It also seems like HPR comments eats backslashes!
Here's my comment showing where backslashes should be.
Would be good if there was a preview comment option:

It seems the rule of quoted literals doesn't apply if the RHS is a variable. So a variable with a quoted "." would try to match a quote followed by . followed by another quote.
If you wanted to match a quote in a literal RE you would have to write {backslash}"{backslash}.{backslash}"
A literal RE "." would be like unquoted {backslash}.
The following Bash snippet illustrates:

#!/bin/bash

v=0
for r in '^a{backslash}.b$' '^a"."b$' "^a'.'b$"; do
  ((v++))
  # matches var 1 only
  [[ a.b =~ $r ]] &amp;&amp; echo match var $v
  # matches var 2 only
  [[ 'a"."b' =~ $r ]] &amp;&amp; echo match double quote $v
  # matches var 3 only
  [[ "a'.'b" =~ $r ]] &amp;&amp; echo match single quote $v
  # all 3 match
  eval "[[ a.b =~ $r ]] &amp;&amp; echo match eval $v"
done