I'll give it a try.
Thanks for the quick response.

Fixed with changeset 50d1cd44b2cb. This will be part of release 23.4.

Note: There is a better way to comment/uncomment a range of lines. Just select the lines 
and press Ctrl-M (to comment) or Shift-Ctrl-M (to uncomment). To toggle the comment state 
of selected lines press Ctrl-#.

issue confirmed

Selecting a block of text then opening the search/replace window to add a '#' to the beginning of each line using regex locks up the editor to the point that only way to close the editor is by killing the process ID.
Steps to reproduce
Create several lines of test.
Highlight the lines of text by holding the shift-left-mouse and dragging across the lines
Open the search/replace window (Ctrl-R)
Select regex, and selection only
In the search window type "^" whithout the quotes.
The editor locks up, even the close icon 'x' in the upper right corner of the window doesn't close the editor.

OS: qubes, fedora-36 vm
This is a fresh install of eric7_ide:
  cd prj
  python3 -m venv eric7_venv
  ./eric7_venv/bin/python3 -m pip install --upgrade eric-ide
  ./eric7_venv/bin/eric7_post_install
APIs:
  MicroPython
  Python3
  QSS
Documentation sets installed:
  PyQt-5.15
  PyQt-6.4
  Python-3.10
  Qt-5.15
  Qt-6.4

Selecting a block of text then opening the search/replace window to add a '#' to the beginning of each line using regex locks up the editor to the point that only killing the editor must be killed using the process ID.
Steps to reproduce
Create several lines of test.
Highlight the lines of text by holding the shift-left-mouse and dragging across the lines
Open the search/replace window (Ctrl-R)
Select regex, and selection only
In the search window type "^" whithout the quotes.
The editor locks up, even the close icon 'x' in the upper right corner of the window doesn't close the editor.