('b', 'B')