diff --git a/src/calibre/ebooks/mobi/writer2/utils.py b/src/calibre/ebooks/mobi/writer2/utils.py
new file mode 100644
index 0000000000..d3f7ff8c32
--- /dev/null
+++ b/src/calibre/ebooks/mobi/writer2/utils.py
@@ -0,0 +1,118 @@
+#!/usr/bin/env python
+# vim:fileencoding=UTF-8:ts=4:sw=4:sta:et:sts=4:ai
+from __future__ import (unicode_literals, division, absolute_import,
+                        print_function)
+
+__license__   = 'GPL v3'
+__copyright__ = '2011, Kovid Goyal <kovid@kovidgoyal.net>'
+__docformat__ = 'restructuredtext en'
+
+import struct
+
+from calibre.utils.magick.draw import Image, save_cover_data_to, thumbnail
+
+DECINT_FORWARD = 0
+DECINT_BACKWARD = 1
+IMAGE_MAX_SIZE = 10 * 1024 * 1024
+
+def decode_hex_number(raw):
+    '''
+    Return a variable length number encoded using hexadecimal encoding. These
+    numbers have the first byte which tells the number of bytes that follow.
+    The bytes that follow are simply the hexadecimal representation of the
+    number.
+
+    :param raw: Raw binary data as a bytestring
+
+    :return: The number and the number of bytes from raw that the number
+    occupies
+    '''
+    length, = struct.unpack(b'>B', raw[0])
+    raw = raw[1:1+length]
+    consumed = length+1
+    return int(raw, 16), consumed
+
+def encode_number_as_hex(num):
+    '''
+    Encode num as a variable length encoded hexadecimal number. Returns the
+    bytestring containing the encoded number. These
+    numbers have the first byte which tells the number of bytes that follow.
+    The bytes that follow are simply the hexadecimal representation of the
+    number.
+    '''
+    num = bytes(hex(num)[2:])
+    ans = bytearray(num)
+    ans.insert(0, len(num))
+    return bytes(ans)
+
+def encint(value, forward=True):
+    '''
+    Some parts of the Mobipocket format encode data as variable-width integers.
+    These integers are represented big-endian with 7 bits per byte in bits 1-7.
+    They may be either forward-encoded, in which case only the first byte has bit 8 set,
+    or backward-encoded, in which case only the last byte has bit 8 set.
+    For example, the number 0x11111 would be represented forward-encoded as:
+
+        0x04 0x22 0x91
+
+    And backward-encoded as:
+
+        0x84 0x22 0x11
+
+    This function encodes the integer ``value`` as a variable width integer and
+    returns the bytestring corresponding to it.
+    '''
+    # Encode vwi
+    byts = bytearray()
+    while True:
+        b = value & 0b1111111
+        value >>= 7
+        byts.append(b)
+        if value == 0:
+            break
+    byts[0 if forward else -1] |= 0b10000000
+    return bytes(byts)
+
+def rescale_image(data, maxsizeb=IMAGE_MAX_SIZE, dimen=None):
+    '''
+    Convert image setting all transparent pixels to white and changing format
+    to JPEG. Ensure the resultant image has a byte size less than
+    maxsizeb.
+
+    If dimen is not None, generate a thumbnail of width=dimen, height=dimen
+
+    Returns the image as a bytestring
+    '''
+    if dimen is not None:
+        data = thumbnail(data, width=dimen, height=dimen,
+                compression_quality=90)[-1]
+    else:
+        # Replace transparent pixels with white pixels and convert to JPEG
+        data = save_cover_data_to(data, 'img.jpg', return_data=True)
+    if len(data) <= maxsizeb:
+        return data
+    orig_data = data
+    img = Image()
+    quality = 95
+
+    img.load(data)
+    while len(data) >= maxsizeb and quality >= 10:
+        quality -= 5
+        img.set_compression_quality(quality)
+        data = img.export('jpg')
+    if len(data) <= maxsizeb:
+        return data
+    orig_data = data
+
+    scale = 0.9
+    while len(data) >= maxsizeb and scale >= 0.05:
+        img = Image()
+        img.load(orig_data)
+        w, h = img.size
+        img.size = (int(scale*w), int(scale*h))
+        img.set_compression_quality(quality)
+        data = img.export('jpg')
+        scale -= 0.05
+    return data
+
+
diff --git a/src/calibre/gui2/convert/regex_builder.py b/src/calibre/gui2/convert/regex_builder.py
index c7b03c9bb4..c41b8786d9 100644
--- a/src/calibre/gui2/convert/regex_builder.py
+++ b/src/calibre/gui2/convert/regex_builder.py
@@ -7,8 +7,8 @@ __docformat__ = 'restructuredtext en'
 import re, os
 
 from PyQt4.QtCore import SIGNAL, Qt, pyqtSignal
-from PyQt4.QtGui import QDialog, QWidget, QDialogButtonBox, \
-                        QBrush, QTextCursor, QTextEdit
+from PyQt4.QtGui import (QDialog, QWidget, QDialogButtonBox,
+        QBrush, QTextCursor, QTextEdit)
 
 from calibre.gui2.convert.regex_builder_ui import Ui_RegexBuilder
 from calibre.gui2.convert.xexp_edit_ui import Ui_Form as Ui_Edit
@@ -16,6 +16,7 @@ from calibre.gui2 import error_dialog, choose_files
 from calibre.ebooks.oeb.iterator import EbookIterator
 from calibre.ebooks.conversion.preprocess import HTMLPreProcessor
 from calibre.gui2.dialogs.choose_format import ChooseFormatDialog
+from calibre.constants import iswindows
 
 class RegexBuilder(QDialog, Ui_RegexBuilder):
 
@@ -134,8 +135,16 @@ class RegexBuilder(QDialog, Ui_RegexBuilder):
                          _('Cannot build regex using the GUI builder without a book.'),
                          show=True)
             return False
-        fpath = db.format(book_id, format, index_is_id=True,
-            as_path=True)
+        try:
+            fpath = db.format(book_id, format, index_is_id=True,
+                as_path=True)
+        except OSError:
+            if iswindows:
+                error_dialog(self, _('Could not open file'),
+                    _('Could not open file, do you have it open in'
+                        ' another program?'), show=True)
+                return False
+            raise
         try:
             self.open_book(fpath)
         finally: