Highlight backslash escapes within $'' strings.

Correct highlighting of backslash escapes within "" strings: highlight only
the four specific escape sequences defined there.

Fixes zsh-users/zsh-syntax-highlighting#196.

highlighters/main/test-data/dollar-quoted2.zsh

@@ -0,0 +1,38 @@
+#!/usr/bin/env zsh
+# -------------------------------------------------------------------------------------------------
+# Copyright (c) 2015 zsh-syntax-highlighting contributors
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without modification, are permitted
+# provided that the following conditions are met:
+#
+#  * Redistributions of source code must retain the above copyright notice, this list of conditions
+#    and the following disclaimer.
+#  * Redistributions in binary form must reproduce the above copyright notice, this list of
+#    conditions and the following disclaimer in the documentation and/or other materials provided
+#    with the distribution.
+#  * Neither the name of the zsh-syntax-highlighting contributors nor the names of its contributors
+#    may be used to endorse or promote products derived from this software without specific prior
+#    written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR
+# IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+# FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
+# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER
+# IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+# OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+# -------------------------------------------------------------------------------------------------
+# -*- mode: zsh; sh-indentation: 2; indent-tabs-mode: nil; sh-basic-offset: 2; -*-
+# vim: ft=zsh sw=2 ts=2 et
+# -------------------------------------------------------------------------------------------------
+
+ZSH_HIGHLIGHT_STYLES[back-dollar-quoted-argument]=$unused_highlight
+BUFFER=": \$'foo\xbar'"
+
+expected_region_highlight=(
+  "3 7 $ZSH_HIGHLIGHT_STYLES[dollar-quoted-argument]" # $'foo
+  "8 11 $ZSH_HIGHLIGHT_STYLES[back-dollar-quoted-argument]" # \xba
+  "12 13 $ZSH_HIGHLIGHT_STYLES[dollar-quoted-argument]" # r'
+)

highlighters/main/test-data/double-quoted.zsh

@@ -29,7 +29,7 @@
 # -------------------------------------------------------------------------------------------------
 
 BUFFER=': "foo$bar:\`:\":\$:'
-BUFFER+=\\\':\"
+BUFFER+=\\\\:\"
 
 expected_region_highlight=(
   "3 6 $ZSH_HIGHLIGHT_STYLES[double-quoted-argument]" # "foo
@@ -41,6 +41,6 @@ expected_region_highlight=(
   "17 17 $ZSH_HIGHLIGHT_STYLES[double-quoted-argument]" # :
   "18 19 $ZSH_HIGHLIGHT_STYLES[back-double-quoted-argument]" # \"
   "20 20 $ZSH_HIGHLIGHT_STYLES[double-quoted-argument]" # :
-  "21 22 $ZSH_HIGHLIGHT_STYLES[back-double-quoted-argument]" # \'
+  "21 22 $ZSH_HIGHLIGHT_STYLES[back-double-quoted-argument]" # \\
   "23 24 $ZSH_HIGHLIGHT_STYLES[double-quoted-argument]" # :"
 )

highlighters/main/test-data/double-quoted3.zsh

@@ -29,10 +29,12 @@
 # -------------------------------------------------------------------------------------------------
 
 BUFFER=': "$" "$42foo"'
+BUFFER+=\ \"\\\'\\x\"
 
 expected_region_highlight=(
   "3 5 $ZSH_HIGHLIGHT_STYLES[double-quoted-argument]" # "$"
   "7 7 $ZSH_HIGHLIGHT_STYLES[double-quoted-argument]" # "
   "8 10 $ZSH_HIGHLIGHT_STYLES[dollar-double-quoted-argument]" # $42
   "11 14 $ZSH_HIGHLIGHT_STYLES[double-quoted-argument]" # foo"
+  "16 21 $ZSH_HIGHLIGHT_STYLES[double-quoted-argument]" # "\'\x" - \' and \x are not escape sequences
 )