diff --git a/README.md b/README.md
index 367d53c..c441163 100644
--- a/README.md
+++ b/README.md
@@ -131,8 +131,9 @@ following exceptions:
 
 - `reducer` (funcref)
     - Reducer transforms the output lines of fzf into a single string value
-- `prefix` (string; default: `\k*$`)
+- `prefix` (string or funcref; default: `\k*$`)
     - Regular expression pattern to extract the completion prefix
+    - Or a function to extract completion prefix
 - Both `source` and `options` can be given as funcrefs that take the
   completion prefix as the argument and return the final value
 - `sink` or `sink*` are not allowed
diff --git a/autoload/fzf/vim.vim b/autoload/fzf/vim.vim
index 2f8204b..7d31a76 100644
--- a/autoload/fzf/vim.vim
+++ b/autoload/fzf/vim.vim
@@ -672,9 +672,17 @@ function! fzf#vim#complete(...)
   let s:eol = col('.') == eol
   let &ve = ve
 
-  let prefix = s:pluck(s:opts, 'prefix', '\k*$')
-  let s:query = col('.') == 1 ? '' :
-        \ matchstr(getline('.')[0 : col('.')-2], prefix)
+  let Prefix = s:pluck(s:opts, 'prefix', '\k*$')
+  if col('.') == 1
+    let s:query = ''
+  else
+    let full_prefix = getline('.')[0 : col('.')-2]
+    if type(Prefix) == s:TYPE.funcref
+      let s:query = call(Prefix, [full_prefix])
+    else
+      let s:query = matchstr(full_prefix, Prefix)
+    endif
+  endif
   let s:opts = s:eval(s:opts, 'source', s:query)
   let s:opts = s:eval(s:opts, 'options', s:query)
 
diff --git a/autoload/fzf/vim/complete.vim b/autoload/fzf/vim/complete.vim
index ca49d1a..2606446 100644
--- a/autoload/fzf/vim/complete.vim
+++ b/autoload/fzf/vim/complete.vim
@@ -58,34 +58,54 @@ function! s:file_options(prefix)
   return printf('--prompt %s --query %s', shellescape(head), shellescape(tail))
 endfunction
 
-function! s:fname_regex()
+function! s:fname_prefix(str)
   let isf = &isfname
-  let pats = []
+  let white = []
+  let black = []
   if isf =~ ',,,'
-    call add(pats, ',')
+    call add(white, ',')
     let isf = substitute(isf, ',,,', ',', 'g')
   endif
+  if isf =~ ',^,,'
+    call add(black, ',')
+    let isf = substitute(isf, ',^,,', ',', 'g')
+  endif
+
   for token in split(isf, ',')
-    if token =~ '[0-9]\+-[0-9]\+'
-      let range = map(split(token, '-'), 'str2nr(v:val)')
-      for i in range(range[0], range[1])
-        call add(pats, nr2char(i))
-      endfor
+    let target = white
+    if token[0] == '^'
+      let target = black
+      let token = token[1:]
+    endif
+
+    let ends = matchlist(token, '\(.\+\)-\(.\+\)')
+    if empty(ends)
+      call add(target, token)
     else
-      call add(pats, token)
+      let ends = map(ends[1:2], "len(v:val) == 1 ? char2nr(v:val) : str2nr(v:val)")
+      for i in range(ends[0], ends[1])
+        call add(target, nr2char(i))
+      endfor
     endif
   endfor
-  let pattern = escape(join(pats, ''), ']')
-  if pattern =~ '-'
-    let pattern = substitute(pattern, '-', '', 'g').'-'
-  endif
-  return '[\w'.pattern.']'
+
+  let prefix = a:str
+  for offset in range(1, len(a:str))
+    let char = a:str[len(a:str) - offset]
+    if (char =~ '\w' || index(white, char) >= 0) && index(black, char) < 0
+      continue
+    endif
+    let prefix = strpart(a:str, len(a:str) - offset + 1)
+    break
+  endfor
+
+  return prefix
 endfunction
 
 function! s:complete_file(command, extra_opts)
   let s:file_cmd = a:command
   return fzf#vim#complete(extend({
-  \ 'prefix':  s:fname_regex().'*$',
+  \ 'prefix':  function('s:fname_prefix'),
   \ 'source':  function('s:file_source'),
   \ 'options': function('s:file_options')}, get(a:extra_opts, 0, g:fzf#vim#default_layout)))
 endfunction