diff --git a/lib/command-error.coffee b/lib/command-error.coffee
new file mode 100644
index 0000000..c9861e9
--- /dev/null
+++ b/lib/command-error.coffee
@@ -0,0 +1,5 @@
+class CommandError
+  constructor: (@message) ->
+    @name = 'Command Error'
+
+module.exports = CommandError
diff --git a/lib/command.coffee b/lib/command.coffee
index 5fb60de..0b5849b 100644
--- a/lib/command.coffee
+++ b/lib/command.coffee
@@ -1,10 +1,7 @@
 ExViewModel = require './ex-view-model'
 Ex = require './ex'
 Find = require './find'
-
-class CommandError
-  constructor: (@message) ->
-    @name = 'Command Error'
+CommandError = require './command-error'
 
 class Command
   constructor: (@editor, @exState) ->
@@ -164,4 +161,4 @@ class Command
       else
         throw new CommandError("Not an editor command: #{input.characters}")
 
-module.exports = {Command, CommandError}
+module.exports = Command
diff --git a/lib/ex-state.coffee b/lib/ex-state.coffee
index 7aff344..e52ea1c 100644
--- a/lib/ex-state.coffee
+++ b/lib/ex-state.coffee
@@ -1,6 +1,7 @@
 {Emitter, Disposable, CompositeDisposable} = require 'event-kit'
 
-{Command, CommandError} = require './command'
+Command = require './command'
+CommandError = require './command-error'
 
 class ExState
   constructor: (@editorElement, @globalExState) ->
diff --git a/lib/ex.coffee b/lib/ex.coffee
index 6a14cd6..0981945 100644
--- a/lib/ex.coffee
+++ b/lib/ex.coffee
@@ -1,4 +1,5 @@
 path = require 'path'
+CommandError = require './command-error'
 
 trySave = (func) ->
   deferred = Promise.defer()
@@ -33,6 +34,23 @@ getFullPath = (filePath) ->
   return filePath if path.isAbsolute(filePath)
   return path.join(atom.project.getPath(), filePath)
 
+replaceGroups = (groups, replString) ->
+  arr = replString.split('')
+  offset = 0
+  cdiff = 0
+
+  while (m = replString.match(/(?:[^\\]|^)\\(\d)/))?
+    group = groups[m[1]] or ''
+    i = replString.indexOf(m[0])
+    l = m[0].length
+    replString = replString.slice(i + l)
+    arr[i + offset...i + offset + l] = (if l is 2 then '' else m[0][0]) +
+      group
+    arr = arr.join('').split ''
+    offset += i + l - group.length
+
+  return arr.join('').replace(/\\\\(\d)/, '\\$1')
+
 class Ex
   @singleton: =>
     @ex ||= new Ex
@@ -148,6 +166,57 @@ class Ex
 
   sp: (args...) => @split(args...)
 
+  substitute: (range, args) ->
+    args = args.trimLeft()
+    delim = args[0]
+    if /[a-z]/i.test(delim)
+      throw new CommandError(
+        "Regular expressions can't be delimited by letters")
+    delimRE = new RegExp("[^\\\\]#{delim}")
+    spl = []
+    args_ = args[1..]
+    while (i = args_.search(delimRE)) isnt -1
+      spl.push args_[..i]
+      args_ = args_[i + 2..]
+    if args_.length is 0 and spl.length is 3
+      throw new CommandError('Trailing characters')
+    else if args_.length isnt 0
+      spl.push args_
+    if spl.length > 3
+      throw new CommandError('Trailing characters')
+    spl[1] ?= ''
+    spl[2] ?= ''
+
+    try
+      pattern = new RegExp(spl[0], spl[2])
+    catch e
+      if e.message.indexOf('Invalid flags supplied to RegExp constructor') is 0
+        # vim only says 'Trailing characters', but let's be more descriptive
+        throw new CommandError("Invalid flags: #{e.message[45..]}")
+      else if e.message.indexOf('Invalid regular expression: ') is 0
+        throw new CommandError("Invalid RegEx: #{e.message[27..]}")
+      else
+        throw e
+
+    console.log pattern, [[range[0], 0]]
+    buffer = atom.workspace.getActiveTextEditor().buffer
+    # This adds an entry to the history for each replacement
+    for line in [range[0]..range[1]]
+      console.log line
+      buffer.scanInRange(pattern,
+        [[line, 0], [line, buffer.lines[line].length]],
+        ({match, matchText, range, stop, replace}) ->
+          replace(replaceGroups(match[..], spl[1]))
+      )
+    # This finds all matches in the buffer, sadly
+    # buffer.scanInRange(pattern,
+    #   [[range[0], 0], [range[1], buffer.lines[range[1]]].length],
+    #   ({match, matchText, range, stop, replace}) ->
+    #     console.log match, matchText
+    #   )
+
+  s: (args...) => @substitute(args...)
+
   vsplit: (range, args) ->
     args = args.trim()
     filePaths = args.split(' ')